Formalised performance specifications for rainscreen cladding

CIMclad 

Computer-Integrated Manufacture of Cladding Systems 

Formalised performance specifications for 

rainscreen cladding 

Document Type: Deliverable 

Status: Final 

Reference: Report 4 (Work Package 2) 

Version: 1.2 

Date: 24 July 2001 

ISBN: 1-897911-20-3 

File: c:\data\cimclad\reports\report 4\final report 4.doc 

Primary Author: Loughborough University 

Authors: Emeka Agbasi 

Chimay Anumba 

Alistair Gibb 

Abdulla Kalian 

Alastair Watson 

EPSRC/DETR LINK project GR/N12770 

The CIMclad project is funded under the Innovative Manufacturing Initiative: Meeting Clients’ Needs 

Through Standardisation. In addition to the two academic partners, the project involves a significant number 

of partners whose input to this report is gratefully acknowledged as are input from other sources. For more 

details visit the project web site: http://www.cae.civil.leeds.ac.uk/current/cimclad 

Department of Civil and Building Engineering 

Loughborough University, Loughborough LE11 3TU 

Tel: +44 (0) 1509 222615 

Email: c.j.anumba@lboro.ac.uk 

CAE Group, Department of Civil Engineering 

University of Leeds, Leeds LS2 9JT 

Tel: +44 (0) 113 2332282 

Email: a.s.watson@leeds.ac.uk

CIMclad: Formalised performance specifications for rainscreen cladding 

CONTENTS 

1.0 Introduction 3 

2.0 Rainscreen cladding 3 

3.0 Entity-based Rainscreen cladding specifications 4 

3.1 Background 4 

3.2 Performance-related specification clauses 5 

3.3 Entity type and requirement based specification 7 

References 31 

July 2001 Page 2 of 2


1.0 Introduction 

The CIMclad project is investigating the feasibility of improving the efficiency and competitiveness of the 

cladding sector through the development of a standardisation framework for computer-integrated design 

and manufacture of cladding systems. This is the fourth report of the CIMclad series, which structures 

relevant coherent rainscreen cladding specification identified in Report 3 - Review of Specifications on 

Rainscreen Cladding. 

It is recognised that there is no generally accepted standard specification for rainscreen cladding. Some 

designers/specifiers have developed their own in-house specification in a generic format but tailored to 

meet the requirements of a specific rainscreen cladding project. In-house standards have often been a 

mixture of existing standard and specification clauses on rainscreen cladding such as Standard for Walls 

with Ventilated Rainscreens (CWCT 1998) and Section H92-Rainscreen Cladding (NBS 2000) 

respectively, and experience. The two documents make references to relevant sections of Building 

Regulations, British Standards, and other guidance documents. 

The CWCT standard and NBS specification are widely used as guidance documents for writing 

performance specification and other requirements for rainscreen cladding in the UK, and both are publicly 

available. The two documents are similar with respect to performance requirements. The CWCT standard 

sets out a framework for the specification of a rainscreen and establishes specific requirements for its 

performance. The NBS specification could be partly regarded as an interpretation of the performance 

requirements in the CWCT standard in terms of specification clauses. It invokes clauses given in the 

CWCT Standard for Walls with Ventilated Rainscreens and other CWCT documents such as Guide to 

Good Practice for Facades [CWCT 1996]. Thus, this report largely focuses on the NBS specification 

clauses. 

Section 2 presents a working definition of rainscreen cladding in the context of this work including its key 

features. The structure of Section 3 on entity-based rainscreen cladding specification is guided by the 

rainscreen cladding product model requirements specification developed in work package (WP) 3. 

Performance and other requirements of the product model entities are extracted from the NBS specification 

and structured for computer implementation. 

2.0 Rainscreen cladding 

In the context of the CIMclad project, rainscreen cladding is defined as a layered cladding system 

comprising of: 

• A visible outer skin (or “rainscreen”) that also forms the primary rain barrier. This layer sheds 

the majority of water down its surfaces, but does not prevent the passage of air into the air gap. 

• An air gap that prevents water ingress into the building. This provides ventilation and, 

depending on the design and dimensions of the rainscreen and air gap, may be intended to provide 

pressure equalisation across the outer skin. 

• A backing wall that forms an effective air barrier and frequently includes an insulating layer. 

Typically the supporting system to the outer skin is fixed to the backing wall. 

A more concise definition is: 

“Rainscreen cladding is a layered cladding system typically comprising an outer skin which incorporates 

air gaps but forms the primary rain barrier, a ventilated air gap that prevents water ingress, and an 

effectively impermeable backing wall”. 

Figure 1 shows the key features of a rainscreen cladding. 



Figure 1: Simplified illustration of rainscreen cladding [CWCT 1998] 

3.0 Entity-based Rainscreen cladding specifications 

3.1 Background 

The product model developed in WP 3 formally structures the context and requirements related rainscreen 

cladding information. The role of the rainscreen cladding product model is to capture the requirements 

against which the rainscreen scheme design was developed, and to transfer all this information into the full 

design stage. The model is entity-based; each entity holds a cluster of information that defines its 

characteristics including relationships with other entities. The model employs the concept of defining 

types-of things (such as tolerance, rainscreen cladding, backing wall performance) associated with each 

entity. Each type-of is itself defined by one or more properties or requirements. Property relates to what 

is proposed (for the cladding) or what exists / will exist while requirement relates to what is required (from 

the cladding). Each property or requirement refers to a governing clause in a specified document, and may 

include required values as appropriate. A requirement for the rainscreen cladding product model is likely 

to be drawn from a generic cladding specification document such as the CWCT and NBS standards or 

from a more particular specification document. However, a property is likely to be taken from a cladding 

manufacturer/installer specification document or from a structural specification document. 

As indicated in Section 1.0, the CWCT standard and NBS specification on rainscreen cladding are widely 

used as guidance documents in the UK. The NBS performance specification is derived from the CWCT 

Standard, thus the performance criteria for the two documents are similar. The route to specifying for a 

generic rainscreen cladding based on the two documents is illustrated in Figure 2. 



"Architect" 

CWCT test methods 

CWCT CWCT testing 

CWCT specification 

NBS specification 

Generic 

Specifications 

Tender Documents 

for a “generic rainscreen” with: 

- the outline layout 

- a basic specification (typically making references to 

external specifications) 

- specific performance requirements 

(the level of detail can differ widely) 

Contractor Proposal 

Contract 

Contractors Specification 

Typically based on external specifications 

with more specific quantitative statements. 

For an existing System 

- may have a System Certificate 

- may be supporting Project Certificates 

For an ad-hoc Solution 

- may be supporting Project Certificates 

Project Certificate Testing House 

System Certificate 

Arrow =“makes reference to” 

may relate to an installation 

test, a factory test (or both) 

Testing Method Statement 

Figure 2: A typical specification route for rainscreen cladding 

Rainscreen Contractor 

3.2 Performance-related specification clauses 

The performance of a building such as its weather tightness (air and moisture penetration resistance, 

condensation) relies on the envelope to act as a barrier to environmental elements in order to provide the 

required performance. Other performance aspects include: thermal and fire resistance; movement (due to 

load, moisture and temperature) and tolerance; structural stability and loading; durability (service life 

design); and others (noise, acoustics, corrosion, infestation) including appearance and fit. A summary of 

the equivalent performance clauses in the CWCT standard and NBS specification is given in Table 1 while 

Table 2 summarises tests for checking compliance. Commentary on the performance clauses vis-à-vis 

CWCT standard and NBS specification document is given in Report 3. 



Table 1 Summary of performance-related specification clauses 

Performance requirement / criteria 

Standard & Clause No. 

CWCT* NBS H92 

Applied loads 2.5 

2.7 

2.9 

Weather tightness 2.13 

2.15 

July 2001 Page 6 of 6 

320 

330 

335 

340 

390 

400 

410 

420 

Cavity ventilation 2.14 

Deflection 2.6 350 

Movement 2.10 

2.11 

2.12 

380 

Thermal performance 2.16 430 

Condensation 2.17 440 

450 

Acoustic and noise performance 2.18 

460 

2.19 

465 

470 

Fire performance 2.20 480 

485 

490 

Fixings and bracketry 2.3 

Tolerances 370 

Testing 

* Standard for Walls with Ventilated Rainscreen 

See Table 2 See Table 2 

As noted in Report 3, a number of CWCT standard performance criteria clauses are not qualified in the 

NBS document. They include: 

2.7 Wind resistance – safety 

2.11 Accommodation of thermal movement 

2.12 Accommodation of moisture movement 

2.14 Cavity ventilation 

2.18 Noise 

2.21 Electrical continuity and earth bonding 

2.22 Bimetallic corrosion 

2.23 Infestation


Table 2 Summary of tests and relevant clauses 

Test CWCT Standard for testing 

ventilated rainscreens clauses 

Standard tests 

H92 NBS 

clauses 

Air permeability tests (in situ) 3.3 and 3.3.2 585 

Air permeability tests (in situ), include test pressure and 

allowable leakage 

3.3, 3.3.2 and 3.3.3 590 

Air permeability tests include test pressure, and allowable 

leakage. The different conditions are: 

Relevant clauses 

595 

• Overcladding a new or existing backing wall with a 

proprietary rainscreen system utilising a drained and 

ventilated cavity 

3.3 and 3.3.1 

• Overcladding a new or existing backing wall with a 3.3, 3.3.1, and 3.3.3 

proprietary 

equalisation 

rainscreen system utilising pressure 

• Overcladding a new backing with a bespoke rainscreen 

system utilising a drained and ventilated cavity 

3.3 and 3.3.1 

• Overcladding a new backing with a bespoke rainscreen 

system utilising pressure equalisation 

3.3, 3.3.1 and 3.3.3 

• An integral system utilising pressure equalisation 

3.3.1 and 3.3.3 

Weather tightness / water penetration tests, large specimen 3.4 610 

Weather tightness / water penetration tests, small specimen 3.4 615 

Wind loading test, large specimen – air barrier 3.5.1 620 

Wind loading test - rainscreen 3.5.2 625 

Fatigue loading test, small specimen - rainscreen 3.6 630 

Discretionary tests 

Building movement / racking test, large specimen 3.10.1 635 

Thermal cycling regime, large specimen 3.10.2 640 

Sparge bar test, large specimen 3.10.3 645 

Hosepipe test, large specimen 3.10.4 650 

Ultimate strength test 3.11 655 

Additional tests 

Soft impact test 

Hard impact test 

Weathering test 

Thermal performance test 

Fire testing 

Destructive testing of fixings to determine the ultimate 

load 

3.12.1 

660/661 

3.12.1 

3.12.2 

3.12.4 

3.12.5 

2.3.2.2 670 

Site fixing test 2.3.3 672 

Site sparge test 3.10.3 680 

Site hosepipe test 3.10.4 685 

3.3 Entity type and requirement based specification 

This section presents the formal structure of the entity types and requirements specification clauses 

suitable for computer implementation in an Access database. It extracts from a specification document 

requirement related specification clauses for the different entity types shown on the product model. The 

requirement is described in text (clause text), and the clause number, document name and aspect are also 

included in the tabular specification format. The example that follows uses NBS specification document 

and excludes detailed explanatory notes relating to each requirement clause, which can be found in NBS 

Section H92 document. 



RequirementID Type Aspect Document Title Clause Number Clause Text Value 

NBS Section H92 370 - Design rainscreen wall: 

- To ensure position and alignment of all parts and features as 

shown on the referenced drawings listed in Type(s) of rainscreen 

cladding clause(s). 

- To accommodate deviations in the primary support structure. 

- Maximum permitted component and installation tolerances: 

1. Permitted deviation of overall panel width +2.0mm -2.0mm 

2. Permitted deviation in panel length 

For panels up to 2400mm +2.0mm -2.0mm 

For panels over 2400mm +3.0mm -3.0mm 

3. Maximum permitted deviation in length 

of two opposite sides of panel +2.0mm -2.0mm 

4. Squareness of panels: 

When the longest of two adjacent sides of the panel is taken as the 

base line, the deviation of the shorter side measured from a 

perpendicular to the baseline at any point along the baseline not to 

exceed +2.0mm -2.0mm 

5. Flatness: 

Deviation under a 1.0m straight edge 

placed anywhere on a flat surface not 

to exceed 3.0mm 

6. Alignment of joints between adjacent 

panels: deviation of panel corner from 

protected lines of edges of adjacent 

panels not to exceed 2.0mm 

7. Alignment of faces to adjacent panels: 

Deviation of panel edge under a 1.0m 

straight edge placed across adjacent 

panel not to exceed 1.0mm 

1 Tolerance Appearance and 

Fit 



GENERAL 

- Comply with CWCT as ‘Standard for walls with ventilated 

rainscreens’, Section 2 - Performance Criteria unless specified or 

agreed otherwise. 

- Project performance requirements specified in this subsection are to 

be read in conjunction with CWCT performance criteria. 

NBS Section H92 310 

Applied Load / 

Structural Integrity 

2 Design 

Requirements and 

Parameters 

HORIZONTAL ZONING OF WIND PRESSURE: 

Reference heights are as follows: (this clause usually deleted – zoning 

not applicable) 

NB - 330, 335, 340 are alternative clauses 

320 

INTEGRITY OF VENTILATED RAINSCREEN CLAD WALLS: 

Determine size(s) and thickness(es) of panels, the size(s), number and 

spacing of fixings, configuration and location of secondary support 

systems and incorporation of other accessories and fittings to ensure 

the cladding system, primary support structure and other elements 

forming the rainscreen wall will resist all factored dead, imposed and 

design live loads, and accommodate all deflections and movements 

without damage. 

- Calculate wind loads on rainscreen walls appropriate to location, 

exposure, height, building shape, and size in accordance with BS 

6399-2 Standard Method, taking full account of existing and known 

future adjacent structures. 

Basic wind speed (Vb): ______ m/s. 

Altitude factor (Sa): ______ 

Direction factor (Sd): ______ 

Seasonal factor (Ss): 1. 

Probability factor (Sp): 1. 

Terrain and building factor (Sb): ______ 

External and internal size effect factors (Ca): 1. 

External pressure coefficients (Cpe): As determined from BS 6399- 

2, clauses 2.4 and 2.5. 

Internal pressure coefficients (Cpi): As determined from BS 6399- 

2, clause 2.6. 

Dominant opening: ______ 

Impact load(s) in accordance with BS 8200: 

Location and category ______ 

Temporary imposed loads: ______ 

(Use this clause where the parameters determining wind loads 

are critical) 

330 




Determine, size(s) and thickness(es) of panels, the size(s), number 

and spacing of fixings, configuration and location of secondary support 






- Calculate wind loads on rainscreen walls appropriate to location, 

exposure, height, building shape and size in accordance with BS 6399- 

2, taking full account of existing and known future adjacent structures. 

- Impact load(s) in accordance with BS 8200: 

Location and category: ______ 

- Temporary imposed loads: ______ 

(Use this clause where the contractor is to determine the basic 

parameters used to calclate wind loads) 


Applied Load / 

Structural Integrity 

3 Design 

Requirements and 

Parameters 

(Contd) 


Determine, size(s) and thickness(es) of panels, the size(s), number 

and spacing of fixings, configuration and location of secondary support 






- Minimum design wind pressures: 

______ Pascals. 



- Design wind pressure: ______ Pascals. 

- External pressure coefficient (Cpe): ______ 

- Impact load(s) in accordance with BS 8200: 

Location and category: ______ 

- Temporary imposed loads: ______ 

- Other design parameters: ______ 

(Use this clause when a more precise control of structural is deemed 

necessary) 

340 



AIR PERMEABILITY THROUGH AIR BARRIER: 

The maximum design value for air leakage through the air barrier at a 

differential pressure of 50 Pascals is ______ m³/hr/m². 

(Normally delete this clause – only used for pressure equalised 

rainscreens) 


Air Permeability 

4 Backing Wall 

Performance 

THERMAL PROPERTIES: 

- Method for calculating the thermal transmittance (U-value) of the 

rainscreen wall: ______ 

- Average U-value of rainscreen wall: ______W/m²K. 

430 

Thermal 

Resistance 

FIRE RESISTANCE OF BACKING WALL: 

To BS 476- 21 and not less than ______ 

480 

Fire Resistance 

INTERNAL SURFACE SPREAD OF FLAME OF BACKING WALL: 

To BS 476-7, Class ______ 

485 

CAVITY FIRE BARRIERS: 

To BS 476-20 and must resist the passage of flame and smoke for not 

less than ______ 

490 



DEFLECTION UNDER WIND LOAD: 

At positive and negative applications of the design wind pressure the 

maximum normal deflection for the listed components must not 

exceed: 

Individual rainscreen panels 

- 1/90 of the span measured between the points of attachment of the 

panel for aluminium, glass, steel, and non ferrous metals, or 

- 1/360 of the span measured between the points of attachment, or 

3mm, whichever is the lesser, for stone,ceramics, and similar 

materials. 


Deflection 

5 Raincreen 

Cladding 


Framing members – 1/200 of the span of the member measured 

between points of attachment to the building, or 20mm, whichever is 

the lesser. 

WIND RESISTANCE - CYCLIC LOADING: 

No reduction in the performance of the rainscreen cladding must occur 

after the maximum effective wind pressure has been applied for 10,000 

cycles. 

(This clause only required when full testing is commissioned) 

360 

Wind Resistance 

AIR PERMEABILITY GENERALLY: 

The average air leakage rate through the listed wall(s) at a differential 

pressure of 50 Pascals must not exceed:(Architect to insert figure – 

typically between 22 and 10m3/hr/m2) ______ 

390 


AIR PERMEABILITY THROUGH AIR BARRIER: 

The maximum design value for air leakage through the air barrier at a 

differential pressure of 50 Pascals is ______ m³/hr/m². 

(Normally delete this clause – only used for pressure equalised 

rainscreens) 

400 

AIR PERMEABILITY: 

There must be no regions of concentrated air flow through the air 

barrier, its closures or interfaces with windows, doors or other 

penetrations through the wall at a test pressure of ______ Pascals. 

410 

WATER PENETRATION onto internal surfaces or into cavities not 

designed to be wetted must not occur when the rainscreen wall is 

subjected to a test pressure of ______ Pascals. 

(Choice of pressure for 410/420 is architect’s – this is a flaw in CWCT 

standard) 

420 

Water Penetration 



CONDENSATION: 

The psychrometric conditions under which condensation must not form 

within or on the interior surface of the rainscreen wall or any surface of 

the wall that is on the warm side of any insulation are: 

- Outdoor notional psychrometric conditions as BS 6229, table 6: 

Winter Summer 

Temperature -5°C 18°C 

Relative humidity 90% 65% 

Vapour pressure 0.361 kPa 1.341 kPa 

Duration 60 days 60 days 

- Indoor notional psychrometric conditions: 

Temperature: ______ °C Relative humidity: ______ % 

Vapour pressure: ______ kPa. (Data available in BS 6229, table 7) 

- Calculated amount of winter interstitial condensate must not exceed 

______ kg/m². Calculated annual net retention must not exceed 5% of 

winter condensate. (0.5kg/m2 for non fibrous and closed cell insulants, 

0.35kg/m2 for fibrous insulants) 


Condensation 

6 Raincreen 

Cladding 


(Contd) 

VAPOUR CONTROL LAYER: 

Determine the interstitial condensation risk of the rainscreen wall using 

the method described in BS 5250 Appendix D. If necessary, provide a 

suitable vapour control layer to ensure that damage and nuisance from 

interstitial condensation does not occur. 

450 

SOUND TRANSMITTANCE BETWEEN INTERIOR AND EXTERIOR 

OF RAINSCREEN CLAD WALL: Minimum sound reduction indices (R) 

to BS EN ISO 140-3: - One third octave band 

centre frequency (Hz) ______ R(dB)______ 

460 

Acoustics 

SOUND TRANSMITTANCE BETWEEN ADJOINING FLOORS 

ABUTTING RAINSCREEN CLAD WALL: Minimum sound reduction 

indices (R) to BS EN ISO 140-3:- One third octave band 

centre frequency (Hz) ______ R(dB)______ 

465 

SOUND TRANSMITTANCE BETWEEN ADJOINING ROOMS 

ABUTTING RAINSCREEN CLAD WALL: Minimum sound reduction 

indices (R) to BS EN ISO 140-3:- One third octave band 

centre frequency (Hz) ______ R(dB) ______ 

470 



GENERALLY: 

- Fabricate and install rainscreen cladding in accordance with this 

specification and the final detailed drawings. 

- Fabricators and installers must employ competent rainscreen 

cladding operatives. Records of their experience are to be provided to 

the CA on request. 

- Select and align all products to ensure uniformity of appearance. 

- Joints must only occur at positions indicated on final detailed 

drawings. 

- Isolate dissimilar metals to prevent electrolytic corrosion. 

- Machine cut and drill all products in the workshop wherever possible. 

- Mark or tag all products to facilitate identification during assembly, 

handling, storage and installation. Do not mark surfaces visible in the 

complete installation. 


7 Workmanship Fabrication and 

Installation 

METALWORK: 

As section Z11, unless specified otherwise in this section. 

912 

TIMBER TREATMENT: 

- As section Z12, unless specified otherwise in this section. 

- Treat surfaces exposed by minor cutting and drilling with two flood 

coats of a solution recommended for the purpose by main treatment 

solution manufacturer. 

915 

GLAZING: 

- As section L40, unless specified otherwise in this section. 

- Fix directional patterned/wired glass generally parallel to surround 

and align adjacent panes where seen together at close quarters. 

920 

FIXINGS/ADHESIVES APPLICATION: 

As section Z20, unless specified otherwise in this section. 

922 

SEALANT APPLICATION: As section Z22, unless specified otherwise 

in this section. 

925 



ASSEMBLY: 

- Carry out as much assembly as possible in the workshop. 

- Joints, other than movement joints and designed open joints, must 

be rigidly secured, reinforced where necessary and fixed with hairline 

abutments. 

- Take precautions to prevent displacement of components in 

assembled units. Obtain approval for any reassembly on site. 


Fabrication and 

Installation 

8 Workmanship 

(Contd) 

INSPECTION: 

- All fabrications and assembled units must be carefully inspected for 

match with approved samples and for compliance with this 

specification and the final detailed drawings before dispatch to site. 

- Give adequate notice of inspection arrangements to enable the CA 

and/or other affected parties to be present. 

935 

PROTECTION: 

- All fabrications and assembled units must be protected against 

damage, corrosion and disfigurement during handling, installation and 

subsequent site operations. 

- Protective coverings must be applied before dispatch to site and 

must not be detrimental to rainscreen cladding products, finishes or 

installation procedures. 

940 

HANDLING AND STORAGE: 

- Do not deliver to site any rainscreen cladding products and units 

which cannot be installed immediately or unloaded into a suitable well 

protected storage area. 

- Store products and units on level bearers clear of the ground and 

separate with resilient spacers. 

945 

SUITABILITY OF STRUCTURE: 

- Not less than ______ weeks before commencement of rainscreen 

cladding installation carry out a geometric survey of the supporting 

structure, checking line, level and fixing points. Report immediately to 

the CA if structure will not allow the required accuracy or security of 

erection. 

- Coordinate geometric survey for rainscreen cladding with any other 

survey(s) for adjacent cladding. 

- Set out erection datum points, lines and levels for a complete 

elevation at a time unless otherwise agreed with the CA. 

950 



PRELIMINARY RAINSCREEN CLADDING INSTALLATION: 

Complete a preliminary area of rainscreen cladding as set out below 

for inspection and approval of appearance by the CA. 

______ 


Fabrication and 

Installation 

9 Workmanship 

(Contd) 

RAINSCREEN CLADDING INSTALLATION: 

- Set out straight, parallel and truly aligned. 

- Tighten all mechanical fixings to manufacturer’s recommended 

torque figures. Do not overtighten fixings intended to permit differential 

movement. 

- Remove protective coverings only where necessary to facilitate 

installation and from surfaces which will be inaccessible on completion. 

970 

IN SITU WELDING is not permitted. 

975 

IN SITU WELDING is permitted, subject to completion of a ‘hot work 

permit’ form and compliance with its requirements. 

977 

INTERFACES: 

Ensure that flashings, closers, etc. (specified in another section) are 

located correctly and neatly overlap the rainscreen cladding to form a 

weathertight junction. 

980 

DAMAGE: 

- Do not repair rainscreen cladding without approval. Such approval 

will not be given where products and units are badly damaged or 

where the proposed repair will impair performance or appearance. 

- Repairs may require additional site testing at the discretion of the 

CA. 

- Schedule repairs or record on drawings for inclusion in the 

maintenance manual. 

985 

Damage 

CLEANING: 

At Practical Completion or when otherwise agreed with the CA, remove 

any protective coverings and thoroughly clean rainscreen cladding 

areas. Cleaning agents for the purpose must be approved by the 

rainscreen cladding manufacturer and incorporated products 

manufacturers. 

990 

Cleaning 

MAINTENANCE: 

Prepare a maintenance manual in accordance with CWCT ‘Guide to 

good practice for facades’, Section 10. Unless otherwise instructed or 

agreed the manual must be completed and handed over to the CA at 

Practical Completion. 

995 

Maintenance 



(The necessity for physical testing should not be required if there is 

substantial real life successful performance of the rainscreen system 

proposed – this is acceptable to CWCT. However, project testing may 

be demanded by the specifier, in which case some, or all of the 

following testing clauses will apply) 

NBS Section H92 

10 Testing Note 

COMPARISON (TYPE) TESTING: 

Fabrication and installation of rainscreen cladding must not commence 

until test results and reports showing compliance with this specification 

have been submitted to the CA. 

(Use this clause when historic data is acceptable) 

510 

Comparison 

Testing 

PROJECT TESTING (LABORATORY): 

- At an agreed stage in detailed design work arrange for laboratory 

testing of specimens of rainscreen cladding and components in 

accordance with relevant clauses of this specification. 

- Fabrication and installation of rainscreen cladding must not 

commence until test results and reports showing compliance with this 

specification have been submitted to the CA. 

(Use this clause as alternative to 510) 

515 

Project Testing 

(Laboratory) 

PROJECT TESTING (SITE): 

- At an agreed stage during preliminary installation on site arrange for 

testing of a section of rainscreen cladding in accordance with relevant 

clauses of this specification. 

- Protect building structure, components and finishes from any 

damage consequent upon testing. 

- Installation of general areas of rainscreen cladding must not continue 

until site test results and reports showing compliance with this 

specification have been submitted to the CA. 

520 

Project Testing 

(Site) 

TESTING AUTHORITY: 

Project testing must be carried out by a United Kingdom Accreditation 

Service (UKAS) approved independent laboratory. 

530 

Testing Authority 

TESTING AUTHORITY: 

Project testing must be carried out by the rainscreen cladding 

manufacturer/contractor and is to be witnessed/certified by ______ 

(Alternative to 530) 

535 



LARGE TEST SPECIMEN: ______ 

- Drawing references: ______ 

- Overall dimensions ______ m high x ______ m wide. 

- Features: ______ 

- Test sequence(s): As clause(s) ______ 

(This clause for complete wall system ie – rainscreen system + integral 

backing wall) 


Large Test 

Specimen 

11 Testing (Contd) 

SMALL TEST SPECIMEN: ______ 

- Drawing references: ______ 

- Overall dimensions ______ m high x ______ m wide. 

- Features: ______ 

- Test sequence(s): As clause(s)______ 

(This clause applicable to rainscreen only system ie – no integral 

backing wall) 

550 

Small Test 

Specimen 

TEST SEQUENCE: To CWCT ‘Standard for testing ventilated 

rainscreens’, Table 2 as follows: 

- Air permeability: As clause ______ 

- Weathertightness/water penetration: As clause ______ 

- Windloading: As clause ______ 

(Use this clause for standard tests) 

560 

Test Sequence 

TEST SEQUENCE: To CWCT ‘Standard for testing ventilated 

rainscreens’ Table 2 and clauses 3.10 and 3.11 as follows: 

- Air permeability: As clause ______ 

- Weathertightness/water penetration: As clause ______ 

- Windloading: As clause ______ 

- Building movement/racking procedure: ______ 

Repeat test(s): ______ 

- Thermal cycling regime: ______ 

Repeat test(s): ______ 

- Sparge bar: ______ 

- Hosepipe: ______ 

- Ultimate strength: ______ 

(Use this clause for standard tests plus choice of other tests) 

570 

AIR PERMEABILITY TESTS: 

- To CWCT ‘Standard for testing of ventilated rainscreens’, clauses 

3.3 and 3.3.1 

- Test pressure: As clause 410. 

- Allowable leakage rates: As clause 390. 

595 


Tests 

Standard Tests 



WEATHERTIGHTNESS/WATER PENETRATION TESTS, LARGE 

SPECIMEN: 

- To CWCT ‘Standard for testing of ventilated rainscreens’, clause 3.4. 



Water Penetration 

12 Testing (Contd) - 

Standard Tests 

WEATHERTIGHTNESS/WATER PENETRATION TESTS, SMALL 

SPECIMEN: 



615 

- Test method: Using a test chamber to BS5368 

(Agreement required on which parts of rainscreen may and may not be 

wetted, prior to test) 

WINDLOADING TEST, LARGE SPECIMEN - AIR BARRIER: 

- To CWCT ‘Standard for testing of ventilated rainscreens’, clause 

3.5.1. 

- Test pressures: As clause 410. 

- Loading directions: ______ 

- Allowable elastic deformation: ______ 

- Allowable residual deformation: ______ 

620 

Wind Loading 

WINDLOADING TEST - RAINSCREEN: 


3.5.2 

- Test pressures: As clause 410. 

- Loading directions: ______ 

- Allowable elastic deformation: ______ 

- Allowable residual deformation: ______ 

625 

FATIGUE LOADING TEST, SMALL SPECIMEN - RAINSCREEN: 


- Test sequence: (Insert as CWCT “Standard for Walls with Ventilated 

rainscreens” clause 2.8.2 or As Clause 360) 

- Maximum design wind pressure: As clause 340 

630 

Fatigue Loading 



BUILDING MOVEMENT/RACKING TEST, LARGE SPECIMEN: To 

CWCT ‘Standard for testing of ventilated rainscreens’, clause 3.10.1. 

Displacements to test specimen: ( Only compulsory when specifying 

earthquake load resistance ) 


Movement 


Discretionary Tests 

Hold test specimen in its final displaced position for the subsequent air 

permeability and/or weathertightness/water penetration tests, before 

returning to its original position. 

THERMAL CYCLING REGIME, LARGE SPECIMEN: To CWCT 

‘Standard for testing of ventilated rainscreens’, clause 3.10.2. 

External temperature limits: 

______ 

Maximum solar radiation intensity on a vertical surface, 

Imax ______ W/m² 

Retain specimen in its final condition for the subsequent air 

permeability and/or weathertightness/water penetration tests. 

640 

SPARGE BAR TEST, LARGE SPECIMEN: To CWCT ‘Standard for 

testing of ventilated rainscreens’, clause 3.10.3. 

645 

Water Permeability 

HOSEPIPE TEST, LARGE SPECIMEN: To CWCT ‘Standard for 

testing of ventilated rainscreens’, clause 3.10.4. 

650 

ULTIMATE STRENGTH TEST: 

- To CWCT ‘Standard for testing of ventilated rainscreens’ clause 

3.11. 

- Test to be at ______ pressure. 

- Termination pressure if failure has not occurred: 


- Load positions: ______ 

(CWCT express reservations re the benefits of such testing) 

655 

Strength 



______ TEST: Carry out in accordance with ______ 

(Use this section for any specialist test required) 



Additional Tests 

IMPACT TEST: 


3.12.1 and BS 8200. 

- Wall category: ______ 

- Test impact for retention of performance: ______ 

Acceptable damage without impairment of performance: 

______ 

- Test impact to ensure safety to persons: ______ 

(This test regarded as compulsory by CWCT for ground floor 

treatments) 

661 

Impact Resistance 

DESTRUCTIVE TESTING OF FIXINGS TO DETERMINE THE 

ULTIMATE LOAD: 

- Number and location of test fixings: ______ 

- Test method: To BS 5080-1 and Construction Fixings Association 

guidance note ‘Procedure for site testing construction fixings’. 

670 

Strength 

SITE FIXINGS TEST: 

- Number and location of test fixings: ______ 

- Test method: To BS 5080-1 and Construction Fixings Association 

guidance note Procedure for ‘Site testing construction fixings’. 

672 

SITE SPARGE BAR TEST: To CWCT ‘Standard for testing of 

ventilated rainscreens’, clause 3.10.3. 

(Use to confirm laboratory test results) 

680 

Water Permeability 



DESIGN 

- Complete the detailed design of the rainscreen cladding and 

associated features shown on the preliminary design drawings to meet 

the requirements of this specification 

- Co-ordinate detailed design with that for all related works. 


Design 

15 General 

Requirements / 

Preparatory Work 

DESIGN PROPOSALS: 

The preliminary design drawings indicate design intent but do not 

preclude submission with tender of reasonable alternative proposals 

for consideration. 

215 

SPECIFICATION: 

- Comply with the latest edition of the Centre for Window and Cladding 

Technology (CWCT) ‘Standard for walls with ventilated rainscreens’ 

and ‘Standard for testing of ventilated rainscreens’ unless specified 

otherwise in this section. 

- Keep a copy of the CWCT ‘Standard for walls with ventilated 

rainscreens’ and ‘Standard for testing of ventilated rainscreens’ 

together with other CWCT publications invoked by these documents, at 

the design office, workshop and on site, readily accessible for 

reference at all times during the course of the works. 

220 

Specification 

INFORMATION TO BE PROVIDED WITH TENDER: 

Submit to the CA the following rainscreen cladding particulars: 

- Typical plan, section and elevation drawings at suitable scales. 

- Typical detailed drawings at large scales, including (insert any 

specific details which should be clarified at tender stage) 

- Technical information and certification demonstrating compliance 

with the specification of proposed incorporated products and finishes, 

including (insert product/finish types generically specified) 

- Certification, reports and calculations demonstrating compliance with 

the specification of the proposed rainscreen cladding. 

- Proposals for connections to and support from the primary support 

structure. 

- Proposals for any primary support structure additional to that shown 

on preliminary design drawings. 

- Schedule of builder’s work, special provisions and special 

attendance by others. 

- Examples of standard documentation from which the project quality 

plan will be prepared. 

- Preliminary fabrication and installation method statements and 

programme. 

- Proposals for replacing damaged or failed products. 

- Areas of non-compliance with the specification. 

225 

Tender Stage 

Information 



INFORMATION TO BE PROVIDED AFTER ACCEPTANCE OF 

TENDER: Submit to the CA within (insert) weeks of appointment the 

following rainscreen cladding particulars: 

- A schedule of detailed drawings and dates for submission for 

comment. 

- A schedule of loads that will be transmitted from the rainscreen 

cladding to the structure. 

- Proposed fixing details and systems relevant to the structural design 

and construction with methods of adjustment and tolerances. 

- A schedule of all fabrication tolerances/size tolerances. 

- A detailed testing programme in compliance with the Main Contract 

master programme (this may not be required) 

- A detailed fabrication and installation programme in compliance with 

the Main Contract master programme. 

- A quality plan in compliance with the CWCT ‘Guide to good practice 

for facades’, Section 6. 

- Proposals to support any outstanding applications for Building 

Regulation consents or relaxations. 


Tender Stage 


16 General 



(Contd) 

INFORMATION TO BE PROVIDED BEFORE COMMENCEMENT OF 

RAINSCREEN CLADDING WORK: Submit to the CA before testing or 

fabrication the following rainscreen cladding particulars: 

- Detailed drawings to fully describe fabrication and installation. 

- Detailed calculations to prove compliance with all 

design/performance requirements. 

- Project specific fabrication, handling and installation method 

statements. 

- Certification for all incorporated components manufactured by others 

confirming their suitability for all locations in the rainscreen cladding. 

- Recommendations for spare parts for future repairs or replacements. 

- Recommendations for safe dismantling and recycling or disposal of 

all products. 

235 

Pre-Works 


PRODUCT SAMPLES: Before commencing detailed design provide 

the CA with identified samples of: 

(usually panel and finish samples) 

Obtain approval of appearance before proceeding. 

240 

Product Samples 

SAMPLES OF FIXINGS: At an agreed stage during detailed design 

work provide the CA with identified samples of each type of fixing, 

together with manufacturers’ recommended torque figures 

250 

Fixings Samples 



FABRICATION SAMPLES: At an agreed stage during detailed design 

work provide the CA with samples of: 

(usually small system samples – not full mock-up) 

Obtain approval of appearance before proceeding 


Fabricated 

Samples 

17 General 



MOCK-UP: At an agreed stage during detailed design work construct 

in an approved location a mock-up of: 

(insert size and details) 

Purpose(s) of mock-up: 

(usually to validate details) 

Arrange for inspection to be carried out jointly with the CA. Obtain 

approval of appearance before proceeding. Retain mock-up in 

undisturbed condition until completion of rainscreen cladding 

installation. 

270 

Mock-Up 

NB – If testing under clause 540/550 is specified, this clause can be 

deleted 



ALUMINIUM ALLOY FRAMING SECTIONS: 

- To BS 1474, alloy 6063 and suitable for the specified finish. 

- Structural members to comply with BS 8118. 


Aluminium Framing 

18 Rainscreen 

Materials and 

Finishes related 

Standards 

ALUMINIUM ALLOY SHEET: To BS EN 485, BS EN 515 and BS EN 

573 in an alloy, temper and thickness suitable for the application and 

specified finish. 

712 

Aluminium Sheet 

MILD STEEL FRAMING SECTIONS/ REINFORCEMENT: To the 

relevant parts of BS 7668, BS EN 10029, BS EN 10113, BS EN 10137, 

BS EN 10155 and BS EN 10210, in a thickness suitable for the 

application, and for galvanizing or other protective coating. 

715 

Steel Framing 

MILD STEEL SHEET: To the relevant parts of BS 1449-1, BS EN 

10048, BS EN 10051, BS EN 10111, BS EN 10131, BS EN 10139, BS 

EN 10140, BS EN 10149, BS EN 10209, and BS EN 10268 in a grade 

and thickness suitable for the application, and suitable for galvanizing 

or other protective coating. 

717 

Steel Sheet 

STAINLESS STEEL SHEET: To the relevant parts of BS EN 10029, 

BS EN 10048, BS EN 10051, BS EN 10095, BS EN 10258, BS EN 

10259 and BS EN 10088-2, austenitic, grade 1.4301 (304) generally, 

grade 1.4401 (316) when used externally or in severely corrosive 

environments, and in a thickness suitable for the application. 

720 

TIMBER BATTENS: 

- Regularised softwood free from decay, insect attack (except 

ambrosia beetle damage) and with no knots wider than half the width 

of the section. 

- Preservative treatment: CCA as section Z12 and British Wood 

Preserving and Damp-proofing Association Commodity Specification 

C8. 

- Moisture content at time of fixing: Not exceeding 19%. 

725 

Timber 

MECHANICAL FIXINGS: 

- Stainless steel to BS EN ISO 3506 grade A2 generally, grade A4 

when used in severely corrosive environments, or 

- Mild steel to BS 4190 and suitable for galvanizing or other protective 

coating, or 

- Aluminium complying with BS 1474 and BS EN 755. 

730 

Fixings 

ADHESIVES must not be degradable by moisture or water vapour. 

732 

Adhesives 



FIXINGS AND FASTENERS must be: 

- Of dimensions not less than recommended by their manufacturers. 

- Capable of adequate three dimensional adjustment to accommodate 

primary support structure and rainscreen cladding 

fabrication/installation tolerances. 


Fixings and 

Fastners 

19 Rainscreen 

Materials and 


Standards 

GLASS GENERALLY: 

- To BS 952 and the relevant parts of: 

BS EN 572 for basic soda lime silicate glass 

BS EN 1096 for coated glass 

BS EN 1748 for borosilicate glass 

BS EN ISO 12543 for laminated glass 

- Panes to be accurately sized, clean and free from obvious 

scratches, bubbles, cracks, ripplings, dimples and other defects. 

- Edges generally undisfigured. Shells and chips not more than 2 mm 

deep and extending not more than 5 mm across the surface are 

acceptable if ground out. 

740 

Glass 

THERMALLY TOUGHENED GLASS that is to be fixed in the following 

locations must be subjected to a heat soaking regime designed to 

reduce the incidence of failure due to nickel sulfide inclusions: 

______ 

All panes must be heat soaked at a mean glass temperature of 280 ± 

10°C for not less than ______ hours. 

742 

INSULATING GLASS UNITS: 

- To BS 5713, hermetically sealed and Kitemark certified. 

- Perimeter taping must not be used. 

- Perimeter seals must be compatible with structural, assembly and 

weather sealants that they come into contact with. 

745 

GASKETS: 

- Noncellular rubber to BS 4255-1 

- Cellular rubber to ASTM-C509. 

- Resistant to oxidation, ozone and UV degradation. 

760 

Gaskets 

WEATHERSTRIPPING of opening units: 

- Noncellular rubber to BS 4255-1. 

Cellular rubber to ASTM-C509 

Polypropylene woven pile, silicone treated. 

- Weatherstripping must be fixed in undercut grooves in framing 

sections and have preformed corners with any joints in the length. 

765 

Weatherstripper 



GENERAL SEALANTS: 

Must be stable and compatible with all contact products and finishes 

and be selected in accordance with BS 6213 from: 

Silicone to BS 5889 

One part polysulfide to BS 5215 

Two part polysulfide to BS 4254 

One or two part polyurethane. 


Sealants 

20 Rainscreen 

Materials and 


Standards 

THERMAL INSULATION: 

- Material: ______ 

Manufacturer and reference: ______ 

Thickness: Not less than ______ mm. 

- Attached to the outer face or supported within the backing wall so as 

not to bulge, sag, delaminate or detach during installation or in situ 

during the life of the rainscreen cladding. 

- Keep dry during installation. 

776 

Insulation 

VAPOUR CONTROL LAYER: 

- Material: Reinforced translucent polyethelene sheeting;minimum 

nominal thickness 0.37mm 

Minimum vapour resistance: 500 MNs/g. 

- Lay as work proceeds ensuring continuity and with the minimum of 

joints. 

- Lap edges of sheets not less than 150 mm and seal with tape. Seal 

with tape to pipes, ducts, structural members, etc. which abut or pass 

through. Prime substrates as necessary to ensure a full bond over the 

width of the tape. 

- Tape: Double sided sealant with vapour resistivity not less than the 

vapour control sheet. 

Size: Mimimum 15mm wide. 

- Before covering check for tears and punctures. Carefully repair, 

sealing with a lapped patch of vapour control sheet and a continuous 

band of sealant tape along all edges. 

780 

Vapour Control 

Layer 



BREATHER MEMBRANE: 

- Material: Spun bonded polypropylene waterproof fabric; U.V. 

stabilised; B2 flame retardancy classification to be achieved under DIN 

4102: tear and wind resistant. 

- Lay membrane over insulation as work proceeds ensuring continuity. 

- Overlap joints not less than 150 mm and seal with tape 

recommended by the membrane manufacturer, to prevent water 

reaching the insulation. 

- Fully seal at penetrations using taping methods recommended by the 

membrane manufacturer. 

- Before covering check for tears and punctures. Carefully repair using 

a patch of breather membrane with 150 mm laps sealed with tape. 

- Ensure that bottom edges overlap flashings, sills, etc. to allow free 

drainage to the exterior. 


Breather 

Membrane 

21 Rainscreen 

Materials and 


Standards 

PROTECTIVE COATING OF MILD STEEL FRAMING 

SECTIONS/REINFORCEMENT: All surfaces must be: 

- Hot dip galvanized to BS EN ISO 1461, or 

- Treated with an appropriate equivalent coating to BS 5493, BS EN 

ISO 12944 and BS EN ISO 14713. 

810 

Finishes 

- Coating 

PROTECTIVE COATING OF MILD STEEL MECHANICAL FIXINGS: 

All surfaces must be: 

- Hot dip galvanized to BS EN ISO 1461, or 

- Sherardized to BS 4921, class 1 coating thickness and passivated, 

or 

- Zinc plated to BS 1706, coating classification Fe/Zn 12 and chromate 

conversion class 2C or 2D. 

820 

ANODISING 

- To BS3987, subject to a local coating thickness of not less than 25 

micrometres and not more than 40 micometres on all significant 

surfaces. 

Processor and reference:………. 

- Prepare base metals, anodise, test samples, protect components, 

and repair damage in accordance with the processor’s 

recommendations 

- Anodise after fabrication is complete. 

840 

- Anodising 



TYPE(S) OF RAINSCREEN CLADDING: 

Aluminium Rainscreen Cladding comprising 2mm thick anodised 

aluminium cassette panels supported by extruded aluminium hanging 

system. The system to be fixed to an inner leaf structural wall and 

incorporate other components such as insulant, membranes, breakers, 

etc. The system to be drained and back-ventilated. 


22 Rainscreen Types Note 

RAINSCREEN CLADDING (insert location or identification) 

- Drawing reference(s): (insert) 

- Primary support structure: (insert brief description of supporting 

background wall eg – blockwork infill panels to steel frame/ insulated 

structural tray system) 

- Rainscreen cladding system: Baffle jointed, secret-fixed, anodised 

aluminium cassette panel and extruded aluminium support system. 

- Type: Drained and back-ventilated 

- Rainscreen panel: Aluminium cassette, stiffened as necessary to 

meet performance criteria for deflection under wind load as clause 350. 

Anti-drumming matting to be fixed to 60% of panel area back. Panels 

must be individually demountable. Panel to be formed into cassette 

format by mechanically secret-fixing panel return flanges at meeting 

points. 

Material: Aluminium to EN1396, 5005H14 

Thickness: 2mm 

Finish: Anodised after fabrication to BS 3987 

Colour: (insert ) 

Joint type: Baffle jointed. No sealants or mastics to be used. 

Joint width: 20mm standard 

- Air gap: (insert____mm: N.B.- minimum 25mm) 

- Support system: Support system comprising extruded aluminium 

vertical support rails with adjustable wall attachment brackets to zone 

range (insert-insert)mm and panel holding hooks; neoprene anti-rattle 

compression gasket. The support framing must allow for calculated 

expansion movement of the entire system both vertically and 

horizontally. 

Material: Extruded aluminium to BS1474 in 6063/T6 grade alloy; grade 

304 stainless steel connecting bolts. 

Fasteners: Secret fix connection of panels to support structure. 

Security fix each panel to support rail (secret fix) with necessary 

expansion allowance. 

Number and location of fasteners: As system manufacturer structural 

design 

- Backing wall: (insert generic description or Structural Tray System 

as Clause 130) 

- Breather membrane: As section 785 

- Thermal insulation: As clause 776. 

120 

Rainscreen 

Cladding 

information 



- Accessories: (Define) 

- Other requirements: (Insert any panel properties not included above 

or in referenced drawings) 


Rainscreen 

Cladding 

information 

23 Rainscreen Types 

(Contd) 

STRUCTURAL TRAY INTERNAL WALL SYSTEM 

Structural tray system, spanning vertically or horizontally across main 

structural members, forming an inner leaf supporting wall for the 

rainscreen system whilst meeting necessary requirements thermally, 

acoustically, and for air permeability. The system accommodates 

window, louvre etc penetrations. Internal finishes are available in a 

range of options 

- Drawing reference(s): (Insert) 

- Material: 0.75mm thick cold rolled steel sheet grade Fe E 350 G, hot 

dipped galvanised to EN 10147 

Finish/colour: To Internal face – galvanised/25 micron bright white 

polester 

- Fixing: Fix to main supports with self-drilling, self-tapping screws 

complete with stainless steel bonded washers. 

- Vapour control layer: Trays to be sealed by side lap gaskets, EPDM 

bonded washers seal fixings, penetrations through the wall to be 

sealed with gun-grade butyl sealant. Trays should thus be sealed to 

form a vapour control layer 

- Insulation:To be fitted within pan of tray, with no gaps or spaced to 

required U-value of (Insert) 

- Breather membrane: Place membrane as clause 785 to front face of 

insulated Tactray 

- Top hat supports: Fit galvanised top hat support bar system vertically 

to front face of trays to carry wall bracket system. Fit to every other 

upstand with 2 self drilling carbonsteel/stainless steel fasteners. 

- Thermal break: Place 6mm thick medium density self-adhesive PVC 

foam break between top hat bar and tactray 90 upstands. 

- Other requirements: 

130 

130A (Use for major accessory items eg – fascias, louvres, etc) 

- Drawing references: 

- Manufacturer: 

- Material: 

Finish/colour: 

- Fixing: 

- Other requirements: 

130A 



References 

CWCT [1996] Guide to Good Practice for Facades. Centre for Window and Cladding Technology 

CWCT [1998] Standard for Walls with Ventilated Rainscreens. Centre for Window and Cladding 

Technology, July, 56 pp 

NBS [2000] Section H92 - Rainscreen Cladding. National Building Specification (NBS) Standard, March, 28 

pp

Formalised performance specifications for rainscreen cladding

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