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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSHOUSE LONGHORN BEETLE (HYLOTRUPESBAJULUS)The house longhorn beetle predominantly attackstimbers with a high moisture content, usually green orunseasoned timber. Few outbreaks have beenassociated with seasoned timber within buildings andthese infestations were usually associated with timberrot. To date there has been one known Irish case ofhouse longhorn beetle infestation, discovered in thetimbers of a building in County Antrim.Treatments of timberTimber treatments involve the application ofchemicals and insecticides. If used indiscriminatelythey may kill off many of the natural predators offurniture beetles and weevils, such as spiders, andadversely affect bats, which are a protected species.Excessive use of such chemicals may also have anadverse impact on the health of the occupants ofthe building.It is important to get expert advice when anoutbreak of fungus or insect attack is discovered ina building. The advice should be independent,preferably from someone who is not trying to sell aparticular product or treatment method. Anytreatment of a fungal attack should include adetailed examination of the building to locate anddeal with all sources of water penetration. Moderndry rot treatments recognise that these fungi areliving organisms that will die off naturally, as anyplant would, if the environmental conditions thatsupport them are removed. Removing the source ofwater and supplying good ventilation will kill offthe fungus. Keeping the building dry will prevent itre-establishing.ASSESSING THE EXTENT OF TIMBERDAMAGEThe extent of timber damage caused by fungal orinsect infestation can be initially assessed using simpletools such as a bradawl or sharp screwdriver pushedhard into the damaged timber by hand. The extent ofunsound timber can be easily judged as the depth thespike enters before resisting moderate hand pressure.Where damage is extensive, or where there are largestructural timber members present, the extent ofdamage will need to be assessed by an independentspecialist who can make accurate assessments of theextent of damage to all the timbers, as well as usingmodern techniques such as microbore drills to checkdeep into larger timber members, particularly at theirbearings. If the extent of damage is severe, then astructural engineer with experience in the repair oftraditional structures will be necessary to design andspecify the strengthening works required.DEFRASSINGDefrassing is a term used to describe the removalof degraded material from timber that is badlydamaged by insect or fungal infestation. Thisprocess was promoted in the latter part of thetwentieth century as a method of assessing theamount of timber of sound structural integrityremaining. Defrassing should not be used onhistoric timbers. The process removes all historictimber marks such as adze marks and thepossibility of accurate dating from growth rings isreduced. Localised removal of damaged timber toallow a structural repair to good sound timber is ofcourse acceptable.An owner or custodian of a historic building shouldbe very cautious about proposed treatments thatrequire the removal of large amounts oftimberwork within a certain radius of the fungalinfection. It should only be necessary to removetimber which impedes the works necessary tostabilise the structure or to reinstate finishes.53
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSProblems arising from previousinappropriate repairsThis series of drawings describes how local repairs toa slate roof can be carried out:The external roof slopes may have been painted with abituminous liquid, either on its own or with a hessianbase, as a ‘quick fix’ to resolve slate slippage and leaks.These types of repair essentially stick the slatestogether and make it impossible to carry out localrepairs. If this treatment, known as ‘turnerising’, hasbeen applied to a roof and there are further leaks,complete replacement of the slates is the only option.Furthermore, slates subjected to turnerising are not fitto be salvaged and reused.The ‘bib’ repair, which is used to re-affix a slipped slateThe process of covering a leaking slate roof with abitumen covering was known as ‘turnerising.’ It isineffectual and damaging and should never becarried out on a historic roof. Furthermore, it preventsthe slates ever being salvaged for reuseThe pitch coating, used on its own, melts andgravitates toward the lower part of the roof. If hessianis used with the bitumen it is more stable on the roofthan the liquid coating on its own but nonetheless hasa negative effect. Furthermore, such treatments causegreater heat and humidity to build up in the newlysealed roof spaces. The ability of the roof to ventilateproperly becomes compromised and this allowshigher levels of condensation to build up. This willmake the roof timbers more vulnerable to insect andfungal infestation, particularly in buildings with highoccupancy levels, as the moisture content of thetrapped air in the attic interior will be higher.Here, a partially folded metal piece is slid under theupper slate as part of the bib repair. The metal ispartially folded to allow the metal to sit above the nailhead. The metal used should preferably be a paintedaluminium or copper but should be matt in textureThe slipped slate (in yellow) and the areas most likelyas a result to spring a leak (in blue): the nail holesand the top of the joint54
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A Guide to the Repair of Historic Roofs - Dublin City Council

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