U-value of a roof terrace with paving on pedestals

An inverted roof under paving on pedestals: the rain correction, the reducing layer and the height at the door.

A roof terrace built as an inverted roof — two layers of 120mm extruded polystyrene (Bauder XPS 300 Inverted, design λ 0.035) over hot-melt waterproofing on a 200mm concrete slab, under a water flow reducing layer, with paving on pedestals above — has a U-value of about 0.14 W/m²K, inside the 0.15 Approved Document L sets for a new roof on an existing dwelling. The reducing layer is what makes that true: paving on pedestals is an “open covering” to BR 443, so without a tested reducing layer the cold rainwater running under the boards adds a correction of f·x 0.03 (rebated boards) or 0.04 (butt-jointed) — 0.20 and 0.22 for the same 240mm at Manchester’s winter rainfall, and 0.26 at Eskdalemuir’s. The paving and pedestals are left out of the calculation.

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

Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report

The build-up (outside → inside)

Inverted-roof XPS at Bauder’s design λ (0.035 for XPS 300 Inverted), as BR 443 §4.8.6 asks. The rain correction is U-Monkey’s inverted-roof option, at Manchester’s October–May mean rainfall (2.16 mm/day) unless a row says otherwise; the hot-melt layer is marked as the waterproofing. The reducing layer, paving and pedestals carry no thermal resistance. The pre-filled link opens with the inverted roof switched on at f·x 0.001.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
2 × 100mm XPS under a water flow reducing layer (f·x 0.001)≈ 0.17Misses the 0.16 and the 0.15
2 × 120mm XPS under a water flow reducing layer≈ 0.14As pre-filled
2 × 120mm, no reducing layer, rebated boards (f·x 0.03)≈ 0.20Paving on pedestals is an open covering
2 × 120mm, no reducing layer, butt-jointed boards (f·x 0.04)≈ 0.22
2 × 120mm, no reducing layer, rebated boards, at Eskdalemuir (4.41 mm/day)≈ 0.26Twice Manchester’s October–May rainfall
2 × 120mm + reducing layer, at Eskdalemuir≈ 0.14With the layer, the rainfall hardly matters (0.144)
2 × 150mm XPS + reducing layer≈ 0.11Just over the 0.11 of the notional dwelling (0.115)
Hybrid: 120mm PIR under the membrane + 50mm XPS over it (f·x 0.03)≈ 0.15Saves height; just misses 0.15 (0.151)
Warm-roof terrace: 150mm PIR under the membrane, pedestals on pads≈ 0.15Just meets 0.15 (0.1497); see the condensation note

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 rain correction is the terrace’s own variable. BR 443 §4.8.6 and the BBA’s Information Bulletin 4 add ΔU = p·f·x·(R1/RT)² for the rainwater that runs between the boards and the waterproofing, where p is the average daily rainfall from October to May (2.16 mm at Manchester, 4.41 at Eskdalemuir), and f·x is 0.02 for roof gardens and green roofs, 0.03 for rebated boards and 0.04 for butt-jointed boards under an “open covering” such as gravel or paving — and paving on pedestals is an open covering. A tested water flow reducing layer laid over the boards cuts f to the BBA’s minimum 2.5% (f·x 0.001), which takes the correction from about 0.06 to almost nothing. The boards are taken at their design λ, not the declared one (ETAG 031-1, as BR 443 says): Bauder’s XPS 300 is 0.035 for inverted use. Ravago gives 175mm of its XPS X ULTRA for 0.15 on the same slab, at its declared λ 0.027 — U-Monkey’s library holds Ravatherm boards at their declared values, so use a design figure in an inverted row. The other limit is height at the door: LABC Warranty asks for upstands 150mm above the finished roof except at door access, and an overflow at least 25mm below the sill, with a finished fall of 1 in 80; a hybrid build-up, PIR under the waterproofing and a thinner XPS layer over it, saves 70mm here for about the same U-value. LABC Warranty also rules out a cold deck under a terrace that forms part of the roof, and asks for spreader plates under the pedestals. Approved Document L sets 0.16 for the roof of a new dwelling (Table 4.1) and 0.15 for a new roof on an existing one (Table 4.2); the notional dwelling assumes 0.11.

Condensation risk on a roof terrace

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month in the inverted rows on this page, with every assumed vapour value tested across its plausible range: the hot-melt waterproofing is a vapour barrier on the warm side of the insulation, and the boards above it stay cold and dry. The reducing layer on top must be vapour-open, so the boards can breathe outwards.

The warm-roof variant is different. With 150mm of PIR on a polythene vapour control layer under a 10mm bitumen membrane, the check finds condensate building up year on year (3 g/m² still there at the end of the year): under a membrane that tight, a warm roof needs a high-performance vapour control layer, such as a reinforced bitumen or aluminium-cored sheet. The hybrid has a conditional pass, so run it with your own products.

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 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 roof terrace with paving on pedestals?

A roof terrace built as an inverted roof — two layers of 120mm extruded polystyrene (Bauder XPS 300 Inverted, design λ 0.035) over hot-melt waterproofing on a 200mm concrete slab, under a water flow reducing layer, with paving on pedestals above — has a U-value of about 0.14 W/m²K, inside the 0.15 Approved Document L sets for a new roof on an existing dwelling. The reducing layer is what makes that true: paving on pedestals is an “open covering” to BR 443, so without a tested reducing layer the cold rainwater running under the boards adds a correction of f·x 0.03 (rebated boards) or 0.04 (butt-jointed) — 0.20 and 0.22 for the same 240mm at Manchester’s winter rainfall, and 0.26 at Eskdalemuir’s. The paving and pedestals are left out of the calculation.

Do I need a water flow reducing layer on a roof terrace?

For the U-value, almost always. With paving on pedestals the insulation sits under an open covering, so rain runs between the boards and the waterproofing all winter and BR 443 adds a correction for it: 240mm of XPS that is 0.14 W/m²K under a tested reducing layer is 0.20 without one (rebated boards, Manchester rainfall) and 0.26 in a wet part of the country. Meeting 0.15 without the layer would take about 400mm of XPS instead of 240mm, because the correction grows as the insulation gets thicker — more than any terrace door threshold has room for. The layer must be the vapour-open kind made for inverted roofs, and the terrace needs drainage at both levels — above the layer and at 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.

Open this build-up pre-filled or call 01202 623236