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

I'm often asked what the difference is between a warm roof and cold roof.
In this video, I explain three different ways of building a flat roof using the warm roof and cold roof methods. Also, I explain the height each roof should be and all the benefits, and drawbacks of each method.
Cold flat roof construction is mare more difficult to get right and I see lots of cold roofs fail. Warm roofs are much more forgiving, the builder must pay attention when doing warm or cold roof construction however its less important in warm roof construction.

If you need any more information, feel free to contact me on:

07802 300 099

Steve at London Flat Roofing

[garvielloken]

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

[przemyslawolejnik]

Well explained. Quick and straight to the point

[Sketch]

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.

[rideforever]

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.

[rexmundi]

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

[robmckenna]

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.

[SteveRoofer]

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.

[michaelholt]

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?

[rohan.london]

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 ?

[reggiewac]

Spot on as always. Keep 'em coming!

[klaeLIFE]

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

[jessedrukker]

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....

[elephantear]

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.

[MaximGrachev]

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!

[UrminnesTider]

How do you lay a warm roof on a A-frame house that has 65 degree angle and make it stick to the surface?

[leojk]

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. 😅

[TomSullivanify]

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

[ArtificialJetleg]

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?

[lukeprice]

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

[SteveRoofer]

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