Technical Specifications - UNDP

SECTION : 6
ROOFING
1 Corrugated Steel Roofs
All corrugated galvanized steel sheeting (whether fixed to wood or steel framing) shall be of 23 or 24
gauge fixed with a minimum of 50mm end laps and with a minimum of one corrugation side lap .
When instructed to have a one corrugation side lap, the sheets shall have a cover of not less than 20mm
and all the lapped sides shall be turned downwards. Where practicable the exposed lapped sides shall be
arranged to face away from the prevailing wind. When instructed to have a one and a half corrugations side
lap the sheets shall be arranged alternatively with a cover of not less than 90mm, the first sheet being
fixed with the lapped sides turned upwards away from the bearer and the cover sheet with the lapped
sides turned downwards .
Sheets shall be secured to purlins at centres not exceeding 300mm by galvanized self parking screws
(minimum 75mm long) with galvanized diamond shaped washers and lead sealing washers
All holes for bolts, self parking screws etc. shall be punched from the underside of the sheets and shall be
on the crown of the corrugations .
Galvanised steel ridge capping shall be supplied and fixed to purlins as described above .
Hook bolts, self parking screws and washers shall generally comply with BS 1494 .
2 Screeds
The provisions of Concrete Work section shall apply to the construction of solid reinforced concrete slab
roofs and to hollow slab roofs. The actual finish will be specifically shown on the Drawings or in the
Particular Specification.
Lightweight concrete screeds for obtaining falls or as an insulation layer shall be of approved type of
foamed concrete. The materials shall be measured, applied and cured in accordance with the manufacturer's
instructions and to the satisfaction of the Engineer.
In all cases the finished screed shall be of an approved proprietary type with a density of not less than
400kg/ m2 to receive the applied roofing . Mixing shall take place using approved mechanical mixers.
Concrete screeds for obtaining falls shall be (1:3:6) mix.
All screeds shall be laid in bays not exceeding 10 square metres and formed between stop boards of the
correct height and cut on each side to indicate the slope required in the roofing . The screed shall then be
trowelled with a wooden float to true and accurate falls or cross falls up to the stop boards . A 10mm side
gap shall be left between each screed bay for the full depth of the screed. The screeds shall be allowed to
cure thoroughly to attain maximum shrinkage. Any cracks which appear due to shrinkage shall be made
good . The gaps between bays shall then be filled in with cold bitumen.
Where the roof screeds are to be reinforced with one layer of galvanised wire mesh , this shall be supported
on top of the base on spacers to ensure that it is maintained at between 10mm and 15mm below the top of
the finished screeds . It shall be at least 100mm wide, securely wired together .It shall be stopped 20mm
from the edge of each bay.
3 Insulation
When asphalt sheets is used on the top of the screeds, it should have the following properties:
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