Ceiling joists are beams laid across the top of the walls of a building. They have two main functions:
1. To tie the walls together and prevent them spreading apart under the load of the roof.
2. To provide a support for the ceilings.
In most residential properties these joists are lengths of solid timber, either fixed to wall plates laid along the top of the brickwork, or secured to joist hangers which are fixed to the walls.
When a building has a sloping roof the joists are also fixed to the rafters, which stops them spreading under the weight of the roof tiles. On flat-roofed buildings the roof boards and waterproofing material are fixed directly on top of the ceiling joists.
Builders now often use pre-fabricated roof trusses. Such trusses are comprised of two rafters and a joist, firmly fixed to form a strong triangular structure. (Additional supports are also inserted to stop the rafters sagging.)
In modern homes ceilings are usually constructed of plasterboard nailed to the joists and finished with a skim coat of plaster. In older buildings the traditional lath and plaster construction may be encountered where the plasterwork is supported on wooden laths nailed crosswise to the ceiling joists.
In recently-built homes the joists will comply with building regulations, but in older properties considerable variations in size and spacing can be found. Under-sized joists can cause problems if they are insufficient to carry any load upon them, or if they are spaced too far apart. Problems may also occur when a loft extension has been added or other alterations have been carried out and the joists have been cut into.
As with any other building timbers, ceiling joists may be subject to decay or damage from woodworm and rot. Problems with damp decay may also occur if loft insulation has been installed without proper care being taken to prevent condensation in the loft space.
A building survey will indicate any apparent problems with the ceiling joists, and may recommend that further specialist surveys are carried out.