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

ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSSURVEY OF ROOF STRUCTUREA survey of the roof should be completed beforecarrying out repair or replacement works. This shouldstart with an assessment of the roof structure:> What is it made of?> Is it structurally stable?> Is it in good condition?> Are there distortions and deflections?The roof profile and the materials used in itsconstruction and covering (including elements such aschimneystacks, roof dormers and lanterns) should berecorded.The cause of any deterioration in condition will needto be determined. Particular attention should be paidto vulnerable areas where water is likely to penetrateinto the interior of the building, for example at theends of the rafters or at the flashings between thegable and the roof slopes and at the base ofchimneystacks. It should also be established whetheror not there is adequate ventilation of the roof timbersand whether fixings are corroded. Checks should bemade for signs of fungal or insect attack in areaswhere ventilation is restricted, such as at rafter ends,behind parapets and around chimneystacks, andwhere pieces of lime parging may have fallen,blocking air flow between the rafters. The opportunityshould be taken to inspect the condition of such itemsas water tanks, access hatches and lights, chimneystacks and aerials, as well as rainwater goods andoutlets. Metal rooflights should be examined for signsof corrosion or distortion and for damaged coverflashings. Similarly, the structural stability ofchimneystacks should be assessed; the capping orflaunching examined for cracks and lifted flashingsreplaced as soon as possible.If the roof covering is to be fully or partially stripped,this provides an opportunity to inspect timbers inusually inaccessible areas around chimneystacks,below parapets and valleys, and along the head of thewall plate where it tends to be warmer and moresusceptible to rot outbreaks.STRUCTURAL APPRAISALIn order to make an informed decision on what repairsare necessary, a detailed survey of individual timbers,their layout, spacing, sizes and condition should becarried out. Once this survey information is gathered, itwill be possible to identify and locate rotten timbersections which need to be supplemented, treated orremoved, and to locate where new timbers need to bescarfed or jointed into the existing structure. The aimof any repair work should always be to retain as muchas possible of the existing historic timber and tosupplement its structural capabilities where necessary.These rafters were originally undersized for theirspan and had deflected, causing slates to lift or slip,and thus allowing water to enter the roof space. Tocorrect the problem, this additional supportstructure was added during roof repair worksIdentifying defectsThe sagging or deflection of a roof should not alwaysbe diagnosed as a roof failure; some unevenness orundulation in an old roof can add to a building’saesthetic and historic character, provided it is not asymptom of ongoing structural instability and is notallowing water to enter the building. However,deflection which has caused the leading edges of theslate to tilt up or protrude is a problem once waterpenetration to the interior roof space occurs. Inextreme cases, roof rafters can deflect to the extent ofstressing the nail fixings and breaking the nail headthrough the slate. Defects of this type should be dealtwith without delay.48