Platform Frame, Post and Beam or Mass Timber

A platform frame organises the building into repeated storeys: panels of closely spaced studs stood on a completed floor, with the next floor built across their tops to form the platform for the storey above. A stud on its own is a prop; the panel is the wall.

A post and beam frame gathers load at a small number of positions and passes it through connections cut into the timber itself, so the frame is sized by what remains at its connections rather than by the members considered alone. A mass timber panel is a plate rather than a member: it carries load down its own height, spans across a room, and resists being pushed out of square in its own plane, doing work a light frame divides between studs, sheathing and joists.

In a platform frame the bracing is the sheathing, and the work happens at the fixings around the panel edge, which is why a sheet stopped short, unfixed at one edge or interrupted by a service run does not brace what it appears to cover. Resisting racking also generates uplift at the ends of a panel, which is what the hold-downs and base plate fixings answer.

A heavy frame is not arranged to rely on sheathing at all: diagonal braces triangulate the bays, and a brace works only when both ends are firmly held, so its value depends on the members it lands into rather than on its own size. In a mass timber building the plates themselves provide in-plane stiffness, but a floor made of several panels is only as continuous as the joints between them.

The platform frame entry stops at the sheathed face. What sits outboard of it, drainage, ventilation, the position of the insulation and the finish, is a separate design fixed back through the structural face at every bracket, so every penetration is a hole in a barrier and a hole in a braced surface at once.

In a post and beam building the question is whether the envelope wraps the frame or fills between it. A wrapping layer crosses the structure in one continuous plane and can be detailed to let the frame move behind it; an infill panel presents a junction at every post and beam, where the weather line, the air line and the movement allowance all have to be resolved together in the same joint.

With mass timber the envelope fixes into the structure. Brackets and rails fix directly into the plate, so every fixing is a penetration of a load-carrying element and of whatever controls water behind the cladding, and the fixing pattern is settled with the panel design rather than after it.

A platform frame offers the stud cavity, competed for by insulation, cables and pipes, and what passes through it is drilled or notched through members that are carrying. The service layout and the structural layout are one coordination exercise rather than two.

A mass timber structure offers nothing through the plate that was not planned. Services run either in holes cut in manufacture or in a battened or suspended zone applied to the face, and deciding which, and where, is what determines how much of the timber can remain visible.

A post and beam frame leaves the walls free of structural duty, so routes are largely an envelope question, but the coordination of services, openings and fixings happens ahead of the point at which the frame is cut.

Mass timber closes earliest and most completely. Openings, service holes, recesses, rebates and connection preparation are part of the manufacturing information, so a hole that was drawn is made in a controlled way within a plate designed for it, and a hole that was not drawn is cut on site through a load-carrying element and through whatever surface was meant to remain visible.

A post and beam frame is cut before it is raised, which means the drawing set is closed early and changes made afterwards are made in an object that has already been fabricated. A platform frame keeps its options longest, but layout changes remain structural changes because the braced wall lengths are part of the layout.

Timber changes dimension with moisture content, mostly across the grain, and each system concentrates that differently. In a platform frame the change gathers in the horizontal timber at each floor band, so a masonry outer leaf, a tall window, a service riser or a stair fixed as though both move together turns an expected change into a distorted opening or a cracked junction.

In a heavy frame the sections dry slowly and go on changing shape long after they are in place, so the design question is not how to stop that but what is allowed to be rigid against it. In mass timber the plates move less than sawn sections but still respond, and where a structure stacks many horizontal plates the accumulated effect at the top of a building is worth designing for explicitly.

All three are vulnerable between erection and enclosure, and they pay differently for it. In a platform frame, board sheathing that has been wetted and dried is not necessarily what was specified, and protection during that period is a planning question whose record belongs with the handover information.

With mass timber the exposure that matters most happens before the building is closed, when horizontal plates collect water and joints hold it. Because the panel is often the finish, marking, impact and water staining during construction stay in the completed building, and water damage to a visible structural surface is generally permanent in appearance.

The entries record each other as alternatives and stop there. What this guide offers instead is the shape of the difference, so that a proposal can be read on its own terms and the awkward questions arrive while the answers can still change something.