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

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFS2. Roof Structures and MaterialsThe shape and appearance of a traditional roof weredetermined primarily by the properties of thematerials chosen to construct and to clad the roof.However, architectural fashions and styles also playeda role and influenced the final roof form. Otherimportant components of the traditional pitched roof– the ridge, the verge and the eaves – could be builtand assembled in many ways, giving rise to the myriadof different forms found in Irish roofs.Roof profileThe capability of the chosen structural material tospan between supports dictated not only the shape ofthe roof, but also the overall depth and dimensions ofthe building. Traditionally, timber was the mostcommon structural material used in Irish roofs and theshape and proportions of these roofs were based onthe span of the timbers between supports, usuallyload-bearing walls.The pitch, or slope, of a roof was determined both bythe characteristics of the cladding material to be usedand by architectural fashion. Different claddingmaterials are required to be laid at different pitches ifthey are to provide a watertight roof that willcounteract wind pressure and avoid becomingsaturated by rainwater. The roof pitch may range froma steep 60 degrees on some seventeenth-centurybuildings to a shallow 30 degrees or less oneighteenth-century roofs designed to be concealedbehind a parapet, but is commonly between 40 – 45degrees on slate roofs. Clay tile can tolerate being laidat a shallower pitch than slate, as low as 22.5 degreesfor interlocking tiles and pantiles. Where stone flagswere used the pitch of the roof rarely exceeded 40degrees. Corrugated iron can be used at very lowpitches. Timber shingles, of which few examplessurvive, were typically used on steep pitches.The pitch of a roof may also hint at its historicalorigins; for example, a steeply pitched corrugatedmetalroof on a vernacular house suggests that theroof was originally thatched, as thatch must be laid atPurlinValley RafterCommon RafterHip RafterCrown RafterCripple RafterChimney TrimmerRidge BoardGable LadderCeiling JoistJack RafterWallplateTerms used to describe the elements of the roof13
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSa steep pitch to prevent rainwater soaking into it. Inmany cases, a thatched roof may still be foundbeneath the metal covering. Similarly, the position ofstructural elements such as chimneystacks can provideclues to the date of a structure and to the internallayout of the building. Exposed chimneybreastsprojecting from the gable ends of a house mayindicate an English influence in the design, while achimneystack in line with the main entrance of avernacular dwelling denotes a central hearthprotected by a jamb wall.Pitched roofs splayed in a curve over a projecting bowwere usually clad in slate but, where the geometryprecluded the use of slate, lead or cut stone wereoften used. Domes and cupolas, with their steep andcurving roofs, were usually clad in copper or leadsheet which could be easily moulded to follow theshape of the roof.Traditional roof formsDOUBLE-PITCHED ROOFThe most common form of traditional roof is thedouble-pitched or ‘A-framed’ roof. The rafter pairs wereusually tied with horizontal tie beams. Ceiling finishes,if applied at all, were fixed directly to the underside ofthe rafters and tie beams.Although a simple double pitch is the most commonform of roof found, roofs may vary from a single, ormono, pitch to more complex arrangements. In theeighteenth century, the desire to reduce the visualprominence of the roof led to the use of two doublepitchedroofs, each with a shallow pitch, forming adouble ‘A’ profile with a central valley, thus reducingthe overall span and the height of the roof.Some typical roof formsRoof with concealedcentral valleyDouble ‘A’ roof withcentral valleyMansard roofHipped roof withmono-pitch to the rearRoof with gable endsand bow frontGambrel roofRoof with gable endsGothic-style roofCurved roofHipped roof withgabled entranceHalf-hipped roofMono-pitch roof14
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A Guide to the Repair of Historic Roofs - Dublin City Council

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