Warm flat roof two layers of insulation will it work? 

Now thats how to answer a question ! 👍 good job mate

Great job, you know more than some architects and engineers I know. 👍

The warm roof generates so many questions and so few definitive answers. As a layman with a keen interest in DIY, I removed my leaking flat roof - a cold roof - two years ago and began rebuilding it from ceiling level as a warm roof. I have just closed it back up with tarpaulin for a second winter, having failed to think of any solution for the problem of water vapour rising from below and condensing into the insulation. Damp insulation (like damp clothes) doesn’t work and it eventually deteriorates. The prospect is made worse by regulations that disallow a warm roof to be dried out by being ventilated. The answer would appear to be to stop water vapour from getting up into the roof in the first place by opening a window. However, in the UK, changing the air, whether in the room below or in the roof above lets out the heat that is keeping you comfortable. If you turn the heating up, the final objective is defeated, i.e. to stop global warming - in which water vapour is strongly implicated. The current regulations (probably originating in the EU) seem to me to be a politically correct compromise, but entirely impractical. They invoke an imaginary process whereby water vapour evaporates but slowly from your roof structure but does no damage while it is there. Most weather proofing, especially EPDM traps vapour beneath it, hence the delamination noted by Steve Roofer in one of his videos. I asked Celotex, the manufacturer of my insulation and their reply was to provide a timber surface on top of the insulation for the EPDM to be stuck down onto. In view of this, I have bought OSB deck board for on top of the joists and 5mm plywood for an upper surface. The main problem then is building your warm roof so as to be thoroughly sealed against hot moist air from below. The problems in building a warm roof are so complex, I am thinking of redesigning my rebuild as a cold roof, ventilated to code and dry as old toast.

My goodness, roofing technology and construction looks to be something of a real science nowadays, construction of roofs was far simpler years ago, the first Victorian houses built in London never had vapour barriers but they’re still going strong!

Question, can you use 50mm on top of joists and 70mm between joists, as sometimes the height is big issue, and cold roof with ventilation isn't an option.

Hiya I'm loving your videos and expertise. I would love some information on a recent build we had done with a flat concrete roof living space with no ventilation or vapour barriers. We are having condensation build up and need some guidance please.

Hi Steve, really good explanation, thank you I have a question please… I am converting a barn with a sloping tin roof into a craft room and live in France so experience huge temp variations I can’t take the existing roof off so want to insulate from the inside - there are a couple of beams holding the roof up which are solid but they are spaced at 5’ so was going to add some extra across struts (to reduce the span of 5’ for extra places to nail the plasterboard) I was thinking after watching lots of videos on here; Vapour barrier against the tin roof (which is in great condition) Then insulation, sealed (builders merchants here have said to use the thin multi layer foil for R value Then more insulation - rock wool Plasterboard to finish I don’t know where my air gap should be - between layer 1 & 2 or 2 & 3 or both I do know I need an escape route for condensation and I have sealed around the gaps in the tin profile with expanding foam Any suggestions please, thank you kindly

IF BM TRADA recommends no timber materials on top does that rule out GRP or liquid roof coverings ?

Doesn't silver foil tape enhance the whole when piecing insulation boards together?

Love your vids, but I'm not quite sure sure I understand how tissue faced insulation helps in this case? The EPDM membrane is fully vapor resistant so any rising moisture has nowhere to go, unless it's vented, like a cold roof. A warm roof needs to be fully vapor sealed below the insulation like you said. As I understand it: cold roof = vapor open construction, vented above insulation, so rising vapor/moisture can get out. warm roof = vapor closed construction, fully sealed on both sides of the insulation so rising vapor/moisture can't get in. Does that make sense?

Interesting as ever! Where you have the Plywood/OSB on to stiffen it, I was wondering why it isn't done in a plastic of some kind. You could design it so it is strong yet allows any water to get out via gravity via a tray. You would think plastic would be a lot cheaper than Plywood too these days.

Multiple layers prevent thermal bridging and leakage Steve?! I see the issue with foil facing though yeah.....

I have a 1960s house with flat roof in south buckinghamshire with basically zero insulation. Was wanting to to upgrade it next couple of years. At the moment there is burnt on felt. Can I use that as the vapour layer? I thought sticking kingspan tr27 to that and then putting resitrix directly onto that would be sufficient. Or a roofer who fixed another problem for me suggested to put 120/150 insulation battened in onto the felt and topped with OSB3 and then EPDM onto that.

still confused....so multi layers are OK but not foil backed? Would that not go for single layers too? And at one point you said not to put ply on top?!?!?

If you don't have any board above the vapour barrier and below the waterproof layer, what do you do when you need a hard surface on the top?

Shouldn't that shiny vapour barrier be on the underside of the ply board?

Hi Steve. Very informative video, as are the others you have done. Quick question; if Im creating a warm roof should I not put a layer of ply or OSB placed ON TOP of the insulation? ie just adhering the EPDM direct to the insulation? Thanks

My understanding is the vapour barriers are not the best, and you should go for a more moisture open vapour control layer. Because let's face it, moisture will get up past the insulation, and it can't escape through the waterproofing layer. Also, if you use a PIR without a foil face but fibre reinforced you can fix some waterproofing membranes straight to it, so no need for a timber deck

Steve, having watched (nearly) all of your videos to help me understand and choose the best solution for my flat roof, there is one thing I cant find an answer for. I have gone for a warm roof as the best solution and, as a carpenter, the roofer is happy for me to do all the fascia boards around the perimeter. Having had a look behind the existing old ones, there is no insulation at the joist ends where they sit on the cavity walls. Should I be sealing the perimeter with insulation before adding the Ply and then fascia boards (I have decided on HPL as is very common here in the Netherlands). If so, should it be of the Rockwool type or solid foam insulation sealed tight all round? Many thanks

Steve my question is would you leave celotex 100mm insualtion in between the rafters and build a warm room over the rafters in the usual process where in you put 18mm board ,vapour barrier then 120mm celotex then 18mm osb board and finally the epdm rubber on the top would this be right way to do it leaving celotex between the ceiling rafters or should we remove it ?

Can I use 120mm celotex boards for my flat roof instead of 120mm kingspan ?

Hi Steve I was wanting to put osb or ply over the insulation Just in case I ever need to put ladders on the roof To clear gutters etc that are on top the single storey extension ! What would you suggest ? Pls

I cannot find tissue faced insulation thick enough to achieve a U-Value of 0.15 which is the requirement for Wales, any ideas steve?

Unfortunately I didn’t get my answer from watching this video. I am from Greece and I am building a warm roof with xps insulation, the plan was to get a 60mm isulation but the factory only produces 30mm insulation because there is no demand for the other. So I’ve been told to use two layers of 30mm. But the thing is xps isolation doesn’t come with a vapor barrier, in fact the vapor resistance is 50-150 μ. My actual vapor barrier that is glued under the insulation is 1500 μ. Can I use two layers of insulation with this built up?

Is there a certain temperature at which water vapour condenses into water on a flat roof?

Hi Steve! I understood the roofing through your videos. Thank you! What I am not clear on is what goes inside, under a warm roof? We are planning to build an A frame house with a warm roof. But how come the first osb layer (close to the ceiling) is not going to get affected by the vapors in the house?

is steve just promoting a vapour barrier? tissue lined insulation must be a new thing, every video Ive seen he used foil back? Allowing moisture past the vapour barrier?

Hello Wonder if anyone can help,I'm building a block man cave ..my roof joist are 7by2 but my insulation is 60mm recticle..Will that be ok having a 5 inch gap in my joists? I'm doing a warm roof soo will osb and use another 60mm above the vapour barrier and then osb again and a rubber roof to complete it. I'm just not sure if that gap is ok in the rafters... Any input will be greatly appreciated Thank you

Very useful information! I have a quick question I was hoping you could help with, could you use insulated plasterboard under a warm roof?

Maybe a stupid question but have to ask.. A builder has suggested I do away with the parapets about yo be built on my new build extension. He suggests a facia and soffit all the way around. Are parapets assential for a warm roof? Sorry if it seems a simple question.

HI please can you help how can I turn my cold roof into a warm roof thanks what would be the layers I would need to turn my cold roof into a warm roof thanks , waiting for your response and I have subscribed and liked your video thanks

not directly about this video, but would it be possible to use 11mm osb for either both decks or the top deck on a warm roof as im using 150mm celotex which was picked at a seconds warehouse?

So moisture can leak through the foil in products like celotex & single ply rubber edpm?

Hi Steve, just been watch several of your videos, very in-depth and easy to follow. I am just about to embark on an extension and I feel warm roof is the way forward. you mentioned tissue faced PIR, with the extra cost of this can you not just peak the foil from the surface of the normal PIR?

Hi Steve, Is there a reason why you do not have any video's of spray foam on a flat roof ?

Hi Steve, just been looking at a company doing garden buildings and they say the roof they do is a "hybrid" cold warm roof, they do videos and explain it here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-soclpj3gGiI.html What are your thoughts on this? From what I have read it goes against what a warm roof and cold roof should be and ends up not being either? Just dreading what might happen with all these garden buildings that people will be getting because of lockdown. Keep up the videos! Always very informative!

Sorry.