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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSEAVES SLATING AND UNDERCLOAKSThe characteristic large slate course at eaves levelshould always be carefully salvaged for reuse as theloss of this detail by re-slating the roof in one regularsizedslate dilutes the historic character of the roof.While the industry recognises this gap in the marketand can provide replacement large eaves slates, theseare not routinely imported into Ireland at present.Therefore an owner may be dependent on salvagedslate to complete any shortfall.Damaged large recycled slates can be cut down andredressed for use as undercloaks. The practice of usingman-made fibre-cement tiles as an undercloak shouldnever be followed, as artificial products are visuallydifferent and age more quickly. They become fibrousover time, leading to water retention and eventuallywater penetration to the top of the wall. In addition,the colour of the fibre-cement washes out over time,giving a pale line at eaves level.UndercloakDiagram indicating the position of the undercloakslate. The undercloak is the only slate on the roof thatis face-bedded (i.e. laid facing downwards) so that itsdressed edge acts like a drip into the gutterPARGING, SARKING AND FELTThe repair of parging may be impractical due torestricted attic space. However, local repairs should beattempted if the roof parging is generally intact. If theparging has become detached and fallen, it should beremoved from between the ceiling joists, as it maycause detrimental loading to the ceilings below.Sarking was the cladding of the roof rafters with dealboards on which battens and counter battens werethen fixed. It may require attention if moisture hasSubstantially intact parging to the underside of slatesis visible from within the attic space and provides asecondary line of defence against rain and windentering the interior of the buildingpenetrated the roof. Historically, sarking wascomposed of high-quality deal; hence, joinerytechniques of timber repair can be used where boardends have decayed. However, where there iswidespread saturation and damage, sarking should bereplaced. White deal is usually specified for internalwork; however where there is a possibility of rainpenetrating the cladding, a higher quality deal shouldbe specified in preference and appropriately treatedafter cutting and prior to fitting.From the 1950s onwards, the use of roofing felt as anadditional barrier below the slate covering was afurther development of sarking boards or limeparging. The use of felt may give rise to a build-up ofmoisture levels and the possibility of condensationforming within a roof space, unless adequateventilation is provided. Unlike breathable membranes,the use of felt adversely affects the ability of a timberroof to regulate and release moisture build–up. Rooftimbers may eventually become vulnerable to timberrot if left in warm, moist, poorly ventilated conditions.REPAIR OF VERTICALLY HUNG SLATINGGenerally, taking out only the damaged slate is asuccessful method of repairing individual slipped,vertically hung slates. However, if the mortar has beendirectly applied onto a rubble stone wall, obtaining anew bed for the replacement slate without looseningthe surrounding slates is difficult, particularly wherethere is a large headlap. It is important to remove theremains of the lime bed of the slipped or broken slateso that the replacement slate can be fitted tight to the81
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSadjoining slates, as vertically hung slating is typicallyfound in areas subject to high levels of wind suction.The careful use of a slate ripper may be appropriate,allowing a new slate to be slipped under the stillintactslate. A proprietary stainless-steel fixing clip orhanger, to take a new slate, may be installed into thejoint between the headlap of the lower slates. A limemortar can be buttered onto the back of the slate andthe slate pressed back into position at the head andclipped at the tail. Any extraneous lime mortar shouldbe removed so it is not visible at the edges or on thefaces of adjoining slates.On this ruinous house in County Cork, the verticallyhung slates, bedded in lime mortar, were laterrendered over. Their existence only came to light asthe external render decayed and fell offIf a large area of vertically hung slating becomesdefective, then all the rows of defective slate should beremoved back to the lime bedding, so that the slateand the lime bedding can be repaired in unison. Thetraditional method of application should be used toapply the replacement slates. The mortar mix forreinstatement needs to be a very rich mix of 2:1(sand:lime) or 2½:1 (sand:lime using a naturallyhydraulic lime, NHL 3.5 or NHL 2), or a pure lime putty.This is a sticky mix and helps in the initial adherence ofthe slate. As the mortar is fully protected by the slate,the very high lime content and potentially lowstrength of the mortar is not an issue.Vertically hung slating should not be repaired withmastic spread over the back of the slate, orcementitious mortar, or any combination of the two.Repairs using the wrong materials can make the slateand the lime backing unusable, potentially wideningthe areas of failure and rendering good slatesunsalvageable.The change in colour of the slate on thechimneystack in the background indicates where anumber of courses were removed and replaced. Inareas of extreme weather exposure, vertically hungslating gives increased protection to the stacksREPAIR OF A SLATE-HUNG ELEVATIONWholesale replacement of vertically hung slating canalter the appearance of a building, and planningpermission may be required before such works may beundertaken. Where there are patterned courses ofslate or specialised detailing, these should bereplicated, and particular care should be taken to saveand reuse any shaped slates.Replacement vertically hung slating should be carriedout in the traditional manner, fixing slate using limeputty mixed with sand applied onto the masonrywalling. Where new fixings are required for the rehangingof vertical slates, opinion is divided on theuse of stainless steel nails or clipped fixing over a largearea. Modern mild steel nails rust too quickly, but astainless steel proprietary fixing, though resistant tocorrosion, does not bond with the mortar in the waythat a traditional wrought iron nail would. This is dueto the initial rust skin on the wrought-iron nail whichbonds with the mortar. Wrought-iron nails can still beobtained through specialist suppliers to the marinetrade. They will eventually corrode, but over aconsiderable time. The traditional lime-mortar bed andthe snug fitting of the head lap of adjoining slates82
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A Guide to the Repair of Historic Roofs - Dublin City Council

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