Timber Platform Frame System

Platform framing organises a building into repeated storeys. Walls are made up as panels of closely spaced studs between a base plate and a head, stood on a completed floor, and the next floor is then built across their tops to form the platform for the storey above. Each level is self-contained, and the junction between levels is a band of horizontal timber laid across the grain.

What carries downward load and what resists sideways force are not the same components. Vertical load runs through the studs and their plates into the platform beneath, while wind and unbalanced loading are resisted by the sheathing fixed across the face of the panel and by the floor and roof planes that tie panels together. A stud on its own is a prop; the panel is the wall.

The nearest relative is cold-formed-steel-frame-system, and the difference shows up at the floor band rather than in the wording. Timber shrinks across the grain as it dries, so the stacked plates and rim members at every storey give a building that shortens slightly and has to be detailed for it, while steel sections hold their length and instead carry heat straight through the wall at every stud.

Balloon framing, in which studs run continuously past the floor line and the floor hangs from them, sits inside this entry as a historic variant rather than as a separate system, because the panel and its sheathing still do the same work. What lies outside the frame - the weather layer, the position of the insulation, the finished face - belongs to timber-frame-external-wall-system.

Sheathing and studs only brace together. The fixings around the perimeter of a sheathing board are the path by which force in the board is handed to the framing, so continuous fixed edges matter more than the area of board present. A panel with an opening near its end may look well sheathed while having very little full-height braced length left in it.

The floor platform collects horizontal force from the walls it runs into and delivers it to the walls that are able to resist it in their own plane. Cut a stairwell or a large service void into that platform and the delivery route is interrupted, which changes what the walls below are being asked to do without anything visible happening to them.

Resisting racking generates uplift at the ends of a panel, and that is what the hold-downs and base plate fixings are answering. Where they are missing or under-provided, a panel that was designed to shear instead rotates at its foot, and the movement shows up as cracking in linings and around openings rather than in the timber.

Shrinkage accumulates in the horizontal timber at each floor band, so the frame shortens slightly while anything rigid and continuous past that band does not. A masonry outer leaf, a tall window, a service riser or a stair fixed in a way that assumes both move together turns a normal, expected change into a distorted opening or a cracked junction.

Notching and drilling reach both duties at once. A hole through a stud takes material from a member that is carrying vertical load and is also part of a braced panel, and a service run that forces sheathing fixings to be omitted breaks the very line where the bracing works. The service layout and the structural layout are one coordination exercise.