Roof deck types and what they mean for a commercial re-roof
Updated 23 October 2026 · SEO Dons Editorial
What is the roof deck, and why does it decide a re-roof?
The deck is the structural layer that carries the vapour control layer, insulation and waterproofing. BSI says BS 6229:2025 covers roofs on heavy supports such as concrete slabs and on lightweight decks, metal or timber-based. On a re-roof the deck is whatever is already there. LRWA says that in refurbishment the structural deck is the existing construction, so its type and condition have to be established before any system is specified. The table covers the decks met most often; the warm-deck re-roof page shows how a new build-up sits on top of them.
| Deck | What it means for a re-roof | Hazards and checks |
|---|---|---|
| Profiled metal (galvanised or coated steel, aluminium) | SPRA describes it as used for lightweight roofs where rapid installation is required. Where a polyethylene vapour control layer is used on a metal deck, SPRA says to seal it with bitumastic tape and support the joints while sealing. For low-risk buildings an unsealed metal deck may give enough vapour control, though air leakage may still call for a layer. | NFRC's torch-on checklist flags refurbishment metal decking where old material may accumulate in the troughs. |
| Concrete (in-situ, precast, pre-stressed) | Precast panels laid to a fall give a simple layout without cross-falls. In-situ concrete is harder to lay to a fall, so SPRA says falls are more often made in the insulation (warm roofs only) or with a screed. | Construction water can stay trapped under a membrane or vapour control layer. NFRC flags exposed expansion joints. The ponding guide covers falls. |
| Timber, plywood, OSB, boarding | SPRA says modern roofs typically use sheets, but traditional timber boarding is not uncommon on re-roofs. | LRWA: a full examination is particularly critical for timber and other degradable decks. NFRC flags an exposed timber deck as a torch-on fire risk. |
| Woodwool cement slab | A rigid timber-and-cement panel, rarely used in new construction. SPRA says specific advice must be obtained from the fastener and membrane manufacturer on resisting wind loading. | On HSE's fragile list; see below. |
| Composite insulated decks | A PIR core factory-adhered to a profiled steel liner, with top sheets from foil to coated steel. For a coated steel top sheet SPRA says to seek the membrane manufacturer's advice. | Insulation is part of the deck, so the thermal and vapour design starts from what the panel already provides. |
Why is woodwool a special case?
HSE's roof work guidance says woodwool slabs may fracture beneath someone's weight, and lists what to treat as fragile: slabs under 75 mm thick without nylon net reinforcement, slabs with net reinforcement but under 50 mm thick, slabs that are wet or previously damaged, and all straw boards, especially wet ones. It also lists woodwool weakened by water damage as a hazard to look for. HSE says work on or near fragile surfaces needs a combination of stagings, guard rails, fall restraint, fall arrest and safety nets, and that all roofs should be treated as fragile until a competent person confirms they are not. The fragile roofs and rooflights guide sets out the owner's duties.
Identification is part of the problem. LRWA lists as a common misconception that all fibrous decking is woodwool, when it is frequently compressed strawboard. NFRC publishes woodwool slab guidance (GN43) on its fragility and the importance of a robust roof survey, which only members and purchasers can read. SPRA also recommends against cement-bonded particle board as a structural roof deck because it has insufficient inherent strength.
What about RAAC planks?
Reinforced autoclaved aerated concrete is a lightweight, bubbly concrete used between the 1950s and mid-1990s, found predominantly as precast panels in roofs, mostly flat. The Department for Education's identification guidance, updated in July 2026 and written for schools and colleges, says the panels are considerably weaker than traditional concrete and are now beyond their estimated service life. It reports sudden collapses of RAAC roof panels that appeared to be in good condition. SPRA tells designers to refer to the SCOSS alert where a lightweight aerated concrete deck is present on a refurbishment.
The guidance describes panels typically 600 mm wide, with a V-shaped groove every 600 mm, and says to check drawings for the makers Siporex, Durox, Celcon, Hebel and Ytong. It also warns that looking behind ceilings or finishes means consulting asbestos guidance first. A suspected plank is a matter for a qualified surveyor or structural engineer before roof work is planned; the RAAC guide covers what building owners should do.
Which deck checks should a survey make before a re-roof?
- Identify the deck from a test cut or drawings. LRWA says a survey must assess the supports, deck, insulation and existing waterproofing, including moisture and condensation.
- Check moisture and stability. SPRA says to check the moisture content and stability of existing materials before laying, and to avoid overlays to wet roofs. The condensation guide explains trapped moisture.
- Treat the roof as fragile. Confirm non-fragility before anyone walks on it.
- Test the fixings. SPRA says pull-out testing should be done for all refurbishment projects, regardless of substrate.
- Check the load. SPRA lists roof loading exceeding the structure's capacity as a disadvantage of overlay. See the overlay versus strip-and-recover guide.
- Check fire risks. NFRC's checklist covers timber, metal-trough and expansion-joint hazards. Approved Document B also asks for a deck of class A2-s3, d2 or better within the 1500 mm zone either side of a compartment wall.
The survey guide describes the methods, and the refurbishment page compares the routes. To compare contractors for a re-roof, send an enquiry through the form on this site.
Sources
- BS 6229:2025 Flat roofs with continuously supported flexible waterproof coverings, BSI Knowledge, checked 22 October 2026.
- Single ply roofing: design guide 2020 (S1/2020), Single Ply Roofing Association (SPRA), checked 22 October 2026.
- Design guide for specifiers 2020 (liquid applied waterproofing), Liquid Roofing and Waterproofing Association (LRWA), checked 22 October 2026.
- Health and safety in roof work (HSG33), fifth edition, Health and Safety Executive (HSE), checked 22 October 2026.
- Roof work, Health and Safety Executive (HSE), checked 22 October 2026.
- Safe2Torch checklist, National Federation of Roofing Contractors (NFRC), checked 22 October 2026.
- GN43 Woodwool slab guidance (summary page), NFRC, checked 22 October 2026.
- Approved Document B, Volume 2 (2019 edition, collated with later amendments), Ministry of Housing, Communities and Local Government, GOV.UK, checked 22 October 2026.
- Reinforced autoclaved aerated concrete (RAAC): identification guidance, Department for Education, GOV.UK, checked 22 October 2026.
Common questions
Can a new roof go over a woodwool deck?
Only on the manufacturers' advice and after a survey. SPRA says specific advice must be obtained from the fastener and membrane manufacturer on resisting wind loading, and HSE says woodwool may fracture beneath someone's weight, so the work needs a safe system as well.
How can I tell whether a roof has RAAC planks?
The DfE guidance lists drawings that mention RAAC or the makers Siporex, Durox, Celcon, Hebel and Ytong, panels typically 600 mm wide with V-shaped grooves, and a white or light grey bubbly texture. Confirmation needs a qualified building surveyor or structural engineer.
Does the deck affect the roof's fire performance?
It can. Approved Document B asks for a roof covering classified BROOF(t4) on a substrate or deck of class A2-s3, d2 or better within 1500 mm either side of a compartment wall. The BROOF(t4) guide explains how the classification applies to the whole roof construction.