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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSPROBLEMS WITH SUPPORTING TIMBERSTRUCTURESometimes damage to lead sheeting has been causedby the subsidence or failure of the supporting timbers.This can be identified by areas of water ponding awayfrom the rainwater outlets or by soft spongy areaswhere the gutter or roof deflects noticeablyunderfoot. The timber boards may have deteriorateddue to an ongoing problem with the supportingsubstructure, or a badly functioning rainwater outletcausing water to back up and to overflow the leadlinings. In these instances the design of the leadworkmay not need altering, but the sheets or gutter liningswill have to be lifted where necessary to allow repairbelow. The reuse of the old metal sheeting may bepossible, but frequently the lead in the damaged areawill need replacing. Where the problem has beenattributed to a blocked outlet, the rainwater disposalsystem should be investigated, as it may need repair orlocal redesign. The fitting of an overflow pipe shouldbe considered, as this will give a warning if futureblockages occur.Ponding of water away from gutter outlets indicatesa problem with the substrate timbers. In this case, alikely cause was poorly functioning flashings at thejunction with the parapet wallLEAD CORROSIONAlthough lead is generally highly resistant to corrosionit is not immune to it. The principal environments inwhich lead is likely to corrode, albeit slowly, arediscussed below.Acids, either from atmospheric pollution (possiblygenerated locally from a chimney on the buildingitself ) or from organic growths such as lichens andmoss, can attack lead. Where there are drip points suchas the end of a slate over a lead gutter, corrosionappears either as grooves or circular patterns on thesurface of the lead. After several years this corrosioncan eventually penetrate the lead. Sometimes thecorrosion forms a powder with discolouration on thesurface, which is apparent as an uneven ridged textureif a (gloved) finger is run across the surface. If theproblem is severe, replacement of the lead will berequired; however, the rate of corrosion is usually slow.This type of acidic corrosion can be avoided by fittingsacrificial lead flashings of a lighter code of leadtucked under the slates and held in place with clips (intime they will need to be easily removed). Theseadditional flashings, which take the full force of thedestructive run-off, protect the main flashing below. Ifthe acidic run-off is caused by lichens or mosses,fitting copper strips higher up a pitched roof causes atoxic wash which kills the organic growth andprevents re-growth. Alternatively, a solution can beapplied (which is more suitable to a flat roof ) that killsthe organic growth and prevents re-growth for aconsiderable time. The choice of solution shouldensure that it is suitable for use on roofs and will notcause additional problems, particularly to otheradjoining roof finishes.The alkalis present in lime and cement mortars caninitiate a slow corrosive attack on lead used as adefence against water penetration, for instance wherea lead damp-proof course is placed below a porousstone coping. It is possible to find lead under stonecopings which has been so badly affected it looks likemoth-eaten cloth. To protect against alkali attack, athick coat of spirit-based, cold-applied bitumen paintshould be coated on all surfaces of lead sheets builtinto masonry which are in contact with lime. Anunderlay sheet should also be used, to allow the leadto move freely with temperature changes. Flashingsturned not more than 50mm into mortar beds orchases in masonry do not need bitumen or underlay,as the carbonation of free lime is rapid, minimising therisk of corrosion of the lead.89
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSMany masonry cleaning agents contain hydrochloricor hydrofluoric acids which can corrode unprotectedlead if they are allowed to lie on the surface of thelead.The organic acids present in hardwoods, particularly inoak, can cause the gradual corrosion of lead. Thiscorrosion can be exacerbated in situations wherecondensation forms on the underside of the leadsheeting and takes up acid from the boards. Whereoak or cedar roofing shingles or hardwood externaljoinery (for example sills on roof dormers) are used inconjunction with lead flashings and gutters, asacrificial flashing or coating of bitumen paint willprotect the lead until the natural weathering of theshingles has leached out the free acid.UNDERSIDE LEAD CORROSION (ULC)Underside lead corrosion is perhaps the most insidiousand hazardous form of lead corrosion. In heatedbuildings, warm moist air rising up from inside thebuilding can condense on the cooler underside of leadsheeting where there is insufficient ventilation. Thismay happen more readily on south- and west-facingslopes in comparison to the more shaded northernslopes, as rapidly alternating sunshine and cloud canlead to the distillation of trapped moisture. It can leadto a thin coating of white corrosion deposits formingon the underside of the lead sheeting, which is not aproblem for the long-term performance of the lead.However, when large amounts of condensation occur,a thick layer of white lead carbonate can form, and inextreme conditions, corrosion can penetrate throughthe lead. The usual signs are white streaks running outfrom under lap joints (not to be confused with run-offstains). Lifting the lead (with a gloved hand) will revealthe underside coated with a white powder. This is ahighly toxic material.ULC may be exacerbated by the action of organicacids if the lead is laid on oak boards, manufacturedtimber products such as plywood or an organicunderlay. Some acids can be corrosive when damp. Ifthe lead was laid onto wet boarding and underlayduring construction the problem can be more severe.Although ULC has been known to plumbers forcenturies, in recent times incidences of more severecorrosion are thought to be increasing, possibly as aresult of the increased use of central heating andreduced levels of natural ventilation in buildingstoday. Painting chalk paste (a slurry of chalk powder inwater) on the underside of new lead just before eachSlight lead corrosion can be seen on the exposedunderside of the lifted lead sheeting on the right. Themain problem here was a poorly detailed gutter.There was no need to change the regime of the flatroof, although good practice would include anunderlay between the deal boards and the sheet leadsheet is laid has shown some positive results inlimiting ULC, but it requires two applications and is adifficult operation, particularly on steeply slopingsurfaces.Adapting the roof to what is termed a ‘cold roof’ mayhelp to reduce ULC. This requires an air and vapourcontrol layer to stop water vapour getting into theventilated space under the lead, and also ventilation ofthis space to the outside only. Generally, if thecorrosion problem is extensive and severe,consideration should be given to the redesign of theroof structure to form a ‘ventilated warm roof.’ This is atype of roof structure more successfully used in newconstruction than in conservation, as it requireschanges to the roof profile.90
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A Guide to the Repair of Historic Roofs - Dublin City Council

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