London & South-East England
Flat Roof Insulation
Warm deck PIR systems that stop the cold, damp extension roof, fitted by our own team.
Is flat roof insulation right for your home?
- Rear and side extensions with a cold, uninsulated flat roof
- Flat roofs due for re-covering anyway, where insulation adds little extra cost
- Dormer and garage roofs over rooms that are in use
- Rooms with condensation or mould on the ceiling in winter
- Owners bringing a house up to a Building Regulations U-value of 0.18
Flat Roof Insulation: key facts
- A warm deck flat roof puts rigid PIR board above the deck, so the whole roof structure stays warm and dry.
- Typical London extension roof (15 to 25 m²) costs £2,500 to £5,000 insulated and re-covered.
- PIR warm roof board from £100/m² (80 mm) to £160/m² (150 mm) supply and fit.
- Reaches a U-value of 0.18 W/m²K with 150 mm PIR, the Building Regulations target for a new roof.
- Installed at 0% VAT as an energy-saving material until 31 March 2027.
- Installed by our own PAS 2030:2023 certified team, with a written guarantee.
What is flat roof insulation?
Flat roof insulation puts a continuous layer of rigid insulation on top of a flat roof deck, under a new waterproof membrane. This is the warm deck method. The roof timbers sit on the warm side of the insulation, so they stay dry and there is no condensation inside the roof. It is the only method we recommend for a flat roof that is being re-covered.
London has hundreds of thousands of flat roofs on rear extensions, side returns, garages and dormers, and most of them have little or no insulation. A cold, uninsulated extension roof shows up as a room that is freezing in winter, hot in summer and prone to black mould in the corners. Insulating from above fixes the problem without touching the ceiling below.
We survey the existing roof, check the deck and falls, and quote for the insulation and the new covering as one job. Our own team fits the PIR boards, a vapour control layer, and an EPDM or similar membrane. Where the roof is sound and only the insulation is missing, we can also quote for insulation between and under the joists from inside.
Who does the work
Flat roof insulation is installed by our own trained team under our PAS 2030:2023 certification for roof insulation. Where the roof covering itself needs replacing, we fit the new deck, insulation and membrane as one job so there is no gap between trades.
Flat Roof Insulation cost guide
Indicative 2026 pricing from our quote book for London and the South-East. Every quote is free, fixed and itemised.
| Job type | Indicative cost |
|---|---|
| PIR warm deck, 80 mm (U 0.30) | £100 / m² |
| PIR warm deck, 100 mm (U 0.25) | £120 / m² |
| PIR warm deck, 150 mm (U 0.18) | £160 / m² |
| Flat roof replacement with EPDM | £110 / m² |
| Typical extension roof, 15 to 25 m² | £2,500 – £5,000 |
VAT: 0%. Roof insulation is an energy-saving material, so the installation is charged at 0% VAT until 31 March 2027 (HMRC VAT Notice 708/6). A full roof replacement that includes insulation is priced with the VAT split shown on the quote.
Systems and materials we install
How flat roof insulation works
Free survey
We check the deck, falls, outlets and any existing leaks, and measure the roof area and edges.
Fixed quote
A written, itemised quote within 24 hours: insulation thickness, membrane, edge details and any deck repair.
Installation
Strip the old covering if needed, lay the vapour control layer, fix the PIR boards, then bond the EPDM membrane and trims.
Sign-off and guarantee
Final inspection, photographs of the build-up, and a written guarantee for the insulation and membrane.
Free survey & fixed quote
Book a free, no-obligation survey and we will send a clear, fixed written quote. One point of contact from survey to sign-off, and a written guarantee on the work.
Get a free quoteFlat Roof Insulation FAQs
How much does flat roof insulation cost?
Warm deck or cold deck: which is better?
Can you insulate a flat roof without replacing it?
What thickness do I need?
Does flat roof insulation need planning permission?
How long does it take?
Areas we cover
We arrange flat roof insulation across Greater London, Surrey, Kent and Hertfordshire. See all areas we cover →
Related guides
In-depth, independent guidance to help you decide.
Last reviewed September 2026 by Stay Warm Insulation.
Get a free quote
Leave your details and we will call you back within one working day to arrange a free survey.