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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFS5. Repairing a Historic RoofTimber structural elementsRepairs to a historic roof should retain its structuralintegrity and historic significance. The aim of repairshould be to minimise the intervention in order to retainas much as possible of the historic fabric of the roof. Theuse of traditional materials, techniques and carpentrydetails is generally preferable, as these keep the integrityof the historical detail and do not detract from thecharacter of the building. Simple repairs such asinserting a new timber alongside a damaged rafter orceiling joist, or splicing in new timber pieces, can beeasily carried out using readily available materials andcarpentry skills. However, it should be borne in mind thattraditional repairs such as the scarf joint have limitedstructural performance and require skilled craft labour toimplement properly. If original material must beremoved to make the repair, then the extent of fabric tobe removed should be guided by a skilled practitioner.Traditional carpentry repairs should be properly labelledor recorded to alert future generations to the date of therepair. Introducing a new structural member to reinforcethe existing historic structure, or even take over thestructural function of the original roof timbers, shouldnot necessarily require the removal of the failed originaltimbers.Repairs to existing structures employing contemporarymaterials, techniques and details should respect thesignificance of the historic timber structure and notcause damage to it. The repairs should not impedefuture research into the roof nor conceal significantdetails or features.The use of a traditional scarf joint repair to damagedrafter ends is particularly worthwhile where the roofstructure is exposed to view (Image courtesy of theIrish Georgian Society)Minimum 6 no. 5 mmdiameter woodscrewsapprox. 80 mm long atstaggered locations.Pre-drill the newtimber before placingit against the existingMinimum 1 metreMinimum 1 metreoverlap to soundoriginal timber butpreferably extendto next supportwhere possiblePlace new timber a min.44 mm thick: depth tomatch existing rafter.Timber to be treatedExtent of rot to original rafter.Only very friable or loose timbershould be removedWhere rafter end are rotten the wall plate is also likelyto be rotten and will also need to be replaced locallyRafter splice detail – for typical rafters up to 125 mm in depthSimilar details can be used for ceiling joists up to approx.3 metre span. When sizes and spans get larger than this ormore than 30% of the rafter ends are rotted, a structuralengineer should be consultedThis drawing shows how a new rafter end can bejoined alongside the existing one. This repair does notrequire removal of the existing timber, but such repairsshould try to avoid obscuring important historicalinformation such as assembly or merchant marksCHOOSING THE RIGHT TIMBER FOR REPAIRSThe choice of timber to be used for repairs is of criticalimportance. A match in timber type should be obtained,for instance oak for oak and red deal for red deal, so thatthe physical performance and structural capabilities ofthe timber are compatible. It is equally important tokeep the use of sapwood to a minimum as it is moresusceptible to insect infestation. Sections of timber needto be carefully selected, and may even need to be cutfrom larger baulks of timber and trimmed down to sizeto remove all sapwood. As new timber is often placed inan area previously affected by timber decay, fungicidalor insecticidal pre-treatment of the timber should alwaysbe considered and only omitted if there is a good reasonfor doing so. New timber should not be placed in directcontact with damp masonry, but should be isolated fromit, either by supporting the timber on new brackets awayfrom the wall or by placing a damp-proof layer betweenthe timber and masonry.59
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSOTHER REPAIR MATERIALS AND TECHNIQUESAdded support has been given to the main hip rooftruss at an internal valley. This repair detail usedprefabricated steel elements and tooth plateconnectors, but avoided the need to open up therecently repaired valley finishes and does not obscurethe original truss detailsWhere necessary, more extensive repairs can be madeusing modern mechanical fasteners such as toothplate connectors, split rings and steel brackets. Thesecan produce a detail whose function is readilyapparent and calculable by current engineeringstandards and which is theoretically reversible.Introducing a new structural member to reinforce theexisting historic structure or even supersede it is oftena viable option and can usually be designed to avoidthe removal of the failed original timber.The use of other more specialist repair techniques,such as resin-fixing steel or carbon fibre plates toexisting timbers, or fixing plates or tensile rods withinthe timber, requires specialist knowledge andexpertise in their design and specification. These repairtechniques are not reversible and their long-termperformance is as yet unknown. However, in certaininstances one or other of these methods may allowmore retention of historic fabric and be visually moreacceptable than any other available method. The useof these techniques in a historic roof will require theadvice of a structural engineer experienced in therepair of traditional buildings.Two consecutive corrective structures to provideadditional support to undersized ceiling joists. Thefirst intervention (upper left) occurred early in theroof’s history and consisted of the addition of atimber beam and timber hangers to give extrasupport to the ceiling joists. The new beam itself thendeflected. Further corrective measures were takenusing a new steel beam (lower right). Note that alloriginal timbers were retained intact and the historyof the roof and successive repairs can easily be readFerrous metal roof structures should be regularlychecked for corrosion, particularly at supports andwhere built into potentially damp masonry. Wherethere is significant corrosion of the ironwork, themasonry is likely to crack, as the products of ironcorrosion (rust) are approximately eight times thevolume of the iron itself60
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A Guide to the Repair of Historic Roofs - Dublin City Council

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