U-value of a wood-fibre sarking roof (a vapour-open warm pitched roof)

Wood-fibre boards over the rafters and batts between them: what each combination reaches.

A pitched roof with 100mm of wood-fibre sarking board over the rafters (λ 0.043) and 200mm of wood-fibre batts between them (λ 0.036), with a plasterboard ceiling, has a U-value of about 0.14 W/m²K — inside the 0.15 for a new roof on an extension and the 0.16 limiting value for a new house. On existing 150mm rafters the same board gives about 0.17, short of the 0.16 Approved Document L asks of a renovated roof. The common 60mm board over 150mm of batts gives about 0.20.

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The build-up (outside → inside)

Wood fibre as in U-Monkey’s library: STEICOuniversal dry sarking board (λ 0.043, made 35–100mm) and STEICOflex 036 batts; rafters at 12% timber. The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
60mm sarking + 150mm between≈ 0.20
60mm sarking + 200mm between≈ 0.160.1646
80mm sarking + 200mm between≈ 0.15Misses 0.15: 0.152
100mm sarking + 150mm between (150mm rafters)≈ 0.17Misses the 0.16 for a renovated roof
100mm sarking + 200mm between≈ 0.14As pre-filled
120mm sarking (λ 0.041) + 200mm between≈ 0.13

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 board over the rafters counts most: it is continuous, while the rafters bridge 12% of the batts between them, and wood fibre’s λ (0.036–0.043) means it takes about twice the thickness of PIR for the same effect. Deep rafters therefore help a lot — on 150mm rafters even 100mm of sarking only reaches about 0.17. The boards go on from outside, so this is usually done on a new roof or when a roof is stripped and re-covered, with counter-battens over the boards to keep a ventilated space under the tiles, which BR 443 leaves out of the calculation. The whole build-up is vapour-open, so the airtight, vapour-control layer on the inside decides the condensation risk.

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 a wood-fibre sarking roof (a vapour-open warm pitched roof)?

A pitched roof with 100mm of wood-fibre sarking board over the rafters (λ 0.043) and 200mm of wood-fibre batts between them (λ 0.036), with a plasterboard ceiling, has a U-value of about 0.14 W/m²K — inside the 0.15 for a new roof on an extension and the 0.16 limiting value for a new house. On existing 150mm rafters the same board gives about 0.17, short of the 0.16 Approved Document L asks of a renovated roof. The common 60mm board over 150mm of batts gives about 0.20.

What U-value does a re-roofed roof need?

Replacing the covering of more than half of a roof is renovating a thermal element (Building Regulations 2010, regulation 23), so in England the roof should be brought to Approved Document L’s 0.16 W/m²K (Table 4.3) so far as that is technically and functionally feasible with a simple payback of 15 years or less (paragraph 4.13). With wood fibre that usually means deepening shallow rafters or using thicker boards: 100mm of sarking over 150mm rafters gives about 0.17, over 200mm rafters about 0.14.

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 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.

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