How durable are the membrane system ?
 In normal conditions found in roofs, the membranes are durable and can be expected to have a life comparable to that of traditional roof tile underlays. Specifiers should however follow the requirements of the relevant Certificate relating to the period for which the underlay can be exposed to the elements before slating/tiling and on the use of eaves guards for open eaves constructions.
With regard to water vapour permeability, the BBA assessment included measurements of the effect of heat ageing, effect of UV light exposure and of accelerated dust/debris accumulation and found no discernible reduction in performance. Do I need a vapour check ceiling ?
 No but it can help. It is important to remember that vapour will pass through the path of least resistance in a ceiling. Gaps and cracks will provide an effective ‘short-circuit’ however high the vapour resistance of the rest of the ceiling is.  What do I do about sources of moisture in the loft space
 Water tanks should have lids on and vent pipes arranged to avoid venting moisture into the loft space. Do I need counter battens ?
 In general, counter battens are not required if the underlay is draped between rafters, ie a nominal 10 mm. If however it is not draped but pulled taut, counter battens are applied. Readers should refer to specific Certificates for the holder’s detailed installation instructions. Can you have unventilated loft spaces when timber sarking is used ?
 Yes, with the traditional Scottish practice of planks nominally 100 mm wide with a 2 mm gap either side. Their use with large OSB, chipboard or plywood timber panels, 2440 mm by 1220 mm, has not been approved by the BBA.

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