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70 EXTERNAL ENVELOPE


control also help reduce problems, but this might mean the building is less able to tolerate issues arising from poor installation or damage to prefabricated elements that might occur.


Traditional build, sarking & primary roof covering Traditional masonry buildings will have higher moisture loading from wet trades and a less precise construction process that places greater emphasis on good workmanship on site. They do however have better fabric heat storage characteristics which can help stabilise temperature swings, and the inherently leakier and more permeable construction can reduce the risks associated with trapping moisture, provided this permeability is accounted for in the design process.


The use of sarking materials, such as plywood or OSB, or traditional softwood planks, will have a big impact on the way moisture flows through a loft. Some underlay membrane certification may impose additional requirements or restrictions. It’s not only traditional slating that will affect condensation in lofts. Even using a standard tiled roof, the air openness of the


outer covering must be accounted for, and if the tiles are particularly airtight or if metal roof coverings are specified, additional venting of the batten cavity may be needed. BS5250 and BS5534 – codes of practice for


slating and tiling – outline when coverings can be considered ‘air open’, what measures must be taken if they are not, and what effect wind uplift will have both on the outer covering and the roof underlay.


Complex layouts


The physical layout of the roof has to be accounted for, and in particular, complicated roofs might pose problems when it comes to ensuring all areas are adequately ventilated and there are no ‘dead spots’ where moisture-laden air can accumulate. Subdivisions of roof areas for example for fire compartmentalisation can further exacerbate this issue, as can specific architectural detailing or the overall aesthetic, which may preclude adding additional ventilation.


BS5250 details the requirements for venti- lating roofs and controlling condensation.


Iain Fairnington is technical director of the A. Proctor Group


WWW.ARCHITECTSDATAFILE.CO.UK


ADF OCTOBER 2018


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