A Guide to the Repair of Historic Roofs - Dublin City Council

ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSFULL RE-SLATINGIdeally, slipped slates and failed fixings should berepaired locally with minimal disruption of the historicfabric. However, if failure is widespread, a decision mayneed to be taken on whether or not the roof needs tobe fully stripped and re-covered. As a rule of thumb, ifan area of between 25 per cent and 30 per cent of theslates have slipped, cracked or are broken, or wherethere is evidence of widespread failure of the fixings, itmay be a more practical solution to strip the existingroof covering and re-slate the roof, rather than repaironly the defective slates. An assessment should bemade as to how much of the original material can besalvaged for reuse, taking into account therequirement for integrity and durability in theproposed roof covering. Stripping and re-cladding theroof of a protected structure or a building in anarchitectural conservation area may require planningpermission, and the advice of the architecturalconservation officer in the local authority should besought when any works are being considered.When the wholesale replacement of a historic roofcladding becomes necessary, it should be carried outusing materials that match the original as closely aspossible in terms of quality, appearance andperformance. Logically this would suggest the samequarry for slate as originally used. However, this is notoften achievable for practical reasons such as costs,availability of material and sometimes also the need tocorrect some design flaws in the historic roof. Adecision may be made as to which is the mostimportant roof characteristic to be maintained, if thesalvaged slate previously turned cannot be reused. Theissues for consideration concern the size, colour ortexture of the proposed slate in relation to the originalslate. The context of the building (whether within aterrace or a freestanding structure) and the rarity ofthe existing roofing material will also have a bearingon the development of an appropriate conservationstrategy for the roof works.A major factor for owners and custodians is the costassociated with providing the full replacementcovering, versus the accumulation of incremental costsfor ongoing piecemeal repairs, insurance, scaffoldingand the like. There will come a day when the decisionto re-roof will have to be made. Compromise is oftenpart of the final decision reached. The cost ofreplacement slate and scaffolding is a sizeableinvestment, which can be underestimated. A welldetailednew or re-covered roof should be fit forpurpose for the next 40–80 years, and commonlylonger with regular maintenance and occasional localrepairs, depending on the slate used in the re-coveringand the quality of the work. Approximately 75 per centof the original slate should be salvageable if the slateshave not previously been turned and are generally in asound condition. If a shared roofing project can beundertaken with an adjoining property the costs ofscaffolding can be reduced, which makes goodeconomic sense as well as having practical benefitssuch as ensuring a seamless repair to the roofcovering. If extensive repair work is envisaged, thepropping of plaster ceilings below is essential, and thedesign of a temporary roof covering and temporaryrainwater chutes should be factored in to the project.STRIPPING SLATESDuring the stripping and sorting of slate from a roof, aloss of 25 per cent of the slates can be anticipated.However, with carefully handling, loss of original slatecan be minimised. The salvaged slate should becarefully sorted according to slate type, size andthickness and left accessible for reuse. Slates are beststored on edge in batches of twenty separated by atimber batten to facilitate ease of installation.Sometimes the storage of slate at roof level isadvisable, and avoids repeated handling of the slates;however, the potential loading on the scaffold needsto be carefully considered. Clay ridge tiles should alsobe carefully salvaged and set aside for reuse wherepossible. If the original roof was laid with diminishingslate sizes or patterns in the slates, this should benoted and recreated. A good roofer will only strip theamount of roof covering that can be protected aswork progresses.76