If part of the edge of a floor becomes springy, and particularly if a gap opens up between the floor and the skirting board, or if cracks appear or plaster begins to flake away from the corresponding area of ceiling on the floor below, the root of the problem is likely to be where the ends of the joists meet the wall. Joists can need attention for a number of reasons:
– they become dislodged or are inadequately supported where they are attached to the main structure
– the strutting has failed or has become inadequate
– they are too small to carry the load
– they have been weakened by rot or woodworm.
Joists can be repaired or strengthened, and if necessary can be replaced.
Remove the floorboards in the affected area. When joists are installed, whether in recesses in the masonry or using joist hangers, wedges of wood or pieces of slate may be used to bring each joist level with its neighbours. The problem could be that these have shifted, allowing the end of the joist to sag. If there are no signs of damage or decay, the levelling pieces can simply be replaced.
If decay is present, then the cause must be determined and eradicated. If only the end of the joist has been affected, it may be possible to replace-just that part of it. Cut away the affected part of the old joist and remove it carefully to minimise damage to the ceiling below. Prepare a new length of joist and a timber ‘plate’, which will be used to attach it to the remaining part of the existing joist. The plate should be long enough to overlap both the original joist and the new end section by at least twice the depth of the joist. Treat the end of the new section with a timber preservative, and, if it’s to be inserted into the masonry, wrap dpc material around it.
Bolt the new section of joist to the plate, then insert the end into the wall or joist hanger and bolt the plate to the existing joist. Make any necessary repairs to the ceiling and replace the floorboards.
For joists which tend to sag in the middle, removing the floorboards may reveal that the cause is failure of some of the struts or noggings rather than of the joists themselves. Replace them, cross-nailing them into the joists. Alternatively, a joist may have a local weak spot, in which case a timber plate can be bolted to the existing joist to strengthen it.