The Building Conservation Directory 2020

50 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 C AT H E D R A L C O MM U N I C AT I O N S 225mm (nine inch) brick wall which has been constructed around the timbers, any voids resulting from the difference in the depth of the timber and the brick is filled with mortar. Where the construction includes jetties these are built in much the same way as the historic timber frames with the jetty joists extending well back into the rooms and securely fastened below a cross wall. In some instances the timber framing is purely decorative and consists of 25 to 50mm thick timbers secured to a solid brick wall, these are normally sunk flush with the surrounding render. Covings beneath overhangs are normally formed from timber laths but these may also be expanded metal lath. INSPECTION The protocol for inspecting this type of building is similar to inspecting a fully structural historic timber frame, although unlike the older timber frames, distortions in the frame are uncommon and should generally be treated with suspicion rather than assuming they are the result of initial drying and shrinkage. Because of the composite nature of some of the walls, brick infill or backing should be considered structural until proven otherwise and load paths should be fully established before preparing any specifications. Particular attention should be given to corbels and brackets, projecting windows and jetties. DEFECTS Unlike the earlier timber frame buildings, problems of water penetration in and around the timber frame were minimised through careful design detailing, however decay remains a problem. Decay occurs preferentially, and typically affects joints and the vulnerable top edges of horizontal timbers. White rots are most commonly found but these can cause extensive damage even in seasoned oak that has been exposed to long-term water penetration. Dry rot is relatively rare in the frame but does sometimes develop internally as a result of defects in the building envelope. Render defects are common and are likely to precipitate decay in the timber frame. Cement appears to have been the binder of choice and this combined with modern paint finishes contributes to the decay that commonly affects the edges of the timber frame. High relief pargetting is found on many buildings of this type and is often moulded in Roman or Portland cement. Where this is confined to the upper part of the gable, in order to keep the weight low the backing is sometimes constructed from timber laths rather than brick, due to the weight of the pargetting and the vulnerability of the laths and nails to the effects of water ingress, these panels can occasionally come crashing down. Due to the nature of the construction, structural problems in the timber framing tend to occur as a result of a failure in the joints around the tie beams and corner posts. However, more serious problems may occasionally occur in the jetty structure. The stability of a jettied storey relies on the ability of the projecting jetty joists to resist the forces applied by the structures above. In the vast majority of buildings the jetty is fairly shallow and free from any defects, occasionally jetties can fail, either because the architect or builder has failed to do their job properly, or because the structure has been compromised in some way. The most likely cause of a jetty failure is a combination of inadequate restraint of the jetty joists and defects in the frame which have caused additional loads to be applied to the horizontal jetty beam. One of the first indications of serious defects in a jetty is a distortion in the floor covering where the jetty joists are rotating over the wall plate causing a characteristic hump in the floor. REPAIRS AND MAINTENANCE The type of repairs required will depend to some degree on the significance of the building. Unless there is a clearly identified design flaw which has contributed to the defect, all repairs should be conservative in nature using techniques and materials to match the original work. Subject to the constructional differences previously described, current best practice methods for repairing historic structural timber frames applies to the repair of timber frame revival buildings. Repairs to the timber frame should be undertaken using traditional timber to timber repairs whenever practical. Subject to approval, isolated pockets of decay in non-structural timbers can be repaired with resin. The use of composite repairs incorporating reinforced shuttered resin and or steel brackets should also be considered for structural timbers where these techniques reduce the loss or disturbance of the surrounding fabric. Depending on the type of failure, repairs to failed jetty structures may require some degree of re-design or substantial strengthening works. The precise repair methods will vary but in order to complete these type of repairs, considerable opening up is often required. Where pargetted panels need to be removed to complete a repair, it may be possible in some circumstances to carefully remove the panel complete with its lath backing and re-fix this after the frame repairs have been completed. Where the pargetting has been applied to brick, a full photographic and drawn record should be made prior to dismantling, this should then be used to replicate the original design. Where extensive repairs are required, there may be a case for replacing brick infill with an appropriate lightweight insulating panel. This decision should consider the historic and visual significance of the brick as well as the structural implications of making such an alteration. Most of these buildings have been painted in the familiar black and white scheme that contributes to the urban grain of many towns and villages. While there are technical arguments in favour of removing paint and other types of finishes from timber frames, in the case of those buildings that are listed, or located in a conservation area, consent to remove the paint exposing the timber is unlikely to be given. RECOMMENDED READING AW Jackson, The Half Timber House , Robert M McBride & Co, New York, 1919 E Hubbard, The Work of John Douglas , The Victorian Society, London, 1991 FH Crossley, Timber Building in England , BT Batsford Ltd, London, 1951 TIM NICHOLSON MScCHE, MCIOB is a historic building surveyor and consultant with Nicholson Price Associates (see page 40). Email info@npaheritage.com Black paint often conceals timber decay Pargetting repairs being carried out by Lizzie Nicholson-Jones to an 1870s house by John Douglas in Chester