U-value of an uninsulated flat roof (and a warm-deck upgrade)

An old felt roof with nothing in it, and what re-roofing as a warm deck achieves.

An old flat roof with no insulation — felt on an 18mm deck, an empty 150mm void between the joists and a plasterboard ceiling — has a U-value of about 1.8 W/m²K with the void sealed. If the void is ventilated to the outside, BR 443 disregards the deck and the felt above it and the roof is about 3.9. Re-roofing it as a warm deck with 150mm of PIR over the existing deck brings it to about 0.14, inside the 0.16 Approved Document L asks of a renovated roof; 140mm gives 0.15, and 120mm about 0.17, which misses.

Open this build-up in U-Monkey — pre-filled, free

Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report

The build-up (outside → inside)

Joists at 12% timber. The pre-filled link opens as an existing roof, judged against Approved Document L’s 0.35 threshold (2021 edition, Table 4.3); switch the work type to “upgrade” and add the warm deck to check it against 0.16.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Void sealed, plywood deck≈ 1.83As pre-filled
Void sealed, chipboard deck≈ 1.86
Void ventilated to outside≈ 3.85The deck and felt are then disregarded
+ 100mm PIR warm deck over the old deck≈ 0.20
+ 120mm PIR warm deck≈ 0.17Misses the 0.16 improved value
+ 140mm PIR warm deck≈ 0.15Meets 0.16 (0.152)
+ 150mm PIR warm deck≈ 0.14Meets 0.16

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

With no insulation, the air in the joist void is most of the resistance, and ventilating it to the outside roughly doubles the U-value. Insulated, the board thickness is nearly everything; the old deck, void and ceiling add only a little. The usual upgrade is to strip the felt and lay a new vapour control layer, the PIR and a new membrane on the existing deck, which keeps the structure warm and the void dry; the existing void must then not be ventilated from outside. The warm flat roof page explains why the vapour control layer under the boards decides the condensation result. Adding 140–150mm raises the roof edge, and Approved Document L allows a lesser standard where upstand heights or the deck’s load capacity make 0.16 impractical (Table 4.3, note 4).

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of an uninsulated flat roof (and a warm-deck upgrade)?

An old flat roof with no insulation — felt on an 18mm deck, an empty 150mm void between the joists and a plasterboard ceiling — has a U-value of about 1.8 W/m²K with the void sealed. If the void is ventilated to the outside, BR 443 disregards the deck and the felt above it and the roof is about 3.9. Re-roofing it as a warm deck with 150mm of PIR over the existing deck brings it to about 0.14, inside the 0.16 Approved Document L asks of a renovated roof; 140mm gives 0.15, and 120mm about 0.17, which misses.

Do I have to insulate a flat roof when I re-cover it?

Usually, yes. Replacing the covering of more than half of a roof is renovating a thermal element (Building Regulations 2010, regulation 23), so the whole roof should be brought up to Approved Document L’s 0.16 W/m²K (Table 4.3) so far as that is technically, functionally and economically feasible — paragraph 4.13 uses a simple payback of 15 years or less. An uninsulated flat roof is far worse than the 0.35 threshold, and a warm deck over the old deck is normally the practical way to do it.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Roofs

Walls

Floors

Different build-up? See the full flat roof U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

Open this build-up pre-filled or call 01202 623236