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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSover time as the render or covering materialdeteriorates. Lead soaker flashings dressed into themasonry projection can tear or lift due to the use of anincorrect weight or length of lead. Unless party wallabutments are historically a feature of the original roofdesign, it is better to avoid introducing them, due toproblems of weathering this detail effectively.Overhanging verges are generally constructed oftimber, with a timber or plaster soffit to the undersideof the over-sailing rafters and gable purlins, and insome cases a decorative timber bargeboard. If notmaintained and painted regularly the timber candeteriorate rapidly. It is important, when scarfing orjointing-in replacement sections to damaged lengthsof bargeboard (as with a decorated fascia), to ensurethat the grain is similar and correctly orientated to theoriginal timber for the most effective repair. Whereentire sections are to be replicated, it is important thatthe quality, section size and profile of the timber arecorrect. The patterns for decorative bargeboards arenormally repeating, so sections can be made to matcha known template.EAVESDefects may be caused by slipped or missing gutters,causing water damage to the eaves fascia and soffit orrafter ends. It is important to repair accurately adetailed decorative eaves fascia, or replicate lengthswhere necessary, by using correct timber profiles orsections.FLASHINGSAn abutment flashing is fitted to weather the junctionbetween a roof and a wall, for instance at a chimney orparapet. Where a new or replacement lead flashing isto be fitted, it should be turned into the joint by atleast 25mm, but generally not more than 50mm.Abutment flashings are usually fixed into the jointsusing lead wedges and then the joint is repointed. Thewedges are simply strips of lead sheet folded severaltimes to suit the thickness of the joint.Flashings slip, lift or tear under constant exposure toweather. Wind rapidly undermines loose flashings andquickly dislodges them. Any failure of a flashing on aroof should be addressed as a matter of priority due tothe likelihood of water ingress and damage. In theshort term, the use of a temporary patch repair toprevent water ingress and to safeguard againstdamage to the ends of major structural timbers in theroof should be considered a top priority. Historic rooftimbers, once softened by water, can be prone toinsect infestation or the reactivation of timber rots.Damage may also be caused to internal fabric andfinishes.FINIALS AND DECORATIVE METALWORKThe poor condition of overhanging verges can leadto deterioration of the ends of the purlins as well asthe loss of sections of decorative timber bargeboardsDecorative embellishments of stone, fired clay orironwork on roofs should be regularly inspected tomake sure they are securely attached and are notcracking or corroding. Finials may also function aslightning conductors or as vents. Repairs willsometimes require that the feature be removed fromthe roof and brought to a workshop. A ceramic orstone conservator or specialist metal worker should beconsulted in the case of the respective type ofembellishment. Repairs will have to withstand thesevere weather conditions of their original location.69
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSA new flush eaves gutter formed in lead along thetop of the wallThis terracotta ball finial was cracked; as part of aroof repair project it was removed, carefully cleanedand repaired before re-instatementRainwater goodsDefective rainwater goods can be responsible for largeamounts of water penetrating into historic fabric.Regular inspection and cleaning of the rainwaterdisposal systems of a building are essentialmaintenance tasks.The life expectancy of lead is in excess of 200 years,and as the material will not rust painting is notnecessary. Where the existing rainwater disposalsystem is made of lead, there is often a temptation touse less expensive substitutes for repair orreplacement. Appropriate high-quality alternativematerials to the correct profiles may be justified insome circumstances. If there is a high risk of theft, forexample where the lead is easily accessible or in aremote building, steps should be taken to reduce itsvulnerability.The most commonly used material for rainwatergoods on historic buildings is cast iron. Cast-ironrainwater systems are far superior to many modernalternative materials. However, iron is prone tocorrosion if not maintained effectively. Cast-iron goodsshould be retained, maintained and repaired wherepossible, as the weight and durability of the originalmaterial is better than extruded metal or plasticsystems. Where pieces of guttering or downpipes aremissing or beyond repair, replacement cast-ironsections matching most profiles are readily available.Most substitute materials look different, perform badlyat exposed heights and rapidly twist or slip off the lineof the eaves causing rainwater to overspill. For adviceon the repair of cast iron, see Iron – the repair ofwrought and cast ironwork in this Advice Series.GUTTERSThe condition of eaves gutters can often besuperficially assessed from the ground. However, amore detailed assessment should be made on aregular basis from a ladder or scaffold, to check thatthere are no underlying issues such as cracking to theback of the gutters next to the masonry, and to assessthe condition of the brackets and the joints betweensections of guttering. The base of the gutters shouldbe checked for soundness, as weak organic acids andsalts in a build-up of bird droppings or other corrosivepollutants can attack the metal. The fall of the guttershould be checked to ensure that it directs rainwaterdown towards the downpipes. The fall can changewith time due to distortion in the roof framing ordamage to gutter brackets from the weight of debrisand water. Damage from ladders, wind pressure orfrom falling branches off nearby trees can alsocontribute to the deterioration of rainwater goods.Water sitting in the gutter or a build-up of debris canweaken the gutter supports, leading to breakage inthe gutter and water penetration into the roof or wall.Ideally, the outer edge of the gutter should be lowerthan the inner edge, so water spills away from the wallof the building if blockages occur.70
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A Guide to the Repair of Historic Roofs - Dublin City Council

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