Timber frame structures

A timber frame resists wind and keeps itself square through racking. The sheathing board fixed to one face of each wall panel, oriented strand board or plywood, turns the panel into a shear wall, and the pattern and spacing of the nails or screws that fix that board is what actually carries the racking load. We calculate the racking resistance each wall can offer and confirm there is enough of it, correctly distributed, to take the wind load on every elevation down to the foundations. Too few sheathed panels on a given face, often where large openings eat the wall, is the usual reason a design fails to stack up.

Connections in timber are made with nails, screws, bolts, dowels and proprietary framing anchors, and their capacity depends on embedment, spacing and edge distances as much as on the timber grade. Uplift matters as much as gravity: wind can try to lift a light roof and light walls, so we design hold-down straps and anchors to tie the roof to the walls and the walls to the foundations, giving a continuous load path in both directions. Metalwork and joist hangers are specified to suit the members they support.

Timber frame suits low-rise housing, flats up to a handful of storeys, schools and light commercial work, and it is well matched to fast, dry construction because panels can be prefabricated and stood up quickly. The frame closes in early, the wall build-up gives good insulation depth, and the low self-weight often allows simpler, shallower foundations than a masonry equivalent on the same ground. It works equally well behind a brick outer leaf or a rendered board finish.

The points to watch are moisture, fire and movement. Timber must be kept dry in service, so the wall build-up has to manage vapour and prevent interstitial condensation, and details at the base of walls keep timber clear of standing water. Fire resistance and, in flats, the spread of fire between compartments are designed in through the lining boards and cavity barriers. Timber also moves as it dries and with humidity, so we allow for shrinkage, particularly the cumulative movement across floor zones in taller frames, so services and cladding are not strained.