26 C O N T E X T 9 5 : J U LY 2 0 0 6 adhesive properties. Proprietary products contain appropriate fillers to control curing shrinkage and to give the desired characteristics to the material in both uncured and cured states. They can be used to fill much larger voids than conventional fillers and are the stock-in-trade of specialist firms offering resin repairs to external joinery as an alternative to traditional timber-on-timber repairs. Some firms offer hybrid systems in which a combination of timber inserts and polyester resin fillers are used to replace decayed timber. Epoxy and polyester resins are completely different materials with different characteristics. Their applications need to be considered separately. Companies specialising in non-structural repairs using polyester resins claim their chief advantage to be cost savings over replacement components (particularly windows) and traditional timber-to-timber joinery repairs. Semi-decayed areas are strengthened with a consolidant and decayed timber is replaced with a Reinforced epoxy resin repair to a decayed beam end as illustrated in Ashurst J and Ashurst N, Practical Building Conservation,Vol 5: wood, glass and resins. Is it better like this… or like this? Repair to the base of post or stud. a) The extent of decayed timber. b) Scarfed repair involves the removal of a significant amount of sound timber. Unless accurately formed, compressive stresses may be concentrated at x.Water penetration and trapping within the joint is a further risk. c) A simple buttjointed repair epoxy resinbonded to the host timber would be easier to form and involve less removal of sound timber.The risks of uneven distribution of compressive stresses and water penetration may also be reduced. polyester resin-based filler. In situations where the joinery is to be painted it could be argued that it does not much matter what the substrate is, provided that it functions satisfactorily (that is, that it remains stable; does not throw off the paint; does not trap moisture: and adheres strongly to the host timber). We would be right to be cautious and sceptical about the ability of polyester resin fillers to fulfil all of these requirements. Theoretically, the most likely mode of failure is cracking at the timber/filler interface caused by differential seasonal movements in the timber and resin, and insufficient adhesion between these two materials. Moisture admitted through these cracks is likely to be trapped behind the repair where it can create the conditions for further decay. However, it should be borne in mind that timber-to-timber repairs may not perform any better if they are poorly designed or specified, executed or maintained. Although there may be initial cost savings, the costin-use of polyester resin repairs will depend on their long-term performance. In the absence of systematic long-term monitoring of the behaviour of such repair systems, it would be prudent to avoid using them in significant and sensitive situations (such as in listed buildings). However, if the existence of such systems, and the companies that apply them, prevent the replacement of a traditional window in an unlisted building (more than likely with a plastic one), then that window survives to be properly repaired another day. Conversely, the use of epoxy resin-based repairs to structural timbers should not be thought of as a cheap alternative to traditional timber-to-timber repairs but as a method that has particular characteristics which may be useful and beneficial in certain circumstances. The repair of timber using epoxy resins has the potential to correct defects with less disturbance and loss of historic fabric than timber-to-timber repair, which involves the removal of decayed, partially decayed and sound timber to form a scarfed joint. Although we are advised by PPG15 that ‘traditional fixing and repair methods should be perpetuated’, this has to be balanced against the importance of preserving significant historical evidence which may be present in the surfaces of a decayed timber element. In addition, traditional timber-to-timber repairs can have a significant visual impact on a building. There is the point of view (to which I subscribe) that much joy may be derived from the craftsmanship of a wellexecuted carpentry repair. This is largely an aesthetic judgement which is not entirely consistent with our desire for repairs in other materials (such as masonry) to be less conspicuous. However, there can be little doubt that the visual character of a timber-framed building is diminished where, for example, elevations are peppered with face patches. Of course, there are many situations where resin repairs are not appropriate, and there are numerous examples of poorly executed and disfiguring resin repairs. However, these should not be reasons to dismiss this type of repair out of hand where it
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