A PIR warm deck flat roof costs £100 to £160 per m² from our quote book, depending on thickness: £100/m² for 80 mm, £120/m² for 100 mm and £160/m² for 150 mm, which is the thickness that meets the Building Regulations target of U 0.18 W/m²K. A typical extension roof of 15 to 25 m² comes to £2,500 to £5,000 with a new covering. Insulation is 0% VAT until 31 March 2027, and the best moment to insulate a flat roof is when it is due to be re-covered anyway.
Key takeaways
- From our quote book: PIR warm deck at £100/m² (80 mm, U 0.30), £120/m² (100 mm, U 0.25), £160/m² (150 mm, U 0.18).
- EPDM rubber covering replacement on its own: £110/m².
- Typical extension roof of 15 to 25 m²: £2,500 to £5,000.
- Building Regulations target for a new or replaced flat roof: U 0.18 W/m²K, which 150 mm PIR achieves.
- Warm deck (insulation above the deck) is the standard method. Cold deck (insulation between the joists) needs a ventilated void and is harder to get right.
- Installed by our own team, with a 10 year manufacturer warranty, 25 year insurance backed guarantee and a Building Regulation certificate at no cost.
How much does flat roof insulation cost?
These are our own quote book rates for a warm deck build-up: a vapour control layer on the existing deck, rigid PIR insulation board, and a new waterproof covering on top. They include labour, materials, stripping the old covering and disposal, and VAT at 0% on the insulation. They exclude repairs to the deck or joists, which are priced when we see the roof.
| Option | Thickness | U-value achieved | From our quote book |
|---|---|---|---|
| PIR warm deck, minimum | 80 mm | 0.30 W/m²K | £100 per m² |
| PIR warm deck, mid | 100 mm | 0.25 W/m²K | £120 per m² |
| PIR warm deck, regulations target | 150 mm | 0.18 W/m²K | £160 per m² |
| EPDM covering replacement only | No insulation added | Unchanged | £110 per m² |
The 150 mm option is the one we recommend and the one Building Regulations expect on a roof that is being replaced. The thinner options exist because some roofs cannot take the height: door thresholds, low upstands at the wall, or a rooflight that would end up below the new surface. Where that applies we say so and price the thickest board the roof can take.
What does it cost for a typical extension roof?
Most of the flat roofs we insulate sit over a kitchen extension, a rear addition on a terrace, a dormer or a garage. They are 15 to 25 m² and land between £2,500 and £5,000 for a full warm deck with a new covering. The examples below are worked from the per m² rates and rounded.
| Roof | Area | 150 mm warm deck (U 0.18) | 100 mm warm deck (U 0.25) |
|---|---|---|---|
| Small rear extension or dormer | 15 m² | about £2,400 | about £1,800 |
| Typical kitchen extension | 20 m² | about £3,200 | about £2,400 |
| Larger extension or garage | 25 m² | about £4,000 | about £3,000 |
Very small roofs, such as a porch or bay, carry the same set-up and edge detailing as a large one, so they are quoted as a job rather than by the metre. Bays are covered in bay window insulation.
Warm deck or cold deck: what is the difference?
The names describe where the insulation sits relative to the timber deck.
Warm deck
Insulation goes on top of the deck, under the waterproof covering. The deck and joists stay on the warm side, so there is no condensation risk in the roof structure and no ventilation needed. It is the method Building Regulations guidance favours and the one we use. The trade-off is height: the roof surface rises by the board thickness, which is why thresholds and upstands are checked at the survey.
Cold deck
Insulation goes between the joists, under the deck, with a ventilated air gap of at least 50 mm above it and vents at the eaves. The deck stays cold. It avoids raising the roof, but the ventilation is hard to achieve on a roof enclosed by walls on three sides, the joist depth limits the thickness, and any gap in the vapour barrier below lets moist air condense on the cold deck. Most rotten flat roof decks we see are cold deck roofs. We only do a cold deck build-up where a warm deck is physically impossible, and we say so in the quote. Roof terraces and balconies use an inverted variation of the warm deck, with the insulation above the waterproof layer under paving or ballast, priced separately.
What U-value does a flat roof need?
The U-value is the rate heat passes through the roof per square metre per degree of temperature difference. Lower is better. An uninsulated flat roof with plasterboard below and felt above is well above 1.5 W/m²K, which is why the room under an old flat roof is cold in winter and hot in summer.
When a flat roof is replaced or renovated in England, Building Regulations expect it to be brought up to 0.18 W/m²K. With PIR board that is about 150 mm. Our 80 mm and 100 mm options, at 0.30 and 0.25, are for roofs where the full thickness will not fit; building control accepts the best that is practical where the target cannot be met. We issue the Building Regulation certificate at no cost to you. For a room built into a pitched roof, see room in roof insulation.
What pushes the price up or down?
- Deck condition. A soft or rotten deck has to be replaced before anything goes on top. We price this as its own line once the old covering is off.
- Height and access. A single storey extension is worked from a ladder or tower. A roof above two storeys may need a scaffold; see scaffolding costs.
- Edges and upstands. Every wall abutment, parapet, rooflight and outlet needs trimming and flashing. A roof with many details costs more per m² than a plain rectangle.
- Thresholds and drainage. Where a door opens onto the roof, the new surface must stay below the threshold. Raising the roof also means raising or replacing the outlets.
- Covering. EPDM rubber is our standard. Fibreglass (GRP) or a felt system can be quoted where a particular finish is wanted.
- Rooflights. Fitting a new rooflight while the roof is open is far cheaper than doing it later.
How Stay Warm insulates a flat roof
Flat roof insulation is installed by our own team under PAS 2030:2023, with a 10 year manufacturer warranty and a 25 year insurance backed guarantee on the installation. Insulation is 0% VAT until 31 March 2027; where any part of the job is not an energy-saving material, such as deck repairs, it is shown at its own rate on the quote.
- Free survey: we measure the roof, check the deck, and note thresholds, upstands, outlets and rooflights.
- Fixed written quote with the thickness, the U-value achieved and the per m² rate stated, and the reason if the full 150 mm will not fit.
- Installation, usually one to three days for an extension roof, with the roof left watertight at the end of every day.
- Building Regulation certificate, guarantee documents and the manufacturer warranty sent to you.
See the flat roof insulation service page for the build-up in detail, or insulating an extension if the walls and floor also need attention. A free heat loss survey covers the flat roof alongside the rest of the house.
Frequently asked questions
How much does it cost to insulate a flat roof?
From our quote book, a PIR warm deck is £100 per m² at 80 mm, £120 per m² at 100 mm and £160 per m² at 150 mm, including a new covering and 0% VAT on the insulation. A typical 15 to 25 m² extension roof is £2,500 to £5,000.
What U-value should a flat roof have?
Building Regulations expect 0.18 W/m²K when a flat roof is replaced or renovated in England. With PIR insulation that is about 150 mm. Where the full thickness will not fit because of thresholds or upstands, building control accepts the best that is practical.
Is a warm deck or cold deck flat roof better?
Warm deck. The insulation sits above the deck so the structure stays warm and dry and no ventilation is needed. Cold deck puts insulation between the joists and relies on a ventilated gap above it, which is hard to achieve and is the usual cause of rotten flat roof decks.
Can I insulate a flat roof without replacing the covering?
Not as a warm deck, because the insulation has to sit under a new waterproof layer. Insulating from below between the joists is possible in principle but needs a ventilated void and a perfect vapour barrier. The practical answer is to insulate when the roof is re-covered.
Is flat roof insulation 0% VAT?
Yes. Roof insulation is an energy-saving material under HMRC VAT Notice 708/6, so it is 0% VAT until 31 March 2027, then 5%. Any part of the job that is not an energy-saving material, such as replacing a rotten deck, is shown at its own rate on the quote.
Do I need Building Regulations approval to insulate a flat roof?
Replacing or renovating a flat roof is controlled work, and the roof is expected to meet the 0.18 U-value target. We handle the notification and issue the Building Regulation certificate at no cost to you.