A joist is a structural member that spans horizontally between the foundations of a building, or between walls or structural beams. In combination with other joists it provides support for a ceiling and / or floor.
In effect, a joist it is a form of beam that typically spans relatively short distances, and is made of solid timber. Joists are often associated with small scale or domestic construction. However they can also be made of composite materials, such as engineered timber I joists, metal web joists and so on which may be used as part of longer-spanning structures.
Series of parallel joists may be given additional rigidity by intermediate bracing running between and perpendicular to the joists, such as noggings (dwangs) or herringbone struts. These prevent joists twisting when loaded. For spans of between 2.5 and 4.5 m one row of bracing is needed. For spans in excess of 4.5 m two rows of bracing are required.
Holes or notches may need to be made in joists so that pipes, wiring and so on can be run through them. This can be done in domestic properties without significantly weakening the joist if notches are within the top 12.5% of the joist and between 7% and 25% of the span (measured from either end). Holes should be within the middle 25% of the depth of the joist, and between 25% and 40% of the span. Adjacent holes should be at least twice their diameter apart and not within 100 mm of a notch. Where more complex structures are involved, or if the joists are more than 250 mm deep, the advice of a structural engineer is required.
Joists may be connected to the supporting structure at their ends using traditional carpentry joints, or they may be placed into pockets, or they may be fixed using hangers, straps or other connectors. The use of hangers rather than pockets in walls tends to reduce air infiltration and noise transmission.
A common problem in older dwellings is that floors sag or feel ‘springy’. This can be because the joists are undersized (particularly where older buildings are converted for modern uses where they may be subject to higher loads than was originally intended), or they have been weakened by decay or by having holes or notches cut through them incorrectly. Joists can be strengthened, by 'sistering' steel, timber or ply reinforcing joists to the side of the existing joists.
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