U-value of a solid (tiled) conservatory roof

Replacing a glazed or polycarbonate conservatory roof with an insulated tiled one: what it must reach, and what does.

A solid replacement conservatory roof with 150mm of PIR between the rafters and 40mm of PIR under them, tiled outside and plasterboarded inside, has a U-value of about 0.15 W/m²K (0.146), meeting the 0.15 Approved Document L sets for a new roof element in an existing home. 140mm between and 40mm under shows as 0.15 but misses, at 0.152, and 100mm between with 50mm under gives about 0.17. Makers of proprietary lightweight roof systems quote their own U-values, so ask for the calculation behind the figure for the build-up you are buying.

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The build-up (outside → inside)

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). Rafters at 12% timber; the tiles and their batten space lie outside a ventilated layer and are not counted (BR 443).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR between + 25mm under≈ 0.22
100mm between + 50mm under≈ 0.17Misses 0.15
120mm between + 50mm under≈ 0.16Misses: 0.155
140mm between + 40mm under≈ 0.15Misses: 0.152
150mm between + 40mm under≈ 0.15As pre-filled: meets (0.146)
150mm between + 50mm under≈ 0.14

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 rafter depth decides how much fits between, and the layer under the rafters covers the 12% that is timber, so the two together set the figure. A conservatory roof is small and full of junctions — the ring beam on the old frame, the box gutter, the wall plate against the house — and those details, with the windows and the walls below, decide how warm the room feels as much as the roof’s U-value does. Many proprietary systems use aluminium or steel beams, which conduct far better than timber; for those, use the U-value in the system’s own certificate or registered detail, which should allow for the metal.

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 solid (tiled) conservatory roof?

A solid replacement conservatory roof with 150mm of PIR between the rafters and 40mm of PIR under them, tiled outside and plasterboarded inside, has a U-value of about 0.15 W/m²K (0.146), meeting the 0.15 Approved Document L sets for a new roof element in an existing home. 140mm between and 40mm under shows as 0.15 but misses, at 0.152, and 100mm between with 50mm under gives about 0.17. Makers of proprietary lightweight roof systems quote their own U-values, so ask for the calculation behind the figure for the build-up you are buying.

Do I need building regulations approval to put a solid roof on a conservatory?

Usually, yes. A conservatory of up to 30m² at ground level is exempt from the Building Regulations (Schedule 2, Class 7) while it stays thermally separate from the house and the house’s heating is not extended into it (Approved Document L 2021, paragraph 0.14). LABC’s guidance on solid conservatory roofs (2013, updated 2018) says that work which significantly reduces the glazing or the translucency of the roof loses that exemption, so the new roof needs approval and should meet current standards as a new thermal element — 0.15 W/m²K in Table 4.2 — with the structure checked for the extra weight and the existing walls and floor left no worse than before.

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.

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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