Flat roof PIR insulation: what thickness do you need?
On a flat roof the thickness of PIR you need depends on the U-value you have to reach, and the standard way to build the roof is as a warm roof, with the insulation above the deck. This guide explains the build-up, what the Building Regulations in England ask for, and what 100mm, 120mm and 150mm boards give you, with live flat roof board prices ex VAT.
Prices correct at 28 October 2026 and may change. All prices are ex VAT; check the product page for the current price.
Quick answer
- A warm roof has the insulation above the structural deck and a vapour control layer below it, with the waterproofing on top.
- For England, Approved Document L (2021 edition with 2023 amendments) gives a limiting roof U-value of 0.16 W/m²K for a new dwelling and 0.15 for a new roof element in an existing dwelling. Check which figure applies to your job with building control or your designer.
- On the indicative figures below, a plain deck needs about 150mm of PIR to reach those values. Thinner boards do not get there on their own.
- Flat roof boards are sold in packs. Compare them per m² (pack price ÷ pack area), not per pack.
Warm roof construction in brief
From the top down, a warm flat roof is usually: waterproofing (single ply, bituminous or similar), the PIR boards fixed or bonded in the way the waterproofing manufacturer specifies, a vapour control layer, then the structural deck (plywood, OSB, concrete or metal) on joists or a slab. The insulation keeps the deck and joists warm, so there is no cold void under the deck to ventilate.
Moisture guidance (BS 5250 and BS 6229, as summarised by roofing manufacturers) says the air and vapour control layer sits on the warm side of the insulation and has to be well sealed, including at laps and upstands. The same guidance does not recommend cold flat roofs. Where one cannot be avoided it asks for a ventilated gap of at least 50mm above the insulation, and your designer has to size the ventilation. See IKO’s technical guidance for a plain summary.
Check the facing on the board against your waterproofing. Mechanically fixed, adhered and torch-on systems have different requirements, and the board datasheet and the membrane manufacturer’s guidance decide what is allowed.
What thickness does Part L need?
In England the limiting U-values for roofs are in Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments):
- New dwelling: 0.16 W/m²K for all roof types (Table 4.1).
- New roof element in an existing dwelling: 0.15 W/m²K (Table 4.2).
- Existing roof being renovated or retained: 0.16 W/m²K (Table 4.3). A note allows a lesser standard on a flat roof if there are problems with load-bearing capacity or upstand height, so ask building control.
These are limiting values. The design often has to do better, and non-domestic buildings are covered by Volume 2. A new edition of Approved Document L takes effect on 24 March 2027 for building work in England, so check which edition applies to your project. Always check with your building control body or designer before you order.
The table gives an indicative U-value for plain boards, using R = thickness ÷ λ and an assumed 0.2 m²K/W for the surface resistances and a thin deck. That 0.2 is our assumption, not a manufacturer figure. Fixings, tapers, upstands and the actual deck all change the real result, so your designer’s calculation is what counts.
| Board thickness | λ 0.022 boards (e.g. TR26, Eurodeck): R-value / indicative U | λ 0.024–0.025 boards (e.g. TR27, TR24, Powerdeck): R-value / indicative U |
|---|---|---|
| 100mm | 4.50 / 0.213 | 4.00 / 0.238 |
| 120mm | 5.45 / 0.177 | 5.00 / 0.192 |
| 150mm | 6.80 / 0.143 | 6.25 / 0.155 |
On those assumptions, 150mm at λ 0.022 comes out at about 0.14, which is inside both 0.15 and 0.16. A 150mm board at λ 0.024 comes out at about 0.155, which just meets 0.16 but not 0.15. 120mm and 100mm boards do not reach either figure on their own. You can try your own build-up in our PIR U-value calculator.
Flat roof PIR board prices
These boards are made for flat roofs and sold in packs. The price per m² is the pack price divided by the pack area. Prices ex VAT, correct at 28 October 2026 and may change.
| Board | Size | Thickness | Pack | Price per pack | Price per m² | λ / R-value |
|---|---|---|---|---|---|---|
| TR26 | 2400 × 1200mm | 100mm | 3 boards, 8.64 m² | £262.13 | £30.34 | 0.022 / 4.5 |
| TR26 | 2400 × 1200mm | 120mm | 2 boards, 5.76 m² | £198.75 | £34.51 | 0.022 / 5.45 |
| TR26 | 2400 × 1200mm | 150mm | 2 boards, 5.76 m² | £249.15 | £43.26 | 0.022 / 6.8 |
| TR27 | 1200 × 1200mm | 100mm | 4 boards, 5.76 m² | £183.95 | £31.94 | 0.025 / 4.0 |
| TR27 | 1200 × 1200mm | 120mm | 4 boards, 5.76 m² | £207.95 | £36.10 | 0.024 / 5.0 |
| TR27 | 1200 × 1200mm | 150mm | 2 boards, 2.88 m² | £129.18 | £44.85 | 0.024 / 6.25 |
| TR24 | 1200 × 600mm | 100mm | 5 boards, 3.6 m² | £141.47 | £39.30 | 0.025 / 4.0 |
| TR24 | 1200 × 600mm | 120mm | 4 boards, 2.88 m² | £116.78 | £40.55 | 0.024 / 5.0 |
| Eurodeck | 2400 × 1200mm | 100mm | 4 boards, 11.52 m² | £243.90 | £21.17 | 0.022 / 4.5 |
| Eurodeck | 2400 × 1200mm | 120mm | 4 boards, 11.52 m² | £287.10 | £24.92 | 0.022 / 5.45 |
| Eurodeck | 2400 × 1200mm | 150mm | 3 boards, 8.64 m² | £265.73 | £30.76 | 0.022 / 6.8 |
| Powerdeck F | 1200 × 600mm | 100mm | 5 boards, 3.6 m² | £91.97 | £25.55 | 0.025 / 4.0 |
| Powerdeck F | 1200 × 600mm | 120mm | 4 boards, 2.88 m² | £85.58 | £29.72 | 0.024 / 5.0 |
| Powerdeck F | 1200 × 600mm | 150mm | 4 boards, 2.88 m² | £101.18 | £35.13 | 0.024 / 6.25 |
| Powerdeck U | 1200 × 600mm | 100mm | 5 boards, 3.6 m² | £97.97 | £27.21 | 0.025 / 4.0 |
| Powerdeck U | 1200 × 600mm | 120mm | 4 boards, 2.88 m² | £89.18 | £30.97 | 0.024 / 5.0 |
| Powerdeck U | 1200 × 600mm | 150mm | 4 boards, 2.88 m² | £105.98 | £36.80 | 0.024 / 6.25 |
For example, the 150mm Kingspan TR26 pack is £249.15 ex VAT for 5.76 m², which is £249.15 ÷ 5.76 = £43.26 per m². The 150mm Recticel Eurodeck works out at £30.76 per m², the 150mm Powerdeck F at £35.13 per m² and the 150mm Kingspan TR27 at £44.85 per m².
The prices are not in a smooth order by thickness or pack size, so compare the boards on the product pages. The right board is the one that suits your waterproofing and reaches the U-value, not simply the cheapest per m².
Before you order
- Measure the roof and add allowance for cutting and for tapered or falls schemes, which your designer may specify.
- Keep boards flat, off the ground, dry and covered until they are fixed. Wet boards should not be used.
- Follow the fixing pattern and fastener length for the thickness you choose; this comes from the waterproofing system, not from us.
If you are comparing with general-purpose boards, see our PIR price per m² by thickness guide. For lofts, see using PIR in the loft.
Frequently asked questions
Can I use ordinary PIR boards on a flat roof?
Use boards that the manufacturer states are suitable for flat roofs and for your waterproofing system. Check the datasheet.
Do I need a vapour control layer?
On a warm roof the usual build-up includes an air and vapour control layer under the insulation. Your designer decides the layer for your construction.
Does thicker PIR always cost more?
Not always per pack or per m². Check the live price for the board you want.
Order flat roof insulation
Browse our PIR insulation boards or the roof insulation range. Prices are shown ex VAT. Most orders over £400 + VAT are delivered free across England, with delivery to mainland Wales and Scotland too. Mainland Wales and mainland Scotland: Celotex insulation and Knauf loft roll have £40 ex VAT delivery on every order; ceilings have a £400 ex VAT minimum plus £40 ex VAT delivery; British Gypsum plaster and plasterboard have a £400 ex VAT minimum plus £40 ex VAT delivery. We do not deliver to Northern Ireland or the Scottish islands.; see our delivery policy. If you are not sure which board suits your job, call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm) or email sales@insulationwarehousedirect.co.uk and we will help you choose.
Sources: Approved Document L: Conservation of fuel and power (Volume 1, Tables 4.1 to 4.3); IKO technical guidance on BS 5250; BMI summary of BS 5250 for flat roofs. Board storage: Recticel Eurothane GP datasheet. Board data from our live product pages.