U-value of tapered roof insulation

How a tapered flat-roof scheme’s U-value is averaged, the thinnest point it must not fall below, and typical thicknesses.

A tapered PIR scheme (λ 0.022) falling 1:60 over a 6m run, from 100mm at the outlet to 200mm, on a 150mm concrete deck averages about 0.15 W/m²K (0.147) — within the 0.15 Approved Document L sets for a new roof on an existing home and the 0.16 for a new home. That is for the usual build-up, tapered boards over a flat base board: laid as a single layer of tapered boards it takes BR 443’s default air-gap correction and averages 0.156, which misses 0.15, so start it at 110mm (0.146). The average has to be taken over the area, as BS EN ISO 6946 (Annex E) does, not by averaging the thickness: the thin end loses more heat than the thick end saves, so 80 → 180mm, which averages 130mm, gives 0.170 — not the 0.162 that the average thickness suggests. BS 6229 adds a limit of its own: no point on a heated roof worse than 0.35, which on this deck needs at least 58mm of foil-faced PIR at the thinnest point (59mm as a single layer).

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The build-up (outside → inside)

The link opens as a new roof on an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2), with U-Monkey’s tapered option on (a one-way fall, BS EN ISO 6946:2017 Annex E; tapered boards over a flat base board, so no air-gap correction) and the deck below as flat layers. The condensation check works on one section through the roof, so it is not run on the tapered scheme: the condensation button below opens the thinnest section instead, as a flat 100mm board.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
60 → 160mm (1:60 over 6m)≈ 0.20Thinnest point 0.34, just inside BS 6229’s 0.35
80 → 180mm≈ 0.17Thinnest point 0.26; averaging the thickness (130mm) would suggest 0.162
100 → 200mm≈ 0.15As pre-filled: 0.147; thinnest point 0.21
100 → 200mm as one layer of tapered boards≈ 0.16BR 443’s level 1 air-gap correction: 0.156, misses 0.15; thinnest point 0.22
110 → 210mm as one layer≈ 0.15Meets 0.15 (0.146)
120 → 220mm≈ 0.13
80 → 230mm (1:60 over 9m)≈ 0.15A longer run lowers the average, but the outlet still sets the thinnest point (0.26)
30 → 130mm≈ 0.29Fails BS 6229 at the outlet: 0.62 at the thinnest point

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions, the tapered course averaged over its area by BS EN ISO 6946 Annex E (BR 443 §8) — laid over a flat base board, so with no air-gap correction on it unless a row says one layer — 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, fall and layout will move the number, and that's what the calculator is for.

What changes the number

The fall, the run and the thickness at the outlet. A fall of 1:60 adds about 17mm per metre of run, so a 6m run adds 100mm and a 9m run 150mm; the outlet thickness sets the BS 6229 check and much of the average. Bauder’s summary of BS 6229:2025 says the old advice to design for 1:40 to achieve 1:80 has gone: 1:80 is the minimum completed fall on most flat roofs, and a design fall of 1:60 a reasonable starting point in many cases. U-Monkey’s tapered option takes a one-way fall over a rectangle — BS EN ISO 6946’s Annex E formula, U = ln(1 + R₁/R₀)/R₁, with R₀ the total resistance at the thinnest point and R₁ the extra resistance at the thickest — which suits a single-fall roof; hips, valleys and sumps form triangles, which Annex E handles with formulas of their own, and a whole roof is the area-weighted average of its parts, so makers design each scheme to the project. The pre-filled scheme is tapered boards over a flat base board, double-layer insulation, so BR 443 sets its air-gap correction at level 0; a single layer of tapered boards takes level 1, about 0.009 more averaged over the fall (0.156 at 100 → 200mm), and needs 59mm rather than 58mm at the outlet for BS 6229’s 0.35 — U-Monkey’s tapered option asks which it is. Fixings are a correction of their own, which the tapered option leaves to you: telescopic fasteners need none (BR 443 §4.8.3), but five galvanised screws per m² through a 100 → 200mm taper would add about 0.02, stainless ones about 0.01.

Condensation risk with tapered insulation

The thinnest section is the worst case, so that is the one to check. Entered as a flat 100mm board under the membrane — the button below opens it that way — the pre-filled scheme’s outlet gives a conditional pass in U-Monkey’s BS EN ISO 13788 check: about 4 g/m² of seasonal condensation under the membrane in February that evaporates in summer. That link is the outlet section on its own, so the U-value and verdict the app shows with it (0.22, a fail) are the section’s, not the scheme’s 0.147. BS 6229’s 0.35 limit exists for surface condensation at the low points — outlets, gutters and sumps — which is why the thinnest point matters as much as the average.

Check the thinnest section 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 tapered roof insulation?

A tapered PIR scheme (λ 0.022) falling 1:60 over a 6m run, from 100mm at the outlet to 200mm, on a 150mm concrete deck averages about 0.15 W/m²K (0.147) — within the 0.15 Approved Document L sets for a new roof on an existing home and the 0.16 for a new home. That is for the usual build-up, tapered boards over a flat base board: laid as a single layer of tapered boards it takes BR 443’s default air-gap correction and averages 0.156, which misses 0.15, so start it at 110mm (0.146). The average has to be taken over the area, as BS EN ISO 6946 (Annex E) does, not by averaging the thickness: the thin end loses more heat than the thick end saves, so 80 → 180mm, which averages 130mm, gives 0.170 — not the 0.162 that the average thickness suggests. BS 6229 adds a limit of its own: no point on a heated roof worse than 0.35, which on this deck needs at least 58mm of foil-faced PIR at the thinnest point (59mm as a single layer).

Can you average the thickness of tapered insulation to get its U-value?

No. Heat flows in proportion to 1/R, not to thickness, so the U-value has to be averaged over the area: BS EN ISO 6946 Annex E gives the formulas, and BR 443 (§8) sends tapered layers to it. Averaging the thickness flatters the result — 80 → 180mm of PIR on a concrete deck gives 0.170 by Annex E, against 0.162 from its 130mm average thickness — and Gradient Roofing made the same point in 2016 with a 1:60 scheme for 0.18 that needed 80 → 200mm by the area method, where averaging the thickness said 70 → 190mm would do. Approved Document L judges the area-weighted average; BS 6229 separately caps any point at 0.35.

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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