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

ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSmore elaborate arrangements includinghammerbeams. This form of roof constructioncontinued into the eighteenth century and was alsofrequently used in Gothic Revival buildings of thenineteenth century.A truss is the name given to a rigid frame of rafters,collar beams and struts capable of spanning largedistances without intermediate support. Historically,there are many different types of trusses, each with itsown name depending on the disposition of thetimbers within the framework, such as king post,queen post, crown post and the like.By the last quarter of the seventeenth century, thetimber roof truss had come into general use, albeitwith historical and regional variations, for the moresubstantial houses, churches and public halls, andremains in use today. Trusses allowed for theconstruction of roofs with larger spans and differingroof pitches, which impacted upon building design inseveral ways. Internally, the use of trusses allowed theroom layout to become more flexible, as fewer internalsupporting walls were required. The use of trussesenabled the construction of lower-pitched roofs, suchas those favoured by the Neo-Classical style ofarchitecture. Roof frames and trusses were frequentlymade off-site, brought up to the roof in pieces, andassembled in situ as it was easier to lift individualtimbers up onto a roof than whole frames. Assemblymarks (sometimes known as carpenter’s marks)ensured that the correct pieces were matched to eachother.THE BELFAST TRUSSThe Belfast truss is the name given to a light, timbertruss formed of timber latticework sandwichedbetween doubled top and bottom timber members,the top member being curved. It is also known as abowstring truss. The timber used for these trusses wasgenerally pine and the trusses were overlaid withtimber decking and various forms of roof sheeting.This form of truss was believed to have been inventedin Belfast in the late-nineteenth century to constructthe roofs of boat sheds. However, earlier exampleshave been discovered, dating from the 1850s, whichwere used by the Malcolmson family to constructeconomical domestic roofs for the houses of millworkers at Portlaw, County Waterford. At Portlaw, theboarding of these curved roofs was covered with aThe ‘Portlaw’ roof was also used further afield forhousing schemes, such as here in Harold’s Cross,Dublin; also in Clara, County Offaly; Blarney, CountyCork; and Carrick-on-Suir, County Tipperary. At onepoint, these trusses were exported to Germanythick woven cotton material called calico, made in thelocal mill. The calico was then waterproofed with a tarcoating, a by-product of the mill owners’ gas works.While there are very few intact surviving examples of acomplete roofing system, these trusses, where found,are an important historical and architectural legacyworthy of conservation.THE USE OF IRON IN ROOFSWrought iron was an integral part of the constructionof timber roof trusses, providing the strapping andfixings required to hang the ceiling tie from the trussand also to secure the rafter/collar tie connections.A later development was the use of wrought ironinstead of timber as the tie beam, particularly inbuildings such as stables and mills that were open tothe underside of the roof. By the early nineteenthcentury, cast- and wrought-iron roof beams andtrusses became commonly used in industrial andpublic buildings. Cast and wrought iron can also befound in the roof structures of features such asconservatories, orangeries and rooflights. Steelgradually replaced cast and wrought iron from theend of the nineteenth century.19