U-value of an inverted flat roof

Insulation laid over the waterproofing and held down by ballast — where the rain correction decides the thickness.

An inverted flat roof with 250mm of XPS laid over the waterproofing of a concrete deck, in two layers and taken at the design λ BR 443 asks for in an inverted roof, achieves about 0.14 W/m²K, provided a certified water flow reducing layer (WFRL) goes over the boards. Leave it out and the rain that runs under the insulation carries heat away: BS EN ISO 6946 adds a correction for it — about 0.06 W/m²K at Manchester’s rainfall for these rebated boards under gravel — and the same roof comes out near 0.20. In an inverted roof it is that correction, not condensation, that decides how much insulation you need.

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The build-up (outside → inside)

Bauder XPS 300 Inverted at its design λ, 0.035 (it declares 0.033), in two layers with staggered joints: BR 443’s air-gap level 0. The table and the pre-filled link use Manchester’s October–May rainfall (2.16 mm/day), U-Monkey’s default station. The rain correction scales with rainfall, so wetter sites lose more: pick your nearest station in the app.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
250mm XPS (150 + 100) + certified WFRL (f·x 0.001)≈ 0.14Meets the 0.15 for a new roof on an extension
250mm XPS, no WFRL, gravel ballast (f·x 0.03, rebated boards)≈ 0.20Same boards, with rain running underneath
300mm XPS (2 × 150), no WFRL (f·x 0.03)≈ 0.17Still short of 0.15: more board doesn’t replace the WFRL
300mm XPS (2 × 150) + certified WFRL≈ 0.12

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

The rain correction is the number to watch. BS EN ISO 6946 adds ΔU = p × f·x × (R1/RT)², where p is the site’s average daily rainfall from October to May, f·x is how much of it reaches the waterproofing and how much heat it takes with it, and R1/RT is the share of the roof’s thermal resistance that sits above the membrane. Without a certified value, f·x is 0.04 for butt-jointed boards under open gravel, 0.03 for rebated boards — Bauder’s are rebated on all four sides — and 0.02 under a green roof or a cast concrete parking deck. A certified water flow reducing layer brings it down to around 0.001, the value this build-up opens with, but take the figure from your system’s certificate. U-Monkey fills p from the nearest Met Office station, so pick yours: the correction scales with the rain. The figures here take Bauder XPS 300 Inverted at its design λ, 0.035, not the 0.033 it declares: BR 443 (§4.8.6, after ETAG 031) asks for the design value in an inverted roof, which allows for the moisture the boards take up in use. Laid in two layers with staggered joints, as here, the boards take BR 443’s air-gap level 0; one layer of butt-jointed boards takes level 1, about 0.01 worse (rebated boards in one layer qualify for level 0 too — choose Perfect under Fitting quality).

Condensation risk in an inverted roof

Turning the roof upside down turns the condensation question around: the vapour-tight waterproofing sits under the insulation, on the warm side, where a warm deck would have its vapour control layer. As pre-filled, this roof passes U-Monkey’s free BS EN ISO 13788 condensation check with no condensation in any month, with the roof surface taken 2 K below the air as the standard asks, and it stays clear at the upland Eskdalemuir station and at humidity class 4. The margin is tight, at about 97% of saturation inside the XPS in December, but the verdict holds with the assumed vapour values for the XPS and the membrane tested across their ranges, one at a time and together.

The same 250mm of XPS over the waterproofing of an 18mm plywood deck on timber joists passes too. What the screening can’t see is cold rainwater running between the boards and the membrane — it works month by month, in steady state — which is why the rain correction in the U-value, and the water flow reducing layer that keeps it small, matter more here than the condensation check.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer, a vapour-open underlay, a ventilated void, a warm deck — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 inverted flat roof?

An inverted flat roof with 250mm of XPS laid over the waterproofing of a concrete deck, in two layers and taken at the design λ BR 443 asks for in an inverted roof, achieves about 0.14 W/m²K, provided a certified water flow reducing layer (WFRL) goes over the boards. Leave it out and the rain that runs under the insulation carries heat away: BS EN ISO 6946 adds a correction for it — about 0.06 W/m²K at Manchester’s rainfall for these rebated boards under gravel — and the same roof comes out near 0.20. In an inverted roof it is that correction, not condensation, that decides how much insulation you need.

Is there a condensation risk in an inverted roof?

Low, as a rule. The waterproofing is vapour-tight, and in an inverted roof it sits under the insulation on the warm side, so it stays above the dew point and does the job a vapour control layer does in a warm deck. U-Monkey’s free check on this build-up finds no interstitial condensation in any month. What a monthly screening can’t model is cold rain running under the boards; the U-value correction above accounts for the heat it carries away.

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.

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