Upper floors: Joist hangers - Modern walls

Modern walls are usually formed in cavity construction and it is common practice to use galvanised steel hangers to support the joists, although they can be built into the wall. Joist hangers are made in a variety of patterns and sizes – a typical one is shown in the diagram below.

When the bricklayers have reached the first floor level, the carpenters position the hangers and joists, and then the bricklayers can complete the next lift of brick or blockwork. The ends of the hangers are embedded in the wall and the joist sits in the hanger and is fixed using nails. The use of hangers means that the joist has the protection of both the internal and external skin of the wall. The bottom of the joist needs to be notched where it fits into the hanger; failure to do this will result in an uneven ceiling line.

The Building Regulations imply that hangers must be used for joist support but
it is still acceptable to build the joists into the internal leaf of the external cavity wall (though not into party walls) as long as either a sealant is used around the joist end or a proprietary joist cover (sealing box) is provided to prevent air leakage and therefore heat loss. The fact that this precaution is not necessary with a joist hanger is an additional advantage to
using them.

The danger of building joists into the wall is the risk that moisture vapour in a damp cavity may, over a long period, raise the moisture content of the ends of the joists. As the moisture content of the timber settles into equilibrium with the damp air in the cavity, conditions may be created which encourage attack by wood-rotting fungi.

Floor joists should not project across the cavity and touch the external leaf of brickwork or stone work. Even slight projections into the cavity can cause problems as mortar droppings can become attached to the timber and cause bridging of the cavity.