Types of Timber Frame.pdf - Build It

Open Panel Timber Frame Construction
Open panel construction is the most widely and readily available method of timber frame construction
available in the UK. It has been used here for around three or four decades, and its popularity is
because it is easy to put together both when the pre-fabricated parts are made on site. In the factory the
essential materials are softwood studs, to form the main structure, and a rigid board such as plywood or
OSB to provide bracing, which are nailed together. The frame is constructed in a series of panels,
designed for easy delivery and assembly. The studs and the panels fixed to them are both crucial
elements of the structure and if either is damaged or missing, the strength of the panel is reduced.
Likewise, if the studs or the panels are not fixed together to the precise specifications calculated the
frame would not perform in the way that is expected. The name ‘open panel’ derives from the fact that
when the panels arrive on the site one side is open and unboarded. It is usually left open until the
building is weathertight at which point service runs and the insulation are put into position. After the
placing of the insulation, the open side is closed in with a sheet of plasterboard or a gypsum-fibre board
such as Fermacell, with a vapour control layer on its inside.
Features of an Open Panel Timber Frame
Fast construction on site
Can be clad in any material
Easy to transport – robust and damage easily repaired
Cost effective use of the materials
Suitable for self builders
High level of insulation possible
Limited alterations on site are possible
In common with most timber frame types, the panels carry out the main load-bearing work and are in an
identical location within the structure to the blockwork inner leaf of a masonry cavity-wall construction.
In the past in the UK, the studs used have almost universally been 90mm by 38mm in size. However,
with the implementation of building regulations that demand increasingly high levels of insulation, the
deeper studs of 140mm are more often being used. This is because the voids between the studs are
filled with insulation and deeper studs mean that more can be fitted within the structure. Because warm
air from the house carries moisture, the internal lining of an outside wall usually has a vapour control
layer incorporated into it, such as a foil backing to the plasterboard. To allow for any vapour that does
find its way into the spaces between the studs where the insulation is, the outer sheathing of an external
wall is covered with a breather membrane, that keeps out water (such as rain) but allows vapour-laden
air to pass through to the outside of the frame.
The timber and structure of an open frame are not
exposed and would not look very attractive even if
this were possible. Beyond the breather membrane,
the cladding is a matter of choice rather than having
to be compatible with the frame. Typical claddings
include softwood boarding, tiles, render and a
separate masonry leaf. If the latter is used, only an
expert eye will be able to spot that there is a timber
frame concealed behind it.
Extracted from “Kit and Modern
Timber Frame Homes” by Julian
Owen, Published by Crowood Press

Stucturally Insulated Panels (SIPs)
Instead of using timber to form a frame, SIPs consist of timber-based boarding, such as oriented strand
board (OSB) sandwiching sheets of a rigid foam insulation such as expanded polystyrene or
polyurethane. The two materials are firmly bonded together so that they effectively act as a single
structural component. The resulting panels are said to be 5 to 10- times stronger than conventional
timber frame constructions. In Europe, they are used mainly for internal and external walls, although they
are being used increasingly to replace rafters, because they provide a clear roofspace that can be used
for bedrooms.
Features of a Structural Insulated Panel (SIP) House
No frame – a ‘flat pack’ house
Very high levels of insulation possible with thin walls
Clear roof space for use as accommodation
Low leakage of air to the outside
Wide range of cladding choices
Cost advantage if standard designs used
Little wastage of materials on site
The panels are mostly self-supporting, although occasionally timber posts may be required. A wide
variety of different materials are used to make up the two components of SIPs including sheets that are
not wood-based, using cement or gypsum alternatives. The vital bond between the core and the
sheathing can be achieved either with adhesive or by using insulation materials that bond themselves to
the board as they are created. The latter is more efficient and generally the preferred method in Europe.
The thickness of the panels available varys between 70 and 250mm. The walls take the same position
as an open panel frame or a block inner leaf and the full range of external cladding materials can be
used, although brick is not one of the most cost-effective options.
For a given thickness, insulation levels that can be achieved are high in comparison with other systems
and masonry construction. In addition to this, the insulation and structure are effectively part of the same
component so that the walls to SIP houses are relatively thin, freeing up more space for use in the rooms
of the house – a factor that could be important on a tight site.
The sheets are cut to size in the factory, with all necessary window, door and service openings formed
before delivery. Lintels are only needed for wide openings, otherwise the panels are self-supporting.
Services can be run either in preformed channels in the depth of the board, or in the gap between the
SIP and the internal lining typically plasterboard on battens. Roof spaces can be kept clear and the
sheets will span up to about 4.8m without the need for supporting purlins.
On site a crane is needed to manoeuvre the panels into place and
the assembly on site is a job that is best left to specialists, who slot
them together and seal the joints. The structure of an average sized
house (e.g.150 sqm) can be assembled in two or three days in this
way. As well as forming the entire structure of a house, the
insulation level of SIPs is so good that they are sometimes used for
the infill panels of oak framed houses.

Oak frame buildings are not the cheapest in comparison to the other types of frame. They are a status
symbol that require a relatively generous budget. However, the cost is matched by the high quality that
runs through the design, selection of materials and care taken in the manufacture and assembly. The
modern oak framed house and the variations on this theme take the practices and principles of
carpenters from the middle ages and re-interpret them using 21 st Century factory prefabrication
methods. Chunky beams, columns and rafters are used, assembled in structural bays, with slotted and
dowelled joints just like the traditional frames assembled hundreds of years ago. The building is clad
either by filling the gaps between the posts with insulated panels, or covering the whole structure with a
cladding such as tiles, timber boards, render or brick.
Features of an Oak Frame Construction
Authentic traditional appearance
High quality construction
Structure is expressed as part of the design
Weathers naturally – no need for finishes
Very durable - no preservative needed in most cases
Suited to ‘open plan’ design
‘Green’ oak, is so called because when relatively freshly cut it is fairly easy to cut and shape. Once
felled, the timber begins to harden and become impervious to water. After about 18months to two
years, it has become so hard it is resistant to insects and also it is difficult to cut or sink a nail into it. As
the green oak matures, it dries out and begins to shrink, twist slightly and even develop small splits. No
one should acquire a green oak house unless they accept that this is part of the character of the material
and a key part of its charm. The frame is made in a factory, either by hand or machine, each element
designed to precisely fit with the others. A system of carpenter’s marks is used to identify how the
separate parts are to be assembled and exercise that is usually carried out in the factory before the
frame is dismantled and delivered to site.
Once the building is weathertight, the frame
usually needs to be sand-blasted or cleaned
with an acid. The colour of the frame
changes from cut wood to a silver grey once
the frame is exposed to the weather. It then
gradually darkens in colour over the years
that follow. Although thanks to its hardness
and strength oak needs no further treatment,
it is possible to darken the external timber
artificially using dye mixed with teak oil.
Inside, it can be waxed, oiled or stained
according to taste. Beeswax is a popular
choice, turning the wood an attractive honey
colour. There are also options for the way
that the surface of the exposed timbers are
treated.
Green Oak Frame Construction
Extracted from “Kit and Modern
Timber Frame Homes” by Julian
Owen, Published by Crowood Press
Photos courtesy of Carpenter Oak
and Woodland