tenon in the mortice. If the end of the joist is badly decayed, it should be cut back to sound timber and a new piece provided scarfed and bolted to the existing joist one end and the tenons re-cut the other end to suit the existing mortices. More likely, however, is that the joint has come apart and needs support, and this can be achieved by the introduction of a steel shoe or strap, the design depending largely on the position of the joists.
A similar problem arises when the joint between either the bridge joists or binder and the storey post is defective. Here, however, the end of the joist can be supported on an oak bracket bolted to the storey post. In addition, if there is any outward movement of the wall, this can be restrained with the introduction of a metal strap or in some cases a tie rod. Metal straps or shoes similar to those above can also be used.
Another common problem is caused by the insertion of service pipes notched into the top of the joists, resulting in a loss of compressionable strength. If the notches are positioned near the bearings, there is not generally any serious problem, but if they occur towards the centre of the span, the result can be disastrous. Where there is sufficient space above the pipes, the problem can be overcome by the insertion of folding timber wedges, but when, as is more often the case, there is little space between the pipes and the underside of the floorboards, it is necessary to insert a metal plate screwed to the joist across the notch. The plate can be slightly bent with the ends notched into the top of the joist, the plate being flattened as it is screwed down, with the ends thrusting outwards against the notches.
The other principal cause of failure in timber floors is the joint between the floor joists and bridging joist. When the end of a floor joist is decayed, the joists should be cut back to sound timber and a new piece scarfed and pegged in and a new joint made to the bridging joists. More often, however, the defect is caused by the lack of end bearing of the floor joists due to shrinkage of the timber. The most satisfactory method of undertaking this repair is to support the joist ends from above with a short iron plate across the bridging joist.
Great care should be taken in the repair of old timber roofs, the old timbers being retained as far as practicable, for roofs are generally the least altered of all in a timber-framed building. Depending on the degree of repair to be undertaken, either the roof-covering can be stripped from the portion of the roof to be repaired or, if the repair is of an extensive nature, the entire roof can be stripped. In either case the exposed timbers should be protected and never allowed to stand open to the weather, for rain and exposure are deterimental to old timbers not previously exposed to the elements, and wind can also cause severe damage. In addition, roofs should be repaired bay by bay. All joints should be carefully examined and defective pegs hammered out and renewed. Joints which have opened but are otherwise sound should be strengthened where necessary with metal straps.
Rafters normally suffer from rot in their feet and can usually be repaired with new timber scarfed and pegged to the existing with the new end shaped and jointed to the wall-plate as required to suit the original. In addition it is possible by renewing some of the rafters to repair the remainder by lengthening them with timber cut from the old rafters. The joint – again a simple scarfed and skew-pegged joint – should be located if possible above the purlin. Sometimes the bridle joint to the apex of the roof is defective, and in order not to lose the existing timber this can be strengthened by the addition of an oak collar-piece pegged on both sides of the rafter below the ridge.
The top face of the wall-plate can also be affected by rot; in some cases, where the rot is not too deep, the top surface can be repaired by ‘piecing in’ a new timber, the ends dovetailed and undercut and glued in position, care being taken to ensure that the new piece has matching grain. If the decay is deep, it should be made up of two or more pieces, each dovetailed and undercut to the existing. New mortices can then be cut to accommodate the ends of the existing rafters. If the wall-plate is beyond repair by this method, the defective section should be cut out and replaced with new, the ideal joint between the new and existing being the bridle-scarf joint.
Another member prone to structural failure or joint failure is the purlin. The most likely position for decay is, as in other timbers, at their ends, especially the tenon joint with the principal rafter. These can be repaired by a false tenon as previously described and strengthened by means of a straddle strap passing over the principal rafter. In some instances the back of the principal rafter is decayed as well, and this can be repaired by piecing in new timbers, dovetailed and undercut as described for wall-plates. Through purlins, either clasped or trenched, are easier to repair. The purlins are often cracked and broken, and isolated repair is often complicated by the bowed condition of the purlin, making it often difficult to scarf on a new length or even to strengthen it with plates. It is therefore probably easier to provide new lengths spanning between the trusses and jointed over the principal rafter. Another vulnerable point is the joint between the principal rafter, tie-beams, wall-plate and main-post. This can be repaired by replacing the defective timbers with new, scarfed and pegged as previously described, and new joints made as necessary.