Insights
Warm Roof or Cold Roof: Which Is Right for My Extension?
Published 19 October 2026
A warm roof puts the insulation on top of the roof structure, so no ventilation gap is needed. A cold roof puts the insulation between the joists, and the roof needs a ventilated gap above it. Which suits your extension depends on how it is built, so it is worth asking for a proper survey before you choose.
What is a warm roof?
On a warm roof, which the Planning Portal calls a warm deck, the insulation is placed on top of the rafters or joists. That means the timbers sit on the warm side of the insulation, inside the heated space.
The Planning Portal says no ventilation is required for these roofs. The roof covering, whether that is felt, a liquid system or another waterproofing layer, then goes over the insulation.
What is a cold roof?
On a cold roof, or cold deck, the insulation is placed between the joists or rafters. The Planning Portal says ventilation is required for these roofs.
For a flat cold roof, it describes a ventilation gap, usually 50mm, between the top of the insulation and the deck above it, with ventilation openings at the eaves or upstand. It also says a vapour membrane should be added to the underside of the insulation, which is fixed to the joists before the plasterboard goes on.
Warm roof or cold roof at a glance
| Warm roof | Cold roof | |
|---|---|---|
| Where the insulation goes | On top of the joists or rafters | Between the joists or rafters |
| Ventilation | Not required | Required |
| Vapour membrane | Not covered on the Planning Portal page | Under the insulation |
| What to ask about | How the insulation is laid and covered | How the ventilation gap is kept clear |
Which is right for my extension?
There is no single right answer. It depends on the existing structure, how much height you have inside, and how the roof meets the house wall. A cold roof needs air to move through the gap above the insulation. That can be harder to arrange on an extension where the roof runs up against the house, so each roof needs looking at on site.
The Planning Portal also notes that where ceiling height would be badly affected, a lower insulation performance target may be appropriate. If your ceiling is already low, say so when you ask for a survey.
What about condensation?
Moisture matters on any insulated roof. The Planning Portal says the risk of condensation in the roof space must be assessed, with provisions made under Part C of the Building Regulations. That is one reason the build-up of a roof, meaning what goes where and in what order, matters as much as the finished surface you can see.
If you notice stains on the ceiling under a flat roof, don't assume the cause. It could be a leak or it could be moisture, and a proper look at the roof is the only way to tell. Look from inside the room and from the ground, and note when the marks appear.
Do I need Building Regulations approval?
Possibly. The Planning Portal says that replacing the roof on an existing building will, in most situations, need Building Regulations approval. It also says that some repairs to flat roofs will not need an application. If a roof with integral insulation is replaced, you may be required to upgrade that insulation, because a roof counts as a thermal element.
We can't tell you what applies to your property. Check with your local council's building control, and ask us when we survey your roof.
Questions to ask any roofer
Before you agree a quote, ask:
- 01Is this a warm roof or a cold roof, and why is it right for my extension?
- 02Where does the insulation go, and how will it be protected?
- 03If it is a cold roof, how will the ventilation be kept clear?
- 04Does the work need Building Regulations approval, and who deals with it?
- 05What happens to the old covering and insulation?
A clear answer to each tells you the roofer has looked at your roof, not just measured it.
Repair or replace?
A flat roof that is still sound can often be repaired, and a roof that has failed needs replacing. The honest answer comes from looking at the roof and what is under it. Wet insulation or stained ceilings point towards a bigger job than a patch. You can check from the ground or an upstairs window. Please don't climb onto a flat roof yourself to look for problems.
How JCS Roofing Solutions can help
We are a flat roofing specialist based in Ingatestone, Essex. We install warm roof systems and cold roof systems, and we work on garage and extension roofs, dormer roofs, balconies and box gutters. You can see what we do on our services page and where we work on our areas page, or look at our projects.
If you are planning an extension or your flat roof is reaching the end of its life, James can talk through the options with you. We will tell you plainly whether a repair will do or the roof needs replacing.
Call James on 07930 520578 or get a quote at jcsroofingsolutions.co.uk/contact
Frequently asked questions
What is the difference between a warm roof and a cold roof?
On a warm roof the insulation sits on top of the rafters or joists, and no ventilation is required. On a cold roof the insulation sits between them, and ventilation is required.
Does a cold roof need ventilation?
Yes. The Planning Portal says ventilation is required for cold deck roofs, and describes a gap of usually 50mm above the insulation on a flat cold roof.
Do I need Building Regulations approval to replace a flat roof?
In most situations, replacing a roof needs Building Regulations approval, according to the Planning Portal, although some flat roof repairs do not. Check with your local council's building control.
Can I check my flat roof myself?
Look from the ground or an upstairs window. Please don't climb onto a flat roof yourself.
Do you fit both warm and cold roofs?
Yes. We install warm roof systems and cold roof systems, including on garage and extension roofs.