U-value of a garden room (garden office) roof

The flat roof of a garden room: a warm deck or insulation between the joists, and what each thickness reaches.

A garden room roof built as a warm deck — 100mm of PIR above an 18mm OSB deck, under EPDM, with a vapour control layer and a plasterboard ceiling — has a U-value of about 0.20 W/m²K; 120mm gives about 0.17 and 75mm about 0.26. Many garden rooms put the PIR between the joists instead, under an EPDM-covered deck, which is cheaper and lower: 100mm that way gives about 0.27, because the joists bridge it. That cold deck build-up is the one BS 6229:2018 (since revised) said is not now recommended, and U-Monkey’s condensation check reports it as a cold deck — the warm deck is the better roof for a room that will be heated.

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The build-up (outside → inside)

Joists at 12%, the void between them unventilated. In England and Wales, detached (stand-alone) garden rooms of under 50m² that are not dwellings are outside the energy efficiency requirements (Building Regulations 2010, regulation 21(3)(d)), so no U-value is required; the figures are for comparing roofs.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
50mm PIR warm deck≈ 0.36
75mm PIR warm deck≈ 0.260.259
100mm PIR warm deck≈ 0.20As pre-filled
120mm PIR warm deck≈ 0.17
75mm PIR between the joists, EPDM on the deck (cold deck, unventilated)≈ 0.33U-Monkey’s check: cold deck
100mm PIR between the joists (cold deck, unventilated)≈ 0.27U-Monkey’s check: cold deck

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

Where the insulation goes matters as much as how much. Above the deck it is continuous; between the joists it is bridged by them (12% here), so 100mm between the joists (0.27) does about as well as 75mm above the deck (0.26). The bigger difference is the deck’s temperature. In a warm deck the OSB sits inside the insulation, at about room temperature; in a cold deck it sits outside it, at about 2 °C in U-Monkey’s check for a February in Manchester, well below the room air’s dew point, so any warm air that gets past the ceiling condenses on it. BS 5250:2021 asks for a cold deck to have a ventilated void at least 50mm deep above the insulation, with cross-ventilation equal to a continuous 25mm opening on each side, and a sealed vapour control layer; a garden room’s roof is rarely deep enough to give the void and still insulate well, which is why the warm deck is the sound choice.

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 garden room (garden office) roof?

A garden room roof built as a warm deck — 100mm of PIR above an 18mm OSB deck, under EPDM, with a vapour control layer and a plasterboard ceiling — has a U-value of about 0.20 W/m²K; 120mm gives about 0.17 and 75mm about 0.26. Many garden rooms put the PIR between the joists instead, under an EPDM-covered deck, which is cheaper and lower: 100mm that way gives about 0.27, because the joists bridge it. That cold deck build-up is the one BS 6229:2018 (since revised) said is not now recommended, and U-Monkey’s condensation check reports it as a cold deck — the warm deck is the better roof for a room that will be heated.

Warm or cold deck for a garden room roof?

Warm. The cold deck — insulation between the joists, waterproofing straight onto the deck above them — is the build-up BS 6229:2018 (revised in 2025) noted is not now recommended, because the deck is cold and any room air that reaches it condenses, and BS 5250:2021 only accepts it with a ventilated void at least 50mm deep above the insulation, cross-ventilated on both sides, and a sealed vapour control layer. Unventilated, U-Monkey’s condensation check reports it as a cold deck, outside the scope of a calculation. A warm deck with 100mm of PIR above the deck gives about 0.20 W/m²K and keeps the deck at room temperature; its check is a conditional pass, with a few grams of seasonal condensation under the waterproofing.

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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