to repair them. When the decision is to repair them, the defective timber should be cut away until sound timber is reached and, in the case of studs, new timbers scarfed and skew-pegged. In the case of the main post, a scissor-scarf joint is recommended. In some cases only part of the sill-beam will need replacing, and in this case the defective length should be removed and replaced with new, an ideal joint in this situation being the bridle-scarf joint. Thin sections, such as struts and braces, should be half-lapped scarf joints preferably ‘sallied’ and pegged or bolted. New braces can be fitted with the use of wedged tenons.
A problem which often arises is when a new stud is required, either because it is defective or because it has been removed at some earlier date, but the adjoining sill-beam, girth or wall-plate is intact and is not to be replaced. This problem can be overcome by various jointing methods; the joint to the top can be formed with a tenon in the usual way and the bottom secured with a false tenon, either a slip or fish-tenon, secured with oak pegs, or with a wedged tenon. Alternatively an elongated mortice in which the bottom tenon of the stud slides into position can be used, the mortice being ‘pieced-in’ with new oak to match.
Existing timber floors are often sloping or springy, and this may be due either to the failure of the bridging-joist or one or more of the common joists or to the distortion caused by the deflection of one of the joists. To replace the defective beams or in some cases even to repair the defective joists would generally entail great disruption to the whole structure, including the destruction of the ceiling beneath. If possible, repair work should therefore be undertaken in situ, thus avoiding serious loss to the building. Original timber should as mentioned earlier only be replaced as a last resort.
Apart from failure due to beetle infestation and rot, most structural failures in timber are caused by overloading. Fractures in beams usually occur in the bottom half of the timber, for the top half of the beam is in compression, while the bottom half is in tension. The simplest method of repairing a fracture in the bridging joist consists of placing metal plates beneath and on top of the fracture and then bolting through the joint vertically. If the joist is exposed, the plate can be painted to match the colour of the oak, helping to hide the repair.
Another likely place of failure is the joint between the bridging joist and binder or side girth. Usually the joint between the two is a double tenon, and often this joint is found to be defective due either to decay, or to shrinkage of the timbers resulting in the loss of bearing of the
348 Timber-Framed Buildings of England
repair of fractured beam 208.