Warm Roof Construction
The 60-second answer: A warm roof is built top-down: waterproof membrane → rigid PIR insulation (typically 100–150mm) → vapour control layer (VCL) → structural deck → joists → ceiling. The whole structure stays inside the warm envelope of the building.
SIX THINGS, EVERY JOB
What you get with Eco Roofers
What we typically see on Nottinghamshire roofs
Nottinghamshire roof-stock splits roughly into Victorian + Edwardian terrace in central Nottingham (NG1, NG3, NG7), inter-war and post-war semis through Mapperley/Sherwood and the West Bridgford suburbs, ex-coalfield estates around Mansfield (NG18-NG20) and Worksop (S80-S81), plus pantile-and-stone period property in Newark and the Trent-valley villages.
Local weather notes. The county catches Trent-valley weather — fewer high-wind events than the Pennines but more sustained-saturation rainfall in extended wet spells. South-westerly storms strip tile fixings on the same nailing line.
Approved Document L 2022 sets the thermal performance baseline for new roofs (U ≤ 0.16 W/m²K pitched, ≤ 0.18 flat). On flat roofs, BS 6229:2018 sets the design and falls standard. Eco Roofers will guide you through any Building Control notification needed on your job.
Get a free estimate for warm roof construction in Warm Roof Construction
Enter your postcode and one line about your roof. Sean replies within 24 hours with a written quote.
What will my new roof cost?
Pick your property type and material. Get a typical UK 2026 cost range in seconds — no email required to see the number.
Market-typical UK 2026 ranges. Eco Roofers’ actual quote is built from a free 30-min on-site survey and delivered in writing within 24 hours, including VAT. 25% deposit, balance on satisfactory completion.
The full warm roof build-up — top to bottom
Working from outside (top) to inside (bottom), a UK domestic warm roof in 2026 has these layers:
“
┌─────────────────────────────────────────────────────────────┐
│ 1. WATERPROOF MEMBRANE │ EPDM 1.2mm / GRP 4-6mm / Single-ply 1.5mm / Felt
├─────────────────────────────────────────────────────────────┤
│ 2. RIGID INSULATION BOARD (PIR, foiled both faces) │ 100–150mm, lambda 0.022 W/mK
├─────────────────────────────────────────────────────────────┤
│ 3. VAPOUR CONTROL LAYER (VCL) │ Polyethylene or aluminium-foiled, taped at all joints
├─────────────────────────────────────────────────────────────┤
│ 4. STRUCTURAL DECK │ 18mm OSB or 18mm WBP plywood
├─────────────────────────────────────────────────────────────┤
│ 5. ROOF JOISTS │ Typically 150-225mm depth at 400mm centres
├─────────────────────────────────────────────────────────────┤
│ 6. CEILING │ Plasterboard with skim, painted finish
└─────────────────────────────────────────────────────────────┘
`
Key feature: the insulation sits above the deck. The deck and joists are inside the warm envelope. No ventilation gap is required because no surface inside the structure ever falls below dew-point.
U-value calculation — what to hit and why
Approved Document L 2022 requires U ≤ 0.18 W/m²K for new flat roofs and major refurbishments (renewing >50% of the roof area).
For typical UK domestic warm-roof construction, the U-value depends almost entirely on the PIR insulation thickness:
| PIR thickness | Achieved U-value | Compliance |
|---|---|---|
| 75 mm | 0.27 W/m²K | Below ADL2022 — non-compliant |
| 100 mm | 0.20–0.22 W/m²K | Borderline — typically OK for refurbishment with minor thermal-bridging consideration |
| 120 mm | 0.17–0.19 W/m²K | Default specification — comfortably compliant |
| 150 mm | 0.13–0.15 W/m²K | Exceeds Regs — best energy-bill payback |
| 200 mm | 0.10–0.12 W/m²K | Passive-house territory — for very-low-energy specs |
For most domestic warm-roof retrofits in Notts/South Yorks, 120mm PIR is the sweet spot — comfortably compliant, energy-efficient, and the £150–£300/m² premium over 100mm is small relative to the lifetime savings.
Falls and drainage (BS 6229:2018)
| Spec | Value |
|---|---|
| Minimum design fall | 1:80 |
| Practical (built) fall | 1:60 typical |
| Minimum 'flat' fall | Not permitted — zero-fall roofs trap water |
| Drainage outlets | At low point of fall, sized to local rainfall + roof area |
| Pooling water tolerance | Eliminate by design — no permanent ponds |
A 4-metre-wide flat roof at 1:60 fall has a 67 mm rise from low point to high point. We typically build the slope into the deck via tapered timber firrings rather than relying on a sloping joist line.
Upstand and edge details — where most warm roofs fail
The membrane must dress up vertical surfaces (parapets, walls, chimneys) and over edges (eaves, drips). UK practice per BS 6229:
Upstand at parapet or wall
`
│ Wall / parapet
│
─────────────┐ │
Cover └────┤ ← Cover flashing dressed into chase or under DPC
Flashing │
│
──────────────── │ ← EPDM/GRP membrane folded up wall, 150mm minimum upstand
│
═══════════════════│ ← Field of roof
`
- Minimum upstand height: 150mm above finished roof level
- Upstand strip bonded to the wall behind a cover flashing
- Cover flashing dressed into a chase cut into the wall, or tucked under a DPC
- Sealant at the top to prevent moisture migration
Drip edge at eaves
`
═════════════════════
\ ← EPDM/GRP folds over edge trim
\
\ ← Edge trim dresses outward 35-50mm
\
\ ← Drip causes water to fall away, not back
══ ═══ ═══ ═══ ═══ ═══════
“
- Minimum drip projection: 35mm beyond fascia
- Edge trim manufacturer-supplied (Permaroof / Firestone (Eco Roofers is a Firestone Elevate approved installer) pre-formed extruded aluminium)
- Membrane bonds over the edge trim with manufacturer adhesive
- Drip prevents wind-driven rain tracking back along the underside of the membrane
Penetration detail (rooflight, vent, soil pipe)
- Pre-formed corner pieces (manufactured EPDM corners) at all four corners of any rectangular penetration
- Membrane dressed up the kerb 100mm minimum
- Cover flashing or counter-flashing over the top
- Manufacturer-approved primer + adhesive
Vapour control layer (VCL) requirements
The VCL is the most-skipped detail on cheap warm-roof installs — and the most damaging when missed. The VCL stops indoor moisture migrating from the heated space below into the insulation, where it would condense and degrade thermal performance.
VCL specification:
- Material: minimum 500 gauge polyethylene OR aluminium-foiled vapour barrier
- Continuous: no breaks, no perforations except where penetrations are properly detailed
- Sealed at all joints: 100mm overlap minimum, taped with manufacturer-approved VCL tape
- Sealed at all penetrations: pipe boots, rooflight kerbs, vents — all need proprietary detail kits, not builder’s foam
- Sealed at perimeter: VCL must continue up upstand walls and tuck under the membrane upstand
A warm roof without a continuous VCL will absorb moisture into the PIR over time, lose thermal performance, and eventually rot the deck below.
Build sequence on site (typical 20m² extension warm-roof retrofit)
| Day | Activity | Duration |
|---|---|---|
| Day 1 AM | Strip existing membrane down to deck. Inspect deck condition. | 3-4 hours |
| Day 1 PM | Repair any deck rot. Lay vapour control layer, sealed continuously, taped at joints. | 3-4 hours |
| Day 2 AM | Lay PIR insulation board, stagger joints, tape PIR foil joints. | 2-3 hours |
| Day 2 PM | Form timber upstands at all walls/parapets/edges. Fit deck-grade plywood over insulation if required by membrane manufacturer. | 3-4 hours |
| Day 3 AM | Apply EPDM bonding adhesive. Roll out and bond EPDM membrane, work out wrinkles. | 2-3 hours |
| Day 3 PM | Form upstand corners with pre-formed EPDM. Dress drips, parapet flashings, rooflight kerbs. | 3-4 hours |
| Day 3 final | Site clean. Final inspection. Hand over written guarantee. | 30 min |
For larger jobs, the sequence extends. For smaller jobs (10m²) we compress to 1-2 days.
Building Control compliance
Warm roof retrofits over the 50%-renewal threshold are notifiable under Approved Document L 2022. Notification process:
1. Roofer is registered with a Competent Person Scheme (CPS) — Bonded Roofers UK, TrustMark Roofing, or equivalent
2. Roofer notifies CPS at start of work
3. Roofer self-certifies on completion (U-value calc, photos of build-up, etc.)
4. CPS issues Building Control compliance certificate
5. Certificate is filed against the property — visible to future surveyors and conveyancers
Eco Roofers handles all CPS notification — homeowner doesn’t deal with Building Control directly.
Common warm-roof construction mistakes
1. PIR over-compressed at fixings. Mechanical fixings (washer plates) compress PIR locally, creating cold bridges. Specify the right washer plate size and load distribution.
2. VCL discontinuous at upstands. VCL on the deck stops at the wall, leaving the upstand zone unprotected. Continue VCL up the upstand under the membrane upstand.
3. Inadequate upstand height. 100mm upstand fails wind-driven rain test. Always 150mm minimum.
4. Wrong adhesive for ambient. EPDM adhesives are temperature-sensitive. Below 5°C deck temperature, bonds fail.
5. No tapered firrings on previously-flat roof. Old cold-roof felt was often laid on a near-flat deck. Warm-roof retrofit must rebuild proper falls.
FAQ
What’s the typical thickness of warm-roof insulation?
100-150mm rigid PIR for UK domestic warm-roof construction in 2026. 120mm is the default specification (U-value ~0.17-0.19 W/m²K, comfortably ADL2022 compliant).
What U-value should a warm roof achieve?
Approved Document L 2022 requires ≤ 0.18 W/m²K for new flat roofs and major refurbishments. With 120mm PIR you typically hit 0.17-0.19 W/m²K. With 150mm PIR you exceed Regs at 0.13-0.15 W/m²K.
Does a warm roof need a vapour control layer?
Yes — essential. The VCL stops indoor moisture migrating into the insulation. Without continuous VCL, the PIR will absorb moisture over time, lose thermal performance, and eventually rot the deck.
Can I build a warm roof from inside?
No — warm roof construction by definition has insulation above the deck. Inside-the-room insulation creates a cold roof. If you can’t access the roof from outside (e.g. listed building, height restriction), other strategies apply (inverted warm roof, hybrid approaches) but those need specialist design.
Can warm roofs be fitted to pitched roofs?
Yes — modern warm-pitched-roof construction puts insulation above the rafters in a sarking layer below the tiles/slates. More complex than flat-roof warm construction; common in retrofit-loft-conversion work.
Need a warm roof built or retrofitted?
Eco Roofers fits warm roofs across Nottinghamshire and South Yorkshire — Nottingham, Mansfield, Sheffield, Doncaster, Chesterfield, Barnsley, Rotherham, Worksop, Newark.
Phone Sean on 07929 379 746 or email sean@eco-roofers.co.uk for a free site visit. Written a clear written estimate. we handle Building Control on notifiable work (private inspector or LABC). 10-year installation guarantee + manufacturer’s 20-year membrane warranty. Every flat roof we fit is designed to BS 6229:2018 — confirmed in writing on every quote.
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