I’d like to insulate a roof. It’s not flat and it’s not pitched. It has a gentle curve to it – presumably many years ago someone felt that would look nicer than flat sloping.

How would one go about turning it into a warm roof?

If I put a wooden structure above the membrane and inline with any insulation is there a risk the wood gets moist and rots?

A warm roof relies on a water/vapour-proof membrane under the insulation. If you are going to have timber as well as insulation above that membrane I think you will be ok provided you have another waterproof membrane on top of that.

it should be ok but does depend what membranes you use, how thick your insulation is / what type of insulation / what type of structure / whats inside heated or unheated. ie how its built up.
the dew point of a roof moves about dependant on all those factors.
try this its german but about the best u-value calculator ive used for free. you can try various build ups and it will show you graphically if you’ll get condensation and where?
https://www.ubakus.com/en/r-value-calculator/
use the demo version

Or wherever you get you insulation from (not merchants) should be able to advise on the best application, similar for roof membrane manufacturer… or ask a local architect/technician but you should pay them for their time for anything more than a quick chat

Thanks! Being slow but what is a membrane layer in the tool can’t work out what to select.

Environment below will be heated and very humid.

muddyjames

the membrane layer is either the vapour barrier, breather membrane or roof covering (single ply/felt) depending on which bit. for a warm roof build up its its generally as follows:

outside

roof membrane (choose either felt or single ply, there should be some generic options under bitumen roofing or epdm depending on what you’re using)

osb3 15mm (this can be omitted dependant on the insulation used)

insulation (thickness up to you but choose generic pir/pur)

breather membrane or separating layer

osb3 15mm

roof structure

vapour barrier

plasterboard

inside

to comply with current regs you’ll need a u value of about 0.15 which will be approx 120-150mm of PIR, you can go less if you’re not going for a building control approval as anything would be an improvement, but the dew point/condensation risk is critical if you do that.

I am no expert but when looking up something similar a year or so ago I found membrane manufacturers very helpful. They tend to want to sell you the full kit. Vapor, breather, etc and will help you design

Actually I’ve just done it for you as I’m using this now.

felt roof 3mm (EPDM would be the same result)
breather membrane or separating layer
15mm OSB3 (could be omitted if you bond/fix roof membrane straight to insulation)
150mm aluminium faced PIR (has to be alu faced stuff) ideally tapped joints)
15mm OSB3
structural zone (assumed 150mm) classed as unventilated air
vapour barrier
12.5mm moisture resistant Plasterboard

gives you a u value of 0.151 and no condensation

all with lots of assumptions and no knowledge of the exact application so no responsibility for this being officially suitable but should give you a starting point….

ok so I forgot about the curved element! might need a bit more thought, but as brick says worth speaking to one of the roofing membrane manufacturers too. they all have technical helplines.

Thanks all. That’s very helpful.

That tool is amazing. Certainly best I’ve seen. Going to be playing with that all afternoon.