BCD 2019

48 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 C AT H E D R A L C O MM U N I C AT I O N S As well as the common structure the other principal structural elements need to be checked for defects and deflections, including purlins, trusses and tie beams. Evidence of movement includes opening of joints, failed tenons or pegs. Check for gaps where purlins meet rafters, trusses and gables, and where rafters and trusses meet wall plates. In particular tie beams spanning between opposing eaves plates need to be checked for opening up. As well as the original structure it is important to try and identify alterations. Purlins can often have struts added from strong points below, or indeed spanning between opposing purlins and cut with a birdsmouth joint. Sometimes truss members and ties are cut to allow better use of the building for accommodation, and elsewhere you may find a rather haphazard arrangement of timbers (illustrated above, top right) which have been added at different times. A common form of medieval roof particularly seen on churches is designed so that the rafters extend down to an outer eaves plate, with an inner eaves plate that supports an ashlar post (a short vertical timber rising to the underside of the rafter) ‘PROBLEM HOLE’ 20mm GAP SOME DECAY AT BEARING POSITION 160 x 190 WALL PLATE 100 x 180 @ 520 CRS OVERHANGS BY UP TO 100mm LOOSE OR MISSING MASONRY IN PLACES LATHE PLASTER REPLACEMENT ASHLAR 100W x 70D (LEANS OUTWARDS) MOVEMENT 165 x 190 WALL PLATE PIVOT POINT A pitched roof thrusting out at eaves level An example of ad hoc timbers added over time to help improve the roof Eaves detail of a medieval rafter roof with inner and outer wall posts: the foot of the rafter lifted slightly leaving the top of the ashlar post free to move outwards as the roof spread. To accommodate the movement indicated in the diagram on the left, these rafters had been cut at the top of the ashlar post, risking collapse. structural component, forming a regular pattern of members at relatively close centres. A purlin roof will direct higher loads at the truss bearing points to the structure below, whereas a rafter roof will normally spread the load more evenly across the wall line. In some cases there may be no need for lateral restraint in the roof structure itself. Sufficient restraint may be provided by thick masonry walls or buttressed walls, or internal structures such as walls and posts may support the roof higher up, significantly reducing the need for restraint. Of course there are historic examples of flat roofs as well, often covered with lead. Although these roof structures can suffer similar problems to pitched roofs, the need for lateral restraint is unlikely. Looking in more detail, the roof assessment will need to include checking the rafters for deflections, fractures or splits and areas of decay, either through water ingress or wood boring beetle attack, or a combination of both, or as a result of the use of second-hand timbers which may have been weakened by old mortices or similar. In many medieval roofs the common rafters are laid so that they are wider than they are deep. This is structurally not so efficient and often we find that the rafters have deflected quite significantly. If deflection exists then firring out (or levelling) may have been carried out as part of re-roofing to provide a reasonable plane for fixing battens and applying the coverings. Of course, finishes such as slate have less tolerance for significant undulations as opposed to handmade plain clay peg tiles. Externally, if a roof has reasonably good lines along the ridge and no significant deflections in the plane, but such deformations are found on the inside, one can say with confidence that the roof has not moved significantly since the last re-roofing. Sometimes a roof can appear to be distressed with regular undulations, but this may be undersized or defective roofing battens with the structure itself being sound. So significant movements to the roof or supporting structure may look concerning, but they may not require any repair work if they are at equilibrium and with no progressive movement. UNTIED ROOF WITH HIGH COLLAR UNDER LOADING Walls pushed out by rafters Walls pushed out by rafters Rafters deflect from tied collar under loading

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