U-value of a built-up (twin-skin) metal roof

A built-up (twin-skin) metal roof — a 0.4mm steel liner sealed as the air and vapour control layer, a rail-and-bracket spacer system with rails on the purlins every 1.8m and a bracket every 1.2m along them, 260mm of glass mineral wool cladding roll (λ 0.040) filling the void, and a 0.7mm profiled steel outer sheet — has a U-value of about 0.17 W/m²K. That meets the 0.18 Approved Document L Volume 2 sets for a flat roof, and 280mm (0.16) meets the 0.16 it sets for a pitched one. The rails and brackets are part of the number: BR 443 says metal cladding “needs to take account of … any metallic components within the insulation”, and by SCI P312, which it gives as the alternative to a numerical calculation for rails and brackets, they add about 0.01 here — enough that 240mm (0.182) just misses 0.18.

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

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

The build-up (outside → inside)

0.7mm profiled steel outer sheet
260mm glass mineral wool cladding roll (Knauf Insulation FactoryClad Roll 40, λ 0.040) filling the void, crossed by the spacer system: 40 × 40mm rails of 1.2mm steel on the purlin lines every 1.8m, and brackets on thermal break pads every 1.2m along them (0.46 per m², 60mm² each)
0.4mm steel liner sheet, laps sealed — the air and vapour control layer

The pre-filled link opens in the England · non-domestic regime with Metal cladding set to Built-up (SCI P312): rails 40 × 40mm of 1.2mm steel, one thickness crossing the quilt, every 1.8m; 0.46 brackets per m² of 60mm² (P312’s example bracket, 50 × 1.2mm), steel at λ 60 as P312 takes it; no liner ribs pressing the quilt. The air round the rails takes BS EN ISO 6946’s value for upward heat flow (0.16), where P312’s worked example, a wall, used 0.215. Heat flows upward (Rsi 0.10); the quilt is at BR 443’s air-gap level 1.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
200mm, rails every 1.8m, brackets every 1.2m	≈ 0.22	Fails 0.18
240mm	≈ 0.18	Just misses 0.18 (0.182); Tata Steel’s 3-D figure for its own system at 240mm is 0.18
260mm	≈ 0.17	As pre-filled (0.169)
280mm	≈ 0.16	Meets the 2021 pitched-roof 0.16 (0.158)
300mm	≈ 0.15	0.149
260mm, 0.8 brackets per m²	≈ 0.18	0.176: SCI P312’s default density
260mm, rails every 1.2m and 0.8 brackets per m² (P312’s defaults)	≈ 0.18	0.177
220mm FactoryClad Roll 35 (λ 0.035)	≈ 0.18	0.176: 40mm less depth than Roll 40 for 0.18
240mm FactoryClad Roll 35 (140 + 100mm rolls, entered as one layer)	≈ 0.16	0.163
260mm, rails and brackets not counted	≈ 0.16	Not allowed: BR 443 §4.9 (0.160)

Figures computed by SCI P312 for the rails and brackets (BR 443 §4.9.1) — the quilt’s rail zone bridged by the rail steel and the air round the rails, the limits weighted by P312’s p, the bracket, air-gap and any liner-rib corrections added — exactly as the pre-filled calculator works them out with Metal cladding set to Built-up, with any pass or fail judged on the unrounded U-value. Indicative for the rails and brackets shown: your system’s spacing, bracket size and quilt will move the number, and the maker’s own 3-D figure is the one to use where it has one.

What changes the number

Twin-skin metal cladding is beyond the plain BS EN ISO 6946 sum because steel crosses the insulation. BR 443 (2019) §4.9: “The U-value of metal-clad walls and roofs needs to take account of joints between panels and any metallic components within the insulation, including through fixings”; its roof table asks for numerical analysis “to allow for the effect [of] metal elements (such as spacer systems)”, with “the method in SCI P312 … as an alternative for rail-and-bracket systems”, as §4.9.1 says too. SCI P312 (Steel Construction Institute, 2002) is what U-Monkey uses when Metal cladding is set to Built-up. It splits the quilt into the rail zone, bridged by the rail steel and by the air the rail squeezes out of the quilt (“twice the width of the rail”), and the rest, crossed only by the brackets, each costing ΔU = 1.6·λ·A·n·(R_i/R_ub)²/d_i on its cross-section A and number per m² n; the two resistance limits are weighted by p = 0.8·R_min/R_max + 0.44 − 0.1·w/40 − 0.2·600/s − 0.04·d/100 (P312’s equation 3, based on the one for light steel framing), and the brackets must sit on thermal break pads. The method is for rail-and-bracket spacers: Z-spacers and standing-seam clips or halters need the system maker’s figures. U-Monkey reproduces P312’s worked example (U 0.255) and lands close to UK makers’ own 3-D figures at their spacings: Tata Steel’s Trisobuild R (computer modelled to BS EN ISO 10211, steelwork at 1.8m, brackets at 1.2m) gives 0.21 at 200mm, 0.18 at 240mm and 0.15 at 280mm against U-Monkey’s 0.22, 0.18 and 0.16; Euroclad’s Elite 1 (brackets every 1.167m on bars 1.5m apart) lists 0.19 at 240mm and 0.16 at 280mm. So the depth and λ of the quilt come first — a 0.035 roll meets 0.18 at 220mm, 40mm less than a 0.040 one — then the bracket density, which the purlin spacing and the wind load set (P312’s method covers rails at least 1m apart and up to 1.3 brackets per m²). Fill the void without over-compressing the quilt: BR 443 §4.9.2 corrects for shallow liner ribs that press it (no deeper than 25mm, at least 250mm apart), and otherwise says to “measure the insulation thickness at its thinnest point”. Approved Document L Volume 2 (2021) sets 0.18 for a flat roof and 0.16 for a pitched one, and its Table 4.1 gives no dividing pitch; U-Monkey treats the low pitches profiled metal roofs are laid to (Tata Steel’s minimum for Trisobuild is 4°) as flat, so check which your building control body applies. The 2026 edition, which takes effect on 24 March 2027 (24 September 2027 for higher-risk building work, with transitional arrangements), sets one 0.18 for every roof. For a new building the test is the whole-building calculation — the BER and BPER against the TER and TPER — where the NCM notional building takes 0.15 for a side-lit building’s roof and 0.18 for a top-lit one’s. Over-cladding an old roof is renovation, with 0.18 the improved value (Table 4.2).

Condensation risk in a built-up metal roof

U-Monkey’s free BS EN ISO 13788 check gives the pre-filled roof a conditional pass: no interstitial condensation in any month while the steel liner works as a sealed air and vapour control layer, but condensate builds up year on year if its laps, fixings and penetrations are left unsealed — the check tests the liner unsealed as well as sealed. With steel on both faces nothing dries out through the outer sheet, so the liner is what keeps the quilt dry.

A perforated (acoustic) liner lets vapour through. With no vapour control layer the same roof collects about 7 kg/m² a year in the check, and Knauf’s guidance is plain: “Where a perforated liner sheet is used then a separate vapour control layer should always be installed.” Which one depends on the humidity class and the liner: Kalzip, for one, recommends its foil VCL (43,000 MN·s/g, about sd 8,600m) under a perforated liner in dwellings of low occupancy, and its BBA certificate asks for a hygrothermal assessment in humidity classes 4 and 5 (sports halls, kitchens, pools). U-Monkey’s check takes the outer sheet as vapour-tight, so it asks a lot of that layer: a polythene VCL on the perforated liner still leaves 52 g/m² a year, and a foil one clears it. Where the check and a system maker’s guidance differ, ask the maker for the condensation assessment behind its recommendation, and for a high-humidity use 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 (f_Rsi) 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 built-up (twin-skin) metal roof?

How many brackets per m² should I enter?

One over the rail spacing times the bracket spacing along the rail, both in metres: rails on purlins 1.8m apart with a bracket every 1.2m is 1 ÷ (1.8 × 1.2) = 0.46 per m², the case pre-filled here. SCI P312’s default is 0.8, and its method covers up to 1.3; average over the whole roof, as edges and corners carry more for wind uplift. On this roof each extra 0.1 bracket per m² adds about 0.002 W/m²K, so the count matters less than the depth of the quilt. Enter the bracket’s cross-section at its thickest part (P312’s example is 50 × 1.2mm, 60mm²). In U-Monkey, set Metal cladding to Built-up and type them in.

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, and for built-up metal cladding SCI P312, which BR 443 §4.9.1 gives as an alternative for rail-and-bracket systems; BS EN ISO 13788 for the condensation 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.