Cold flat roof vents

17 Dec 2008
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United Kingdom
I've seen various comments on this site about the requirement for ventilation in a cold flat roof. Our new flat roof has moved from being a warm roof to a cold one after architect was required to lower the height of roof as it was in excess the actual planning application! As a result the roof height has had to be lowered and insulation put within the beams to compensate for lower height.

Building Control recently refused to OK the new roof as it was now a cold roof and therefore required ventilation. I'm told it will require 3 days of work by builders to construct vents and I am expecting to be asked to pay for this additional work as it will be classed as a building control issue. Should the architect have known though when re-designing the roof that ventilation as a cold roof would be required and therefore cleared with building control the ventilation requirements, rather than just building it with no ventilation and only realising the problem after the roof was complete? I ask as everywhere I look it seems fairly clear that cold flat roofs require ventilation.
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Yes its pretty fundamental he should have informed you & or the builder assuming he was apointed to detail it up/get you building regs. A decent builder should have queried it too really.
No, its not the case.

You can have a cold deck, with the void between the joists filled fully with insulation and it will be acceptable

If you don't have enough space (depth ) to fit sufficient insulation to meet the b/regs, then you will have to put a layer (of celotex) across the bottom of the joists too. But otherwise, timber will not act as a thermal bridge.

The issue with a cold roof is that warm air can potentially condense on the underside. So to deal with this you remove any air voids by ensuring the insulation is tight up against the deck
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I think in this case there is a limited gap for insulation because of the need to reduce roof height but still maintain adequate ceiling height. I would expect all of the void area to be used. Also, there is a lot of glass in the room with folding glass doors about 6m in length and 3 2m x 1m glass roof panels. Building Control was quite clear that the design would require some form of ventilation as designated a cold roof.
The insulation requirements for a flat roof are something like 120mm of Celotex/Kingspan either over/under/in the roof void or combinations of these.

You presumably have at least this amount of space between the joists.

Despite what building control may say or think, a cold or a warm roof are not the only ways to construct a flat roof.

It all hinges on the need prevent condensation (and possible subsequent rot) to the roof. This is the key consideration and the requirement of the building regulations. The regulations are not that you must have either a cold or a warm roof.

If the potential for condensation is removed then the roof construction has to be accepted by building control - as they will not be able to argue to the contrary.
If a cold roof per se has a void between the insulation and the underside of the outer roof deck it should be ventilated. If any moisture gets into the void it needs to be able to escape.
Yes if condensation can form then it needs ventilation to deal with it. But the point is, if the potential for condensation is removed, then the need for ventilation is also removed.

If the OP does not construct a cold roof, then it will not need ventilating
True but I read his original post that he'd had the roof built already and the builders quoted an additional 3 days to rectify.
If already built, then it should be just a case of altering the insulation before plasterboarding?
As Woody, if there is no air to vent, it doesn't need a vent. Pack it with insulation.
I have been in this exact situation before on several occasions and it is down to the BCO at the time, the old methods of constructing a warm roof were to either insulate over the deck or to completely fill the void, some no longer feel that completely filling the void constitutes a warm roof and that warm air can still get up to the underside of the cold roof deck,
Now whilst my personal opinion is that if insulation is coreectly fitted, tight, and even sealed using expanded foam and a membrane, then how can condensation form. Some BCOs however see this as un acceptable, I would stick to your guns,completely fill the void and fit a membrane and if they are not happy ask why and ask for a second opinion.
after all it's all about moisture condensing in the roof space and if moisture can't get in how can it condense
If a vapour barrier is placed below the rafters the moisture shouldn't be able to get above the insulation layer and thus removes the need for ventilation and as woody says, if insulation is tight up against the roof deck then there is no way to ventilate it anyway, there is a lot of confusion depending on who you talk to as what designates a warm roof and it can cause all sorts of problems depending on who has the final say.
This type of roof will not be termed as a warm or cold roof or any of the standard terms, and the nearest description will be a "composite roof".

The principle is that the roof is one single unit - no separate deck, insulation or joists ... just a single combined element.

No aid voids and a vapour check are the essential things, and there can be no argument from the BCO, as the design will satisfy any applicable b/regs

Having seached for further discussion/ideas on Woody's ''composite roof'' idea, which just happens to fit my requirements, does anybody have any further thoughts on fully filling the joist void?

I'm considering applying this to an existing flat roof (140mm deep joists) with a mixture of rock wool and Kingspan and adding a further 50mm or so of Kingspan to the underside of the joists. Obviously I'll install a vapour barrier/foil backed plaster board.

I know this subject has generally been talked to death but I haven't been able to find other similar threads so any comments/thoughts would be greatly appreciated.

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