U-value of a warm flat roof on a profiled metal deck

Single ply over PIR on a trapezoidal steel deck — the commercial flat roof, with the fasteners counted as BR 443 asks.

A warm flat roof on a profiled (trapezoidal) steel deck — a polythene vapour control layer on the deck, 150mm of PIR fixed through to it with five 4.8mm galvanised steel fasteners per m², and a mechanically fixed PVC single-ply membrane — has a U-value of about 0.18 W/m²K, just inside the 0.18 that Approved Document L Volume 2 sets for a new flat roof on a building other than a dwelling (0.176). The fasteners matter more than on any house roof: they add about 0.023, so the same roof held down with telescopic tube fasteners, whose steel screw sits recessed in a plastic tube and which BR 443 lets you leave out, reads 0.15 — and 140mm with galvanised fasteners misses, at 0.19. The air in the deck’s troughs counts for nothing (BR 443 §4.7.7).

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Every layer editable · live U-value · Part L Volume 2 targets (buildings other than dwellings) · free PDF report

The build-up (outside → inside)

Fasteners as U-Monkey’s “Roof fasteners, galvanised” preset: 4.8mm steel screws (18.1mm², λ 50), five per m² for board and membrane fixings together, through the full board; enter your own density from the wind-load calculation. Telescopic rows use the “Telescopic fasteners” preset, which records BR 443’s exemption. The pre-filled link opens in U-Monkey’s England · non-domestic regime (Approved Document L Volume 2), at BR 443’s air-gap level 1 for a single layer of boards.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
120mm PIR, 5 galvanised fixings per m²≈ 0.22
140mm PIR, galvanised fixings≈ 0.19Misses 0.18 (0.188); a table without the air-gap term reads 0.18 here
150mm PIR, galvanised fixings≈ 0.18As pre-filled: meets 0.18 (0.176)
140mm PIR, stainless fixings≈ 0.17Stainless (λ 17) costs about a third as much as galvanised steel
140mm PIR, telescopic tube fasteners≈ 0.16No fixings correction (BR 443 §4.8.3)
150mm PIR, telescopic tube fasteners≈ 0.150.153
160mm as 2 × 80mm, staggered, telescopic fasteners≈ 0.13Two layers with staggered joints: no air-gap correction either
150mm PIR, 10 galvanised fixings per m² (a corner zone)≈ 0.20Corners and edges carry more fasteners for wind uplift
150mm tissue-faced PIR (λ 0.024), galvanised fixings≈ 0.19Boards for bonded or torch-on membranes, λ by thickness band
200mm stone wool roof board (λ 0.039), galvanised fixings≈ 0.22Non-combustible

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

On a metal deck the fasteners are part of the U-value. BR 443 (2019) §4.8.3 counts both the board fixings and the membrane fixings, each by BS EN ISO 6946 Annex F — λ of the fastener, its cross-section, the number per m² and how deep it runs into the insulation — and five 4.8mm galvanised screws per m² through 150mm of PIR add about 0.023 W/m²K. The same clause lets one kind off: “No correction need be applied in the case of fixings in a flat roof where the metal part … is recessed by at least 50% of the length of the fixing and the density of fixings … does not exceed 15 fixings per square metre” — the telescopic tube fastener. Sika, which makes one, says standard steel fasteners “can add in excess of 10mm to the required thickness”; here they make the difference between 150mm and 130mm for 0.18. Stainless screws (λ 17) cost about a third of the galvanised correction. The density comes from the membrane maker’s wind-uplift calculation and is higher at edges and corners. Published tables differ on the air-gap term: NSAI Agrément 23/0436 (Unilin) gives 140mm with five galvanised insulation fixings per m² for 0.18, which U-Monkey matches with no air-gap correction (0.178); with BR 443’s level 1 for a single layer of butt-jointed boards it reads 0.188, while two layers with staggered joints, or boards with lapped or sealed joints, take level 0. The air in the profile troughs counts for nothing, “No allowance for these should be made in U-value calculations” (§4.7.7), and composite panels and twin-skin metal roofs are outside this kind of calculation — BR 443 sends them to numerical analysis. Approved Document L Volume 2 sets 0.18 for a new flat roof (Table 4.1; the 2026 edition keeps it). Over-roofing or re-covering more than half of an existing roof is renovation (regulation 23), and the renovated roof should reach 0.18 too (Table 4.2, column (b)) where that is technically, functionally and economically feasible, with a lesser standard allowed where the frame’s load-bearing capacity or the upstand heights prevent it — common when an old metal roof is over-roofed; the table’s 0.35 threshold is for a roof kept through a change of use or of energy status.

Condensation risk on a metal deck

U-Monkey’s free BS EN ISO 13788 check gives the pre-filled roof a conditional pass: a trace of seasonal condensation (a peak under 0.1 g/m²) that dries out in summer, but condensate builds up year on year if the PIR, the polythene vapour control layer and the deck all sit at the unfavourable ends of their tested ranges at once — the deck taken with its laps unsealed. On a profiled deck the vapour control layer does the work, so it must be sealed at every lap, penetration and upstand, and the fasteners that pass through it make that harder.

The membrane moves the result: a PVC single ply (sd about 30m) lets the roof dry upwards a little, while a polyolefin or bitumen membrane is ten times tighter and needs a stronger vapour control layer below it. For a pool, a sports hall or anything else with high humidity, raise the humidity class and run the check again.

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 building'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 a warm flat roof on a profiled metal deck?

A warm flat roof on a profiled (trapezoidal) steel deck — a polythene vapour control layer on the deck, 150mm of PIR fixed through to it with five 4.8mm galvanised steel fasteners per m², and a mechanically fixed PVC single-ply membrane — has a U-value of about 0.18 W/m²K, just inside the 0.18 that Approved Document L Volume 2 sets for a new flat roof on a building other than a dwelling (0.176). The fasteners matter more than on any house roof: they add about 0.023, so the same roof held down with telescopic tube fasteners, whose steel screw sits recessed in a plastic tube and which BR 443 lets you leave out, reads 0.15 — and 140mm with galvanised fasteners misses, at 0.19. The air in the deck’s troughs counts for nothing (BR 443 §4.7.7).

Do roof fasteners really change the U-value?

On a warm roof over a metal deck, yes — enough to decide a pass. Five 4.8mm galvanised steel fasteners per m², the board and membrane fixings together, add about 0.023 W/m²K to 150mm of PIR (0.153 becomes 0.176), so 140mm, which would pass without them, reads 0.19 against Approved Document L Volume 2’s 0.18. Stainless fasteners add about a third as much; telescopic tube fasteners, with the steel recessed at least halfway down a plastic tube, need no correction at all at up to 15 per m² (BR 443 §4.8.3). In U-Monkey, open the insulation layer’s fixings sheet and pick the galvanised, stainless or telescopic preset, then set your own density.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (Approved Document L Volume 2 for buildings other than dwellings, and the Welsh and Scottish equivalents), 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? For a building other than a dwelling, U-values feed the SBEM calculation that checks the building’s emission and primary energy rates against their targets (BER and BPER against TER and TPER) — and the non-domestic U-value page has the Volume 2 limits. 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