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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFScheck the tile condition, colour, size and generalsuitability before making a purchase. It is important toestablish the provenance of any salvaged tiles toensure that they have not been taken from anotherhistoric building that should not have beendemolished in the first place.Concrete tile repairThe lifespan of a concrete tile is not as long as that of aclay tile, particularly in the case of ridge tiles, whichbecome more porous over time. Concrete tilesweather, making single tile replacement difficultunless a reclaimed tile can be sourced which has asimilar weathered appearance. The standard size is 15x 9 inches (380 x 230mm); while a few are 18 x 10inches (460 x 255mm). Care should be taking whensourcing replacement tiles to establish whether a lefthandedor right-handed tile is needed. This is becausethe interlocking part of a tile can be handed, or turnedback to front. The tile may appear identical in all otherrespects.There is a wide range of clay ridge tiles available,including ornamentals, which enhance the characterof a roof. These should be repaired or replicated asnecessary, but not introduced unless there is evidenceof their use on the buildingWhen trying to source appropriate replacement tiles,photographs can be a convenient way to circulateinformation on the existing tiles to be matched. Whenproviding photographs of an existing roof tile topotential suppliers, every attempt should be made todepict the measurements of the tile accurately byusing rulers or tapes to measure them within theimage; a sketch noting the exact measurementsshould also be provided.A Marley concrete roof tile on this modest 1940scottageMAJOR REPAIRSWhere the condition of a concrete tiled roof is so poorthat stripping and re-tiling is considered necessary, theroof should first be surveyed and photographed toensure that the existing details will be properlyreplicated. It is important to salvage as many aspossible of the sound tiles for reuse. These should becarefully selected, cleaned and mixed with new tiles toensure a good colour blend. When carrying out analteration or extension, the old concrete tiles could bestripped from the rear of the roof to make up ashortage on the front pitch, using similarly weatheredtiles for the sake of uniformity.87
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSLead and leadwork repairLead is the best material for weatherproofing complexroofing details and junctions which cannotsuccessfully be made watertight over the long termwith any other roofing material. However, leadworkingrequires considerable expertise. Lead has ahigh coefficient of thermal expansion. This means thatit expands when heated and contracts when coldsignificantly more than other adjacent buildingmaterials, and by as much as three times more thanbrickwork. Lead is often positioned in parts of abuilding that are subject to the greatest range oftemperature changes such as roof surfaces. Therefore,good detailing and work practices are essential toensure that thermal movement in sheets of lead doesnot cause opening of joints or splitting of the sheets.The weight and low strength of lead sheets can also, ifnot correctly detailed, result in tearing at fixing points,or in creeping and sagging of sheets, particularlywhere fixed to steep pitches or used as verticalflashings.WORKING WITH LEADLead has been used in construction for centuries.However, lead and its corrosion products arehazardous to health and it is essential to takeprecautions when working with the material. The LeadSheet Association provides helpful advice on the safeuse of lead which can be downloaded fromwww.leadsheetassociation.org.uk.Lead is very dense and therefore even the smallestsheets and rolls of new sheet can be exceptionallyheavy. Proper lifting equipment should be madeavailable to assist in the lifting of any weights over25kg. Lead when ingested can build up in the body totoxic levels; therefore it is essential not to eat, drink orsmoke in a place liable to be contaminated by leadand to wash thoroughly at the end of each workingsession. Likewise, gloves should be worn whenworking with lead to prevent fragments becomingembedded in the skin.hazardous white dust. Therefore, masks must be wornwhen removing old lead sheeting and the lead sheetmust always be handled with the utmost care. All oldlead sheet and any off cuts generated during theworks should be collected, segregated from othermaterials and disposed of safely or recycled.PUNCTURES AND GASHESLead is a soft material that is easily damaged and tornif subjected to heavy foot traffic. Punctures and gashesfrom walked-in nail heads or slipped slates are thesimplest types of damage to repair. Short-term repairscan be made using a proprietary sealing tape or bysoldering/solder wiping. Lead soldering involves eitherdropping solder onto the damaged area (which hasbeen prepared using flux) or soldering on a small leadpatch. A soldered repair has a smooth wiped look.Because the solder is formed of approximately twoparts lead to one part tin, it can behave differentlyfrom the base lead, and thus a soldered repair shouldalways be regarded as temporary. However, solderingcan produce an effective repair without the use ofexcessive heat, thus avoiding the risk of burningthrough the lead sheet and damaging not only thesheet but the substrate below.Lead welding (traditionally called ‘lead burning’ andnot to be confused with soldering) is a skilled job andshould be carried out by an experienced lead worker.To reduce fire risks, lead welding, or indeed theapplication of any heat on a building, may be bannedon many sites or subject to permits and agreedworking procedures. If the lead is old and has sufferedfrom metal fatigue or crystallisation (see below), leadwelding will not be successful. Hot work at roof level,where permitted, should only be carried out by askilled operative and carefully monitored, as it maycontinue to smoulder. The area of work should becarefully monitored for an extended period of at least24 hours in order to detect as early as possible anoutbreak of fire.When lead is heated it can begin to fume attemperatures above 500°C. Hence, when lead weldingit is essential to wear masks and to only heat lead inwell ventilated areas. In certain conditions lead cancorrode, forming lead carbonate in the form of a88
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A Guide to the Repair of Historic Roofs - Dublin City Council

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