Fortunately, joists do not develop as many faults as floorboards, but defects can occur, especially if unseasoned wood was used in the first place. A joist can twist out of true. A symptom of this would be that a floorboard would be seen to be twisted, but could not be nailed down flat. Or a joist can be attacked by woodworm or dry rot. The floor will seem weak or unstable if this is the case. If you suspect a weak floor you should always lift a board or two to discover what is wrong, it must be stressed that floor joist repair is a major undertaking, and not to be embarked upon lightly. They involve a great upheaval, especially on upper floors, for your work will almost certainly damage the ceiling below. Since each joist carries every board in the room, a large part of the floor, if not the whole lot will have to be taken up, which will mean the room will be unusable, as will any room below if you interfere with its ceiling.
A joist might be damaged along part of its length — a tendency for the floor to sag or feel unstable at one point is an indication of this, in such a floor joist repair case, saw out the damaged section as detailed below, buy a new length of timber of the same size, and bolt it to the cut ends of the old joist. If a joist is twisted or bowed it is often possible to straighten it by fitting struts between it and the joist on each side. Should all the joists be twisted, struts of fairly strong timber, 25 mm thick, and almost as wide as the joists are deep, will have to be fitted between each of them. They are cut to a length equaling exactly the spacing between the joists, and are fixed by skew—nailing. If this remedy does not work, the joists will have to be replaced.