The use of a load bearing wall reduces the span of the joists and precludes the need for long deep-section joists which were (and still are) expensive and difficult to obtain. The joists do not penetrate the party wall thus helping limit the spread of fire and the passage of airborne sound. In many houses the joists can be found running front to back. If they are securely built into the external wall, they provide a measure of restraint and prevent buckling and bulging in the wall. They are, however, at risk from damp penetration.
Where issues of restraint and fire protection are not important the joists will normally be found spanning the shortest distance across a room. This provides minor savings in cost because the joist depth can be reduced.
Terraced houses with large bay windows, for example, sometimes have joists running parallel to the bay. The joist direction can be determined quite easily; they always run at right angles to the floor- boards.
To stop the joists from twisting or warping (and possibly damaging the ceiling finish) it is usual to find a line of strutting fixed at right angles to the joists. Strutting also helps to ‘tighten up’ a floor, thus reducing ‘bounce’. The struts can form a herringbone pattern or can be ‘off-cuts’ of timber; usually staggered so that they can be easily nailed to the joists.
Fixing the Joists to the External Wall
When joists are built into solid external walls there is a danger of damp penetration, which can ultimately cause rotting of the timbers and collapse of the floor. If the wall is over 1 brick thick then this problem is less likely to occur. However, the majority of houses built before the introduction of cavity walls have walls 1 brick thick and, in these cases, the joist ends are only protected by 100 mm or so of masonry. General deterioration of the bricks and mortar can lead to damp affecting the ends of the timbers; subsequent problems of rot can be expensive to resolve.
In some properties a strip of timber known as a wall plate is inserted under the joists in order to provide a level surface on which to fix the joists. This can be found where the brickwork is of poor quality or where stone rubble or irregular sized bricks preclude level courses. In this situation an outbreak of rot in the wall plate may ultimately lead to instability of the wall.
Openings in Timber Floors
Nearly all upper floors have openings in them to accommodate stairwells and, possibly, chimney stacks. Forming the opening is known as trimming and is a relatively simple exercise.
The diagram below shows a modern floor that has been trimmed to form a stairwell. The joists in the diagram are supported at the wall by steel hangers, but could be built into the inner leaf of the blockwork. Either side of the stairwell the joists are doubled up and bolted together. This provides extra strength, as these two joists are carrying