The Building Conservation Directory 2021

124 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 2 1 C AT H E D R A L COMMU N I C AT I ON S Cost and benefit: where the cost of repairing a component is likely to be greater than the cost of replacing it, some cost-benefit analysis may help prioritise work. In other words, more time and care (and so cost) should be afforded to a feature that’s more special. Reinforcement: reinforcing pieces may be attached to individual members to strengthen them, preferably invisibly. For more extensive damage, old pieces can be supported or reinforced by attaching them to a substrate board or new timber, so they are in the right places and the new substrate does the work. Generally, the support would be hidden behind the original material, but if a great deal of the original was lost, it may be necessary to create an honest repair showing the support. On items of any significance, the new timber repair can be dated in a concealed place, so it can be easily identified as a repair in the future. Letting in Where new timber is to be let in, any damaged material that can’t be incorporated in the repair is removed, taking the item back to original material or to a point where the parent material is sound enough that the new material can be attached. Previous repairs may be kept or discarded once their intrinsic value, condition and aesthetics have all been carefully considered. Sometimes quite a complicated shaped original timber can result. Timber selection The correct choice of timber is vital for any repair. The new and old timber junction will expand and contract depending on temperature, moisture content or grain direction and so it is important to choose as similar a type of timber as possible and direction of grain. The new timber should be allowed to acclimatise significantly, so that the moisture content of both pieces is very close, before it is cut and shaped. Shrinkage or expansion may occur afterwards otherwise and the junction of the two pieces would then be poor. If differential movement occurs on an item that is painted, cracking of the paint often results. If the repair is then subject to rain, water will find its way into the crack, resulting in decay and further loss of material and another repair may soon be needed. Joining When it comes to joining the new and the old, a variety of techniques can be employed. It may be that the new piece can simply be screwed to the old using stainless or non-ferrous fixings to minimise corrosion. Also, screw heads can be hidden beneath a wooden pellet of a similar material, positioned so the grain is in line and planed or fitted to a similar finish. Traditional carpentry jointing techniques such as mortice and tenons, splices and bridle joints may be used. Dowels may also be used and they can often be discreetly positioned along the edge of the pieces to be affixed in the repair so that they are almost invisible. Sometimes a repair may be as simple as injecting glue to a split, then clamping or wedging the split together until it’s firm. Polyurethane (PU) glue is good for this as any voids will be filled as it expands during curing and any excess can be removed afterwards. Any movement of the new timber may cause warping or cracking of the timber. If that’s likely to be a problem, then the new could be joined to the old using joints or joining pieces that allow for differential movement. Resin repairs Resin can also be used to join a repair piece to the original. This technique works particularly well if the repair requires a degree of movement between the pieces, for example ensuring that overpainting will not crack. In these instances a minimum of 5mm of epoxy resin is recommended. This technique has the further advantage that a timber profile can be made up and then joined to the original. More good timber doesn’t have to be lost in order to create a suitable cut off point for an easier timber repair. The resin joining the two infills allows some flexibility and can be shaped easily after joining, producing a very hard to detect joint which is unlikely to crack. Resin can offer quicker, easier repairs to timber and also allows complex in situ repair, which in turn minimises the potential for damage during dismantling. Repair of items that would simply be impossible or incredibly difficult with timber can be achieved with resin if well specified. While resin definitely has a place in repair, it is not popular with some carpenters. It suffers from bad press associated with earlier poor performing resins which didn’t bond with the substrate timbers at a molecular level and unfortunately are still being sold today. In particular, a four-part epoxy system is far superior to a two-part one as it incorporates a fluid two-part resin which flows into the wood to stabilise it, allowing the later two-part repair to bond more firmly with sound timber. Poor application of resin repairs, bad technique, and moisture have led to mistrust. In reality there are probably as many poor timber repairs, but craftspeople are not so keen to talk about them and the failures aren’t as spectacular. Resins also lend themselves to consolidation. Low viscosity epoxy for example can be poured or injected into cracks, crevices and worm holes of an important historical piece, binding a fragile surface and saving it from further losses. Additional treatment for insect or fungal attack may of course be required and temporary filler, clay or plasticine may be needed to plug up any places the resin would run from until it’s cured. As distinct from epoxy resin, acrylic resin has the advantage of being reversible and as the compound is prepared by dissolving the solid resin in a volatile organic solvent, Scarf repairs to a painted boarded door using an epoxy resin: the thickness of the joint is to allow flexibility so that the paint will not crack. (Photo: Robin Russell) A casement window being repaired in the workshop: the bottom rail has been replaced and the two styles have been cut short to allow new sections to be let in. The same window installed – the repairs enabled maximum retention of original fabric. (Both photos: Robin Russell)