we are rebuilding a timber framed extension (not brick) 3.6m x 3m, clad in weather board, roof is obs board covered with 3 layers of felt, 100mm kingspan between joists and have 50mm foil backed board to go under(not up yet), we have a vented soffit board but there is no vent at the highest point/abutment of the roof, so the roof wont vent, can I retro fit mushroom vents? is so do you need 1x per joist space or can I fit 1x every other space with holes between joists?, look forward to any replies thanks
venting cold flat roof?
- Thread starter CHOWDER
- Start date
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sorry just to add there is a 50mm air gap between the kingspan and board
Read more: //www.diynot.com/forums/building/venting-a-cold-flat-roof.371659/#2832893#ixzz2aDq6TKVf
sorry just to add there is a 50mm air gap between the kingspan and board
Read more: //www.diynot.com/forums/building/venting-a-cold-flat-roof.371659/#2832893#ixzz2aDq6TKVf
The rule of thumb is.........
The eaves vent needs to be one thousandth of the whole roof area and the ridge / abutment vent needs to be one and a half times this. This ensures a positive flow of air through the roof space to eradicate the possibility of a build up of warm, moist air within the roof space.
Some ventilation is better than none but if you deviate from the rules you risk the durability of the roof structure.
The eaves vent needs to be one thousandth of the whole roof area and the ridge / abutment vent needs to be one and a half times this. This ensures a positive flow of air through the roof space to eradicate the possibility of a build up of warm, moist air within the roof space.
Some ventilation is better than none but if you deviate from the rules you risk the durability of the roof structure.
Hi thanks for the reply, the felt has already been laid, can I still fit mushroom vents, 4x 1" holes have been cut towards the top of each joist 100 mm aoart and would fit vents every other space, also have taped the kingspan to the joists to give a better vapour barrier, then still have to put up the 50mm therma board underneath , how died that sound!??
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Probably should be ok.
Remember you don't need a vapour barrier in a cold roof.
Remember you don't need a vapour barrier in a cold roof.
No! The vapour barrier is the most important part of a warm roof!!!!! 
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