How To Build A Flat Roof Warm Or Cold Construction. What's The Difference? 

Steve bro! This was priceless info! Thanks for taking the time to share!

I just wanted to say that you are a master of your trade and that while my project is not exactly a flat roof but so far your videos have helped my planning process immensely. I am about to build a metal frame, roof over a partially underground concrete floored/walled trench that used to be a parking lot, and I want to make the ultimate yard machine shop (I already have an industrial lathe that I plan to move there, expect a gorgeous new milling machine as soon as the construction of the space is complete and plan on adding a bunch of high precision measuring equipment which pretty much leaves no room for insulation errors...so much more for condensation and leaks). The initial plan was to just add some screw through composite panels (standing seam ones don't exist in my country), but I later realized that doing only that in itself is a great way to create a huge moldy mess that would potentially consume my machines and tools. Also I realized that 3cm of insulation is not nearly enough to maintain comfortable temperatures in the windy and cold winter, and I wouldn't stand a chance at maintaining standard test conditions (23°C) for more than 2-3 hours a day without some massive energy bill My current plan after all the information you have shared (unfortunately I didn't have time to assimilate even a fraction of it since I have a 1 month deadline for the construction) is to first set the metal beam frame up with some raised joists, on which I will place the standardized screw through composite panels on top of the frame, taking advantage of the fact they don't need any subplate beneath, and then slide some extra insulation between the joints and directly above the beams. My questions are if the vapor barrier have to be directly adjacent to the first insulation layer, or if it's a good idea to apply it under the beams and seal it with some interior panels, and should I install an EPDM rubber layer between the 2 insulations, or is building a deck below them necessary to stick the rubber to? Would that be enough to make sure my hybrid "warmish" roof stays dry and leak free (vapor barrier from bellow the beams and EPDM ontop of them and under or between the insulation layers) Thanks in beforehands and keep the investment saving lessons coming! PS: After having seen your videos I might never trust a "professional roofer" again tbh, since now that I know what to look for, I know that nobody around here ever does it correctly (this is why the factory I work at has half a dozen leaks per square meter, leading to micrometer accurate precision tools, fixtures and jigs rusting over a single weekend). Also code here is pretty loose and not even remotely up to date with modern building practices so anyone is allowed to do whatever they feel like.

Well explained. Quick and straight to the point

Spot on as always. Keep 'em coming!

Spot on as usual Steve. Could you advise on a SIPS flat roof and which category of roof this falls into. Seems people are starting to use these SIPS panels fastening plasterboard direct below and rubber roof direct on top. Wondering about ventilation voids in this situation. Keep up the videos, awesome to share your knowledge.

Hi Steve thanks for the videos they are great! they have been a guide for me as I got to design bc drawings for a loft conversion for a terraced house , would like to go with warm roof but height will be problem after proposing new flooring level as well :/

Hi! I am going to do a cold metal frame what is the best way to insulate it and thermal bridling it . Maybe do a video for some metal framing too that be nice.

I see. Look forward to the video. How do you know how much insulation you need?

Hi, other than additional cost is there any reason not to have insulation between the joists as well as external insulation on a warm roof?

Yes it stops the hot air from rising making the bottom of the decking colder and in some conditions condensation may form. If you have insulation in between the joists you have to place more insulation on the roof to counterbalance Im going to do a video on this shortly as it seams silly that insulation between means more insulation above. This only becomes a problem when conditions are adverse

Steve, thanks for this explanation. Am I correct that your diagram of the hot roof does not include any ventilation? Why is condensation not a problem for the hot roof? I would have thought an air space would be required between the insulation and the roof tiles....

Hi just wondering can you use 11mm osb on top of joist to keep cost and weight down ?

I have a cold roof construction that needs a new epdm ( rubber) or bituminous covering can I also add a lair of insulation like on the warm roof example? Or does that create other problems?

Very informative, thank you.

Hi Steve, with the added layer of Insulation underneath the joists where would you then put your vapour barrier? Under the joists or under the extra insultion with battens? Thanks

I am looking to convert my lean to style conservatory roof from the polycarbonate to a solid roof. I was looking at a cold roof system, to allow for ventilation I thought about having batons running perpendicular to the joists so I could have ventilation at the sides instead of some obscure system against the house wall. Does this sound practical? Would I perhaps be better to go for the warm roof system as I think I'd have the height available. Many thanks, great video.

Hello I'm glad I've cone across your videos. I'm building a garden room and been watching someone who puts the insulation on a cold roof but butts it up to the osb and has a gap between the insulation and vapour barrier that's not vented like a hybrid roof. Now will the fail or not he says that with the insulation up tight to the osb that it cant condensate up there. (Its a rubber roof) cheers.

Very helpful. I’m going with the warm roof.

Hi Steve, my cold roof design has no ventilation where the roof meets the house. It has vented soffit all round and a cross battening to achieve cross flow as well as air flow from the front too, but no ventilation at the gmhouse end. Do you think this is ok

Hi Steve. Nice explanation. I discovered tonight from my architect that insulation can go om top or under. What’s the best for keeping a house cool/confortable? I live in the Caribbean, where the sun is always shining with temps of about 32/33 Celsius. -> warm or cold roof?

Hi Steve I am currently building a garden room and I am limited to 2.5M I wanted a warm roof but the build up is aot. My question is does a sips panel roof solve my problem as in it is the same as a warm roof but without the supporting timbers. Cheers Lee

Thanks Drukker, no ventilation is needed in a warm roof, the gap between the joists is now internal in fact you don't need a ceiling. So do we vent our rooms? Yes we have a changed of air to keep it fresh and healthy. Do we get condensation in our rooms. Yes on cold spots such as windows and sometimes on walls. So wont we get condensation on the underside of the decking especially as we haven't got a ceiling? No as the performance of the warm roof is so good we don,t get cold spots.

Hello i have a question i have a flat roof made of Rodman liner. And wanted to insulate better. I’ve been proposed dense packing the ceiling. And but can possibly accumulate rote and mold. I was wondering whether I should dense pack below my ceiling. Thank you

In warm roof can you also put insulation plasterboard on cilling

Great video

Hi Steve, amazing videos and info! We have an addition in Utah with 16" BCI joists. We have dry summers and winters. The old roof was an insulated/vented 10" cold roof. We planned to copy that method but many suggest the hot roof methods. With a 6 " difference in old and new roof heights, we'd like to avoid more height. Is it feasible to put foam insulation between the flanges (underside osb) and leave an air channel between the foam and a standard bat insulation for venting since there is so much height in the 16" joist? Or would we be better off to do fully spray foam insulation inside?

Legendary. Some helpful

Hi great video. A question for you, with a warm roof do I have to insulate in between the joists as well before plaster boarding. ?

Thermal bridging on cold roofs can be quickly resolved using thin rubber glued to the top fo the joist and the deck screwed through or even around the top 3 sides of the joist with the insulation stuffed between. No need to lower ceiling and rapid to do. The major thermal bridging is from direct contact with the outside, airflow bridging is much lower.

Steve wonderful video. I live here in Central Florida in a house built in 1958. The house has a 1/12 pitch roof and several rooms with flat roof. The framing is at least 2x6 or 2x8 on 16 inch centers. The roof material is "torch down". I have zero insulation inside the roof framing and the roof was put down in 2002. We desperately need insulation. Right now we have a Whirlybird and that attic roof turbine draws a lot of heat out of the attic in the summer since the framing in the soffits each have 2 inch holes in each rafters bay. What would be our best solution? How should we go about insulating? Thanks. Great video. Jeff

Very good

Perhaps update Spec now 150mm ? ..can sound deadening mineral slabs be added..R

hi steve, how would you run cable and install downlighter on a cold roof without slicing channels in the insulation boards?

Fantastic videos. I have a outbuilding which I am converting into a gym with a new warm roof. The building is 2.5m by 7m long, I was looking to use 50mm by 200mm joists running the 2.5m length with spacing between the joists of 400mm (can i have a larger gap?) 18mm osb board on top of the joists, dpd membrane, 120mm insulation, epdm roof. The slope needs to run down towards the longer end and I was wondering if i could just angle the joist hangers which will be attached to the brick wall at an angle to get the desired slope or do i need to have Firrings? Keep up the good videos.

Hi Steve. Great video. Very informative. What do you, think. Would it make sense to make a mix of both? Meaning insulate from the inside (cold roof), and also from the top (warm roof). The insulation heights would be less, because of having them on both sides, and the disadvantage of the height problem in the warm roof would not be there. Would it be necessary to have also ventilation? Since it is a mix, maybe it could be something like the best of both worlds? Again, great video!

Thanks for this great information 👍

Great video Rick!!

Hi Steve, excellent level of detail gone into all your videos. Really interesting. I am going to construct a warm roof but I have a bit of a height restriction, I plan to use osb Vaper barrier - Insulation - osb combination. I will probably only get about 100mm of insulation fitted and was wondering if I could use foil type multlayer insulation in place of the vapour to act as extra insulation in a thiner design and as a vapour barrier which brands like superfoil claim to do anyway. Thanks in advance. Gary

Hi Steve live the videos can I ask what software do u use to draw up you examples please

Steve, I wish we had someone like you up north (north Manchester). I spent£4000 on a warm roof with what is supposed to be the top warm roof installer around here. Okay it hasn't leaked, but warm? No way! Biggest mistake was they did not put insulation behind the fascia boards between the roof joists or at the sides (it's a dormer). After watching your videos the only way i can see around the problem is to take part of the ceiling (a 2ft width) down all around the front and rear dormer and fit insulation from below. Unless you know any other way?

Guys insulated my bedroom 'ceilin' which had a flat roof. Looking at your sketches, is that why my room is so cold? It's the coldest room in my house!

Great video! I’m building a warm roof on top of the existing decking on my summer house. Am I right thinking I need to put a breathable membrane on both sides of the insulation board? thanks

Hi , can you recommend as guide on constructing a warm sloping timber roof

Hi Steve, Excellent video. If the roof is flat concrete, is there an issue with laying timber firrings directly to the roof then constructing a typical OSB/VCL/PIR/OSB/EPDM build up ? There is obviously an air gap between the firrings and the first OSB layer. Is this an issue ?

Hi Steve, you have convinced me to use a warm roof for my garden room. The make up I'm going for is as follows EPDM Roof Membrane 18m T&G OSB 100mm PIR Non-breathable plastic sheet 22mm Egger Protect Waterproof covered chipboard T&G 125mm x 50mm roof rafters 12.5 plasterboard Plaster Skim with downlighters cut in Does this look ok? (ignoring the roof rafter size as they are adequate for the span in the span tables for C24) Would i need another vapour barrier between the plasterboard and the rafters like in a cold roof? Many thanks

Great visualization, much appreciated, had a question, what goes on top of the warm roof insulation and is warm or cold best for a living space/patio on top of your roof? Thx.

Could you use spray form if you use a warm roof system and the then spray the ceiling joist and leave it open? I just want tin increase the R value because I live in canada.

I need help. I think my carpenter placed the joists in the wrong direction and has no ventilation on either end of them to create a cold roof. He won't admit his mistake and now I have to have a ventilation on top and drill holes in my joists to create air flow. can someone please help me know how big of a hole I need to make. I think he also used 2x6 for joist. I am so done with him I am willing to make the holes myself.

Steve have you ever used vacuum insulation with a warm roof to save thickness?

Hi Steve, I've built a shed 3m x 3m but due to height restriction have opted for the cold roof. My question is noggins that are in place because of the span of the build and to keep straight the joist they also now block airflow. To create adequate airflow through the roof whats the best way to do it.. is it a case of creating holes through the noggins but above the insulation.. any advice would be much appreciated Cheers

Hi Steve, I'm trying to design a cold roof for a garden office (not subject to building regs) It has to be a cold roof because of height restrictions. The joists are 150 and will have 100PIR what I can 't get my head round is am I better to push the hard PIR up to the OSB so that there is no ventilation but also seal the any gaps so that no moisture can pass also or just leave a 50mm between the PIR and OSB to vent gap and not worry too much about the holes the downlighter will need...??

Hi Steve my builder have done the warm roof fiber glass roof on 6 meter extension after few weeks cracking noises start to come early morning and in the evening same time during the day . Can you please let me know what is the problem way this cracking noises are coming?

Hi Steve, you really convinced me on the cold roof. I'm building a timber frame man cave in my garden. The problem I have is the 2.5m height limit. My bas is already about 10" of the ground. I need at least 2 meter ceiling height . The recommended roof joists are 8" then I would like to put firring strips on top of those so I don't have a sloping ceiling. How would you solve this problem? Could sips be laid across 4.5 meters unsupported? Thanks Steve, your videos are really helpful. I wish I could afford you to do my roof. Thanks again.

Hi, i have a question, i live in florida, i have a one car garage with a flat roof like the one on this video, no soffit, so no air coming in or out of the roof when close wit drywall . my plan is to install a mini split in the garage, Summer is to hot here, i know i have to insulate my garage door, now, how should i insulate this roof and what to use, my rafters are 16" x 2" and like 15 or 16" a part, left and right walls are block and stucco, anynof the two walls are exterior, left wall has my kitchen on the other side, and right wall meets my neighbors garage, the rafters goes right thru the blocks, so no wood against any of the walls. I want to do this my self, and im scared of moisture on the roof and floors.. I hope this info its enough for you to help me out, if you can, thanks in advance...

Hi Steve, what software do you use to do these 3D models?

What fixings should b used on warm roof installation with bottom and top decks being 18mm OSB?

What do we do now when in the cold its really cold even in hot climates and vice versa

Hi Steve I have a kitchen extension that has been converted to a pitched roof. What they have done is take of the felt and leave the flat ply base. And create a pitched roof above it with a wooden frame and tiled it. The sides have been closed up using pvc cladding, it looks very nice but there is no insulation in the roof. Would I be ok to create a warm roof on the plywood base but obviously not put the felt on as it’s already waterproof. If I insulted between the rafts the space above my kitchen would still be very cold because of the cladding on the sides. Any advice would be appreciated. Thanks

Hi Steve, I am having an extension built flat warm roof with a felted finish. Would it be ok to go for 18mm t&g osb staggered, vapour barrier, insulation board, finished felt product. regards

What do you think about foil insulation products? Can they be used to insulate an old flat roof from below, making it a cold roof and retaining airflow underneath the deck due to the insulation being thinner than foam or rockwool? Would love to see a video on them like the one where you show traditional hot and cold roof buildups side by side on your workbench.

Is there any disadvantage to installing insulation between joists and having a warm roof construction. This way there is maximum insulation?

Hi Steve - I need to go the cold roof route on my workshop due to height restrictions. I have been led to believe that the roof has to be vented across from one side to the other with a 50mm gap between the insulation and underside of roof deck - my question is that when I fit noggins this will effectively stop any cross ventilation as the noggin is the same size as the rafter - any ideas of how I get round this or have I misunderstood the entire concept ! Thanks

Great explanation as always. I am planning to convert my existing cold flat root to warm flat roof (EPDM rubber). I have called the council (Solihull) about the planning permission and they said if the height is going to be more then 150mm then existing roof then you need to apply for PP. As going to cold roof to warm roof height is going to go up , do you think it be more then 150mm ? I will need BR in any case. PS if only you were to provide your services in midlands :)

Can I ask what the OSB is used for in the cold roof? I'm doing a cold roof in a conservatory and I don't doubt my skills. I spoke to a private building control firm and they said OSB isn't really needed.

Where does the Vapor go on the plywood or on top of the insulation?

The other big advantage of Warm Roof is that Every MEP engineer loves to run all services inside the empty roof. When they see it, they just love it, because you can basically run there everything from pipes to lamps. Also, with the cold roofs there is another issue: trying to keep the integrity of insulation. Once the Electricians start to drill holes in the lower insulation, then the problem starts. It's nearly impossible to make it perfect with cold roofs. And in 9 out of 10 roofs contractors somehow manage to forget about ventilation until the last moment. We spent most of our year in correcting cold roofs ventilation by simply drilling holes in firrings after a few years when the moisture starts accumulating and become visible. 😅

Pretty wild seeing all these comments talking about height restrictions. In my area, can't go higher than 10 meters above grade without a variance, which is easy to get. What would you say is the highest performing roof assembly from an energy standpoint?

Hi, do you recommend adding insulation below the joists as well as between them? In your lighting video ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-L-hXgV44omc.html I can only see the insulation between the beams and not under them/on top of the plasterboard. Many thanks.

Could you not just have the insulation between the rafters pushed tight up to the board. Would be like the best of both.

cool bob the builder