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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSDRY ROT: SERPULA LACRYMANSDry rot is less common in roofs than wet rot;nonetheless it can, in some instances, cause seriousproblems. Dry rot requires damp, humid conditions tothrive. The name ‘dry rot’ is misleading as this funguswill survive only where timber moisture content is inexcess of 20 per cent. An advanced outbreak of dry rotis easily distinguished from wet rot in that it producesan abundance of white mycelia – a fluffy mass ofcotton wool-like threads. Mature fruiting bodies arerust red in colour and rust-coloured spore dust onnearby surfaces can be an indication of its presence.Timber that has been the subject of dry rot attack willhave lost its structural integrity and look as if it hasbeen formed in separate cubes with cracks runningacross the grain. This cuboidal cracking is a distinctivecharacteristic of dry rot infected timber.The conservative method of treating dry rotconcentrates on removing the source of water andimproving levels of ventilation to the roof space. This isknown to halt the spread of dry rot and leave thefungus unable to thrive and reproduce. Leakingflashings and gutter linings should be repaired andappropriate pointing or rendering carried out toadjoining masonry. The removal of timber affected bydry rot should be limited to that timber which isstructurally decayed and any small remaining partsthat need to be removed to allow a structural repair.There is no need for wholesale removal of soundtimber in an area of dry rot outbreak, nor is extensivechemical irrigation of masonry adjoining the outbreakalways advisable, due to the toxic nature of someproducts and the need to avoid introducing moremoisture into the fabric of the building. New timbersused in repair works should be pre-treated andisolated from masonry by wrapping the ends in adamp-proof membrane. Local fungicidal treatment ofthe timber immediately surrounding the rot outbreakmay be appropriate to prevent future outbreaks. Thistreatment will need to be of a deep penetratingformulation, or else injected into pre-drilled holes inthe timber. An independent timber treatmentspecialist should be consulted to advise onappropriate action.The fruiting bodies of the serpula lacrymans funguscan be identified by their rust or orange colour,usually with a white edgingINSECT INFESTATIONIn Ireland, there are four types of insect capable ofmaintaining an infestation in building timbers.These are:> Furniture beetle (commonly known as‘woodworm’)> Wood-boring weevil> Death watch beetle> House longhorn beetleThe last two species are extremely rare and have onlybeen found in a handful of buildings in this country.FURNITURE BEETLE OR ‘WOODWORM’(ANOBIUM PUNCTATUM)The furniture beetle can be a serious pest in buildingtimbers. It lays its eggs in crevices in the sapwood ofboth hardwoods and softwoods. The larvae hatchwithin weeks and spend the next few years (betweenone and five years, depending on conditions)burrowing through the sapwood of the timber. Thelarvae then make their way back up to just below thesurface of the timber and form a pupal chamber. Theypupate for about eight weeks. The adult beetlesemerge between late May and August. Active furniturebeetle infestation can be recognised by the presenceof sharp-edged 1 to 2mm diameter emergence holeswith interiors the colour of freshly cut timber and freshwood dust. Particularly in the summer months, deadbeetles 5 to 6mm long can be seen near or on affectedtimbers.51
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSThe burrowing larvae cause damage by eating thecellulose in the timber and leaving the remainingstructure in a weakened honeycombed state. Thefurniture beetle prefers wood that has a moisturecontent of above about 12 per cent, whichunfortunately includes the timber in most roof spaces.Furniture beetle almost exclusively attacks thesapwood rather than the heartwood of any timberelement. Hence in oak construction, and many of theearlier softwood roofs, the extent of damage is limited,as the amount of sapwood left on the timbers wasminimal. The exceptions to this are timber boarding,such as that used in valleys, and roofs constructed ofunwrought timber members, as were sometimes usedto make vernacular thatched roofs. The percentage ofsapwood in these timbers is usually high and theextent of damage caused by the furniture beetle maybe of greater significance.Where furniture beetle damage is found in twentiethcenturyroofs, it is likely to be more extensive and ofgreater structural significance, as the timber used inthese buildings came from managed plantations thatwere harvested after about only 50 years of growthand so contain large amounts of sapwood.A simple rule of thumb is that if there is activefurniture beetle and there is any extent of sapwoodpresent then treatment will be required. Wherepossible, the treatment should be localised to the areaof outbreak. Within roofs the treatment should be inthe form of a spray or brush treatment of all accessiblewormed surfaces with a spirit-based preservativecontaining a contact insecticide. It is possible to killfurniture beetle by heating the entire building fabricslowly to 55˚C whilst maintaining constant humiditythroughout the fabric. However this remedial processis suitable for museum-standard conservation projectsonly.WOOD-BORING WEEVIL (EUOPHRYUM CONFINE/ EUOPHRYUM RUFUM)The wood-boring weevil attacks only timbers thathave been modified by fungal decay and thus it canbe regarded as a secondary pest. It attacks all types oftimber and plywood. The emergence holes are about1mm diameter and the adults are between 3mm and5mm in length. The adults can be found all year roundand can be distinguished from the furniture beetle bytheir cylindrical body shape and elongated head. Theinsects are attracted to light and so quantities of deadweevils on window sills can indicate the presence ofconcealed wet rot decay.DEATH WATCH BEETLE (XESTOBIUMRUFOVILLOSUM)Furniture beetle has damaged only some of thefaces of these timbers – these are the sapwood facesof ceiling joists. The slight shrinkage of the joistswhere they are mortised into the ceiling beam wasnot seen as problematic in this case but, if thedisplacement were any greater, it would needthorough investigationThe death watch beetle is so called because of thesuperstitious belief that the tapping noise theyproduce with their heads was an omen of impendingdeath. The death watch beetle predominantly attacksdecayed oak or other timbers where an infestation inthe oak has already occurred. Because of the lack ofsurviving oak in Irish roofs, the death watch beetle hasbeen identified in only four buildings in Ireland todate, all of which are located in the Dublin area.52
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A Guide to the Repair of Historic Roofs - Dublin City Council

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