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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSSNOW BOARDS AND ACCESS BOARDSSnow or access boards are sometimes found in hiddengutters. These running boards are laid in both valleyand parapet gutters and are like gangways orduckboards on a boat. They are typically constructedof good quality softwood as hardwood tends to beslippery when wet. Modern materials may providealternative, longer-lasting solutions depending on thesituation. The primary function of snow and accessboards is to allow blown debris to remain on top ofthe boards whilst not impeding the water flowingunderneath, but they also afford a level of protectionto the roof covering, preventing puncturing frompedestrian traffic.After heavy falls of snow, when the depth of snow islikely to exceed the height of the sides of the gutterlinings, snow boards allow the melt water to flow awayfreely beneath the accumulated snow. In most areas ofIreland, a properly detailed tapered valley or parapetgutter will rarely experience problematic snow andmeltwater levels. However, where snow conditions areknown to be severe, access is difficult or the buildingfabric or contents are particularly valuable, proprietaryelectric heating tapes laid in the gutters, catch pits andhopper heads can be considered as an alternative, oran addition, to snow boards.Snow and access boards need to be maintained asthey can frequently trap debris, and if allowed todeteriorate can end up blocking the gutters andrainwater outlets they were meant to protect. It shouldbe noted that they can constitute a significant trip orfall hazard at roof level, particularly when they are inpoor condition.RIDGES AND HIPSA missing ridge cap allows the penetration of waterinto the roof at a very vulnerable point. Replacementsare generally available for historic clay ridge tiles,though care should be taken, as the angle of eachridge cap may vary and the colour or glazing maydiffer. Where there are no equivalent modern versionsin production, some companies may be able to matchunusual profiles. Clay ridge tiles should be bedded inlime mortar and a consistent ridge line formed bytight, well pointed-up joints. The ridge and hip tilesshould not be fitted with wide joints in order to avoidmaking up a shortfall of salvaged tiles. This is a falseeconomy as it results in fat, highly visible and morevulnerable mortar joints.Incorrect bedding and detailing, with fat joints to theridge and hip tiles, are visually very disruptive andinappropriate to a historic buildingOn hipped roofs, to reduce the impact of the weight ofthe clay hip tiles and to prevent them slipping downthe roof, a decorative wrought-iron strap known as ahip iron was sometimes fixed to the top of the hiprafter to support the lowest tile. Less decorative strapswere concealed in the bedding for the ridge stone orhip tile at eaves level. Where the irons have corroded,the hip ridge tiles can start to move, creating gaps andallowing water penetration. This movement eventuallyresults in the hip tile falling off the roof. Pig tail hipirons (so called because of their decorative shape) arecommonly available but should not be used for repairsunless there is a historic precedent for the design. Aplain, buried fixing should be used instead. In mostcases, provided the hip and ridge tiles have not beenpreviously fixed with concrete, it should be possible toremove the tiles without damage to the underlyingslate. The hip irons can be fixed directly to the rafterprovided that the underlying timber is in soundcondition. Where a lead ridge or hip roll has slipped ortorn, the replacement should follow the detailillustrated here.Additionalfixings whichoccur underthe leadThe Lead Sheet Association advises a modification tothe profile of the ‘broom handle’ timber roll andpromotes instead the use of a more elongatedsection in treated softwood to reduce the likelihoodof the pinched lead cracking along the roll67
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSNote that this does not apply to the installation of theundercloak slate. The slate overhang at the verge shouldnot be over-long as, in an exposed location, wind can liftthe last row of slates. Wind uplift wears the nail holefixings, eventually resulting in slate slippage.In many areas, particularly those noted for high winds,a stone gable coping was used, set in mortar and ontop of the roofing material. A more modern version ofthis detail is a lime- or cement-based gable coping,cast in situ.Interleaved soakers: a variation of the external leadroll is the cloaked version seen on hips of roofs, whichconceals much of the leadwork below the slatecourses. In this detail, the lead flaps are interleavedwith the slate on the ridge or hip. If this detailrequires repair or replacement, local re-slating of theroof will be necessary to access and replace thedamaged leadworkGABLES, VERGES, COPINGS AND ABUTMENTSThe slate covering at a flush gable was traditionallyfitted tightly on top of the wall by bedding the outerslates in a lime mortar. As this detail is vulnerable tosevere weather, the use of lime mortar for repairs is ofparticular importance to ensure that there is sufficienttolerance for the slates to move. To preserve historicdetails and character, the salvage of large slate sizes toachieve the ‘slate and a half slate’ detail at the gableverges is an important aspect of any re-roofing process.By virtue of their size and weight, these large slates,bedded in lime and held by the adjoining slates, tend tooutperform details comprising smaller slates. Where‘slate and a half’ sizes are not available and a half slate isused at the gable verge, it should be fixed twice toprevent wind lift at the edges. It is good practice, whenlaying slate to a closed verge, to ensure that there is aslight fall back in the last slate towards the roof to directthe water away from the edge of the verge. This can beformed by packing out the lime bedding below.A gable detail where the verge is held or ‘locked’ inplace with a stone coping. A particular difficultyarises with this detail if there is damage to slates setbelow the coping, as the coping will need to beremoved to allow for repair of the roof coveringWhen a gable coping is set directly onto a corrugatediron roof, problems may arise with corrosion of thecorrugated sheet in contact with the lime or cementproducts. A protective layer of priming paint canreduce the likelihood of corrosion where the sheet isin contact with these products.Where a gable or party wall is raised over the roofslope it is known as a roof abutment or a gableparapet. These parapet walls, having exposed faces onboth sides, are vulnerable to extremes of weather, andthe masonry can become susceptible to water ingress68
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A Guide to the Repair of Historic Roofs - Dublin City Council

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