Windows and doors

Forming openings in timber frame construction.

Window and door openings in prefabricated timber frame panels will be formed in the factory to the dimension given on the drawings. Each opening will typically have a CLS cripple stud fitted to each side to support the timber lintel over and the sill piece underneath. Fitting the windows and doors and their frames into the openings then present challenges of making sure that they prevent (a) the ingress of moisture, (b) the passage of uncontrolled air and (c) cold bridging of the thermal insulation.

Ideally, the window or doorframe should be fitted in line with the cavity wall insulation and fixed securely to the frame around it. Any cold bridging needs to be dealt with by adding strips of rigid insulation where necessary.

Then all the edges of the frame must be sealed with an approved flexible sealant before applying air sealant tape around the frame and its surrounding timber frame members. This makes it sound easier than it actually is, but the fact remains that airtightness tests often reveal leaks around openings which haven’t been sealed with the utmost care.

Added to these challenges is the need to incorporate movement joints to take care of differential shrinkage between the timber frame, the external cladding and the windows and doorframes; and you soon see why this part of the build requires time, care, knowledge and skill.

Any site operatives carrying out these jobs should therefore have received proper training and be suitably experienced. Some contractors won’t like to have their most skilled and experienced people being tied up for long periods on a job which in the past took only minutes and was left to the window suppliers.

You should also check whether the window manufacturer employs trained window installers and if their warranty covers the carrying out of remedial work in the event that inspections or airtightness tests indicate any problems. It is best to keep alert to the possible pitfalls on this part of the job and find out how to do it yourself if need be. There are various short courses available which address the issues of airtightness, cold bridging, moisture risk and so on; and some training providers will even train you up on site.

In areas of severe or very severe exposure to driving rain, that is throughout NI and in most of ROI, masonry should form a rebate at the reveals of openings to avoid a straight through joint where the frame abuts the masonry. Damp proof courses (DPCs), preformed cavity trays and cavity drainage weepholes should be installed over all openings in external walls.

Differential movement

As the timber frame structure dries out, it will shrink and the overall height will reduce. The extent of the differential movement increases with the height of the building, and will be found to occur between the timber frame and other parts of the construction.

This movement should be expected to occur in both prefabricated and stick built timber frame structures and will be more pronounced where green timber has been used. Common areas which require particular attention include:

� At openings in the wall panels.

� Interfaces between the timber frame and other types of construction.

� At each floor level.

� Where the roof meets the walls, i.e. at eaves and verges.

� Where materials with different rates of expansion or shrinkage are positioned together.

Movement joints must safely accommodate the anticipated amount of shrinkage or expansion, be weatherproof and durable and be protected by a cover strip where the movement gap is expected to be more than 35mm.

There are tables available in the timber frame standards which offer general information on the expected gap sizes due to movement, although these cannot cover all eventualities, so exact gap calculations can be carried out based on guidance within EC5.

Where to incorporate movement joints

In the case of wall openings in walls with a timber frame inner leaf and an outer leaf of brickwork or blockwork, gaps need to be considered due to both upward expansion of the brickwork and downward movement of the timber frame.

A movement gap above a window will therefore increase after movement occurs and the gap under the sill will compress. To allow for this movement, sills should be built with a gap between them and the masonry; and outer lintels need to be built in mind with a gap that will expand.

Where outer lintels are attached to the inner timber frame leaf, they must be fitted to allow for movement between the lintel and the timber.

For external walls with lightweight cladding on the face of the inner timber frame wall panels, there should be no differential movement at window heads and sills.

Where a roof supported on a timber frame wall abuts a masonry wall, accommodation for movement should be incorporated behind the flashing so as to retain a minimum of 75mm of upper flashing cover over the roof flashing upstand. This situation can apply to sloping or flat roofs abutting a masonry wall or parapet.

At doorways, where a suspended timber ground floor meets the outer concrete or masonry structure, a wedge shaped timber or composite transition piece should be left between them until the floor settles down. The transition piece can be removed when the timber parts of the structure have ceased to shrink and the inner and outer levels are about equal.

At eaves and verges, the roof which is carried on the inner timber frame structure will tend to move down towards the external brickwork or blockwork outer walls. For this reason, suitable gaps are left under the soffits so that they do not become deflected under the movement. These gaps should be filled with a compressible infestation-proof mesh.

Any services such as pipes, ducts or vents passing through an inner timber frame and outer brickwork wall must also be fitted to anticipate the relative movement between the wall leafs. This should be addressed in a similar way to the window openings, with a gap under the service opening which should decrease and a gap over which should increase.

At suspended timber floor levels where the external wall is of lightweight cladding on an inner timber frame, horizontal gaps should be provided in the external sheathing of the inner wall panels at the head of the lower frame and the bottom of the upper frame.

A horizontal gap must also be provided between the upper and lower cladding sheets and if they are fixed on vertical battens on the face of the inner wall, the batten must also be broken with a similar gap. Where the timber floor is constructed using solid timber joists, these gaps should be 15mm and in the case of I-joists, the gaps can be reduced to 10mm.