Sheathing diaphragm

The basic idea

A board is very stiff in its own plane — far stiffer than in bending. Fix that board to the frame and the plane of the wall, floor or roof behaves as a bracing panel.

This is the diaphragm effect. It is what makes it possible to use a whole wall surface for bracing rather than only diagonal members.

Where it appears

In the wall

The external sheathing board, sometimes the internal one, works as a bracing panel. Useful above all where there is no room for a diagonal strap.

In the floor

The floor board gathers the horizontal loads and delivers them to the bracing walls. That role is often invisible, but without it the bracing walls never receive the load.

In the roof

The same role in the plane of the roof.

What its capacity depends on

Board type and thickness

A cement-bonded board, a gypsum fibreboard and a wood-based panel give different capacities. The board material is structural data.

The screw pattern

The most important parameter and the easiest to get wrong. The capacity of the diaphragm rests largely on the number and position of the fixings — at the edges of the board especially.

Board size and layout

The position of the joints, the orientation of the boards and staggering them all affect the stiffness.

The edge connection

The fixing to the section running along the edge of the board, and how that is passed on at the connection.

What tends to happen in practice

The most common mistake is substituting a different board or screwing it at wider centres, because “it is only lining”. A large part of the bracing then disappears without anyone noticing.

So the drawings have to mark clearly where the lining counts as structure, and to what pattern.

It combines with strap bracing

The two do not exclude each other: bracing part strap, part board is common. The design, however, has to be explicit about which carries what.

The point

Where the lining braces, the lining is structure. The screw pattern is then as fixed a value as a section size.