Many domestic flat roofs fail due to leaks in their coverings and all are destined to fail at some future point but are there other reasons for their failing? Any failure of the deck will cause the coverings to prematurely fail.
Flat roofs are susceptible to damage from condensation forming under or interstitially within their wood based decking boards, in some cases I believe that roof decks have been taken out by the “pumping” * of moisture from outdoor air in through flat roof vents, into the ventilation void of a cold deck roof and condensing on the underside of the decking boards or even interstitially within decking boards themselves. The temperature of the top surface of a flat roof can fall to well below zero even in summer due to something called a “radiation frost” which happens on clear nights when the roof radiates its heat to the clear night sky cooling it dramatically. This mechanism will operate whether or not the ceiling has a vapour barrier and irrespective of the use of the spaces beneath.
A brief explanation of what I mean by ‘pumping’ is in this context – it is easiest to visualise by thinking of condensation streaming down a window pane in winter. The room feels comfortable and warm and condensation forms on the cold glass surface, the water is literally being ‘pumped’ from the air in the room onto the window pane and it will continue to collect on the window until the level of moisture in the air in the room falls to below level necessary for condensation to occur on the window. That is to say the problem will continue until the cooled air nearest to the window is above the dew point. Please note that there is no air movement necessary for this process to take place simply contact with the air between the two places.
We have all seen cats covered in condensation in the morning and some of us may be able to recall seeing dew form on car roofs in the summer on clear nights caused by the same mechanism as radiation frosts. In hot deserts radiation frosts are well known and help erode wet pours stone by repeated freeze thaw cycles.
This note will hopefully trigger a major rethink on the regulations concerning protection and ventilation of flat roofs.
Dr Tony Cowling 2018
