BCD 2019

BUILDING CONTRACTORS 2 49 C AT H E D R A L C O MM U N I C AT I O N S 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 1 9 and forms a small triangular void at eaves level secured with tie beams at regular centres (see diagram opposite). Again there can be variations, such as no wall plates, with each rafter having a stub tie which supports its foot and the post. With the deflection mentioned above occurring in this situation, the posts can become a pivot point, and in extreme cases the foot of the rafter lifts off the eaves plate. A void may open up in the core of the wall, and sometimes the posts can lean back at an angle. Often the repair will look to tie the rafter foot to the inner eaves plate or properly triangulate the joint to help resist this action, with appropriate packing to ensure the plate is supported. TIMBER DECAY In general terms and pre the mid 18th century, timbers are more likely to be oak, whereas after this date softwood (Scots or Baltic pines) are much more common. However, there are always variations to this including the use of elm, and occasionally black poplar and other local hardwood species. Timber from North America was not imported until the 19th century. Of course the different timbers have different structural characteristics, and A micro-drill may be used to investigate the condition of timber: variations in the speed at which a 2mm drill bit penetrates timber gives a reliable indication of the depth and extent of decay. (Photo: Hutton + Rostron Environmental Investigations Limited) A scarf repair to a tie beam with concealed fixings Structural fixings over the existing rafters A rafter foot with a partner timber. varying resilience to decay or beetle attack. Useful tools for inspecting a timber roof structure include note pads, torches, camera, tape measure, a moisture content reader and a probe to check for concealed decay. This may be a bradawl, penknife or screwdriver. Of course, depending on the significance of the building or the particular elements, specialist equipment for investigations may be warranted. Particularly useful can be a micro-drill (see illustration above) which measures and records the density of the timber, thus identifying areas of concealed decay. As it only uses a 2mm drill bit the damage is minimal. A hand drill can also be used in much the same way, but relies on the experience and skill of the user to assess its progress through the timber, to distinguish between sound and decayed areas. With regard to beetle attack, the general approach is to try and ensure that the moisture content in the timber is kept at low levels (below 15 per cent) in which case beetles will not thrive and the timber will not require treatment. REPAIRS AND ALTERATIONS Where structural defects are located and repairs required or where proposed alterations will affect the performance of an existing roof, careful consideration is needed to establish the most appropriate methodology and philosophy for the works. This is influenced by the significance of the fabric affected, the buildability, the cost, and the views of the statutory authority in the case of a listed building. For a decayed member the two principal options are either to scarf a new section in and joint it appropriately or to apply additional members. These may be in timber or potentially steel. With the former the repair timber may be carefully detailed to work in with the existing (see illustration above), or alternatively the repair may be made obvious, such as a flitch plate repair (illustrated on the next page) or the reinforced rafter feet (illustrated top left), so it is clear that this is a repair, often termed ‘an honest approach’. With a deflected rafter, it is possible to improve this by installing structural firrings. These are new timbers, often oak for strength,