Roof void requirement in warm deck roof construction

7 Aug 2010
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United Kingdom
I am building a warm deck flat roof. I understand that there is a requirement to include a roof void in the construction beneath the insulation layer, tho it need not be ventilated. I cannot find a minimum requirement for the size of the void required - that is, for a cold deck, you require a 50mm ventilated void; for a warm deck, there does not appear to be a minimum requirement. Does anyone know the definitive position?
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The whole point of having a warm roof is that anything beneath the layer of insulation will be within the insulation bubble therefore it will be (or very close to) the same temperature as the inside of the room therefore no ventilation void beneath the insulation is required. That is not say you may not end up with a natural void beneath the structure and the ceiling finish but there is no requirement for a void.

The reason a cold roof has a ventilation void is that because when warm air that seeps out of the insulation there is invariably a gap between it and the roof finish where it can condensate hence the the need for ventilation above the insulation.

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