Fire safety for flat roofs has never been more high-profile. The Grenfell Tower tragedy exposed the consequences of flammable materials and led to sweeping reforms in UK building regulations. As roof spaces become multifunctional – hosting terraces, solar panels and plant – designers, owners and contractors must be fluent in the evolving rules.
This guide explains the key fire safety requirements in 2025 and offers practical advice to keep your projects compliant.
Key Regulations to Understand
Approved Document B (Fire Safety)
Approved Document B (ADB) supports the Building Regulations 2010 and applies to all new builds and refurbishments in England. It’s split into Volume 1 (dwellings) and Volume 2 (non-dwellings), and was most recently updated in 2025. The document covers:
- B3: Internal fire spread (structure)
- B4: External fire spread (roofs, cladding)
Changes in 2025 include: – Removal of national fire classifications (e.g. AA, AB) – Exclusive use of BS EN 13501-5 for roofs – Clearer guidance on compartment walls and roof junctions
Building Safety Act 2022 & Fire Safety Act 2021
These Acts implement Hackitt Review recommendations. They extend duty of care across the building lifecycle, impose 15–30 year liability periods, and strengthen the legal obligations of specifiers, clients and contractors.
Regulation 7 (Combustibles Ban)
Since 2018, combustible materials have been banned from external walls of buildings over 11 m. Balconies are included in the ban, and aside from waterproofing layers, must be constructed from non-combustible materials.
Fire Safety Order 2005
This regulation mandates that building owners perform fire risk assessments and maintain adequate precautions. It applies to most non-residential premises.
BS EN 13501‑5: External Fire Performance for Roofs
All roof systems must now be tested under BS EN 13501‑5, using the CEN/TS 1187 Test 4 method. This simulates external fire exposure from burning brands, wind and radiant heat. The test evaluates the entire roof build-up: membrane, insulation, deck, adhesives, fixings, and finish.
Roof Classifications:
| Classification | Description | Minimum Boundary Separation |
|---|---|---|
| B_ROOF(t4) | Highest rating: 60+ min resistance, low flame spread | None – unrestricted |
| C_ROOF(t4) | 30+ min resistance, medium spread | ≥6 m from boundary |
| D/E/F_ROOF(t4) | Reduced or untested performance | 6–20 m depending on use |
Important: Ratings only apply to the tested build-up. Substituting components invalidates the result.
Compartment Walls & Roofs
When a roof crosses a fire compartment line: – A 1.5 m wide strip on each side must be B_ROOF(t4) – Fire stopping must connect the wall to the underside of the roof or deck – Double-skin metal decks should include a 300 mm non-combustible zone over the wall
Roofs Near Boundaries
Section 12 of ADB states: – B_ROOF(t4) may be used within 6 m of a boundary – Lower ratings (C–F) need greater separation – Roofs pitched >70° are treated as walls and must meet non-combustibility standards
Balconies vs Terraces: BS 8579:2020
BS 8579 distinguishes between: – Balconies: Over open space, treated as wall projections – must be A1 or A2-s1,d0 (non-combustible) – Terraces: Over internal space, treated as roof coverings – must meet B_ROOF(t4)
Surfaces like ≥50 mm paving or ≥40 mm ballast are deemed compliant with B_ROOF(t4) by default.
Grenfell and Flat Roofs: Not the Same
Grenfell involved flammable cladding and failed cavity barriers on a vertical façade. The combustibles ban applies to external walls and balconies, not horizontal roof areas. Roofs are regulated under BS EN 13501-5, not -1.
Flat roofs can include combustible materials (e.g. PIR insulation, membranes) as long as the complete system passes B_ROOF(t4). Cladding and terrace rules should not be conflated.
Green Roofs & Fire Risk
A 2013 UK government study on green roofs recommends: – 1.5 m B_ROOF(t4) strips at compartment lines – 500 mm fire breaks around rooflights – 1 m fire breaks every 40 m across large roofs – Substrates <50% organic with ≥80 mm depth – Designated irrigation and maintenance zones
These measures reduce lateral fire spread through vegetation.
Solar Panels & Plant Decks
Solar PV systems must be carefully detailed: – Mounts must not compromise the B_ROOF(t4) rating – Clear firefighting access should be maintained – Cables/inverters must be separated from combustibles – Fire engineers and insurers should be consulted
Practical Advice for Specifiers
- Use fully tested build-ups – Don’t swap layers unless listed in the certification.
- Engage building control early – Discuss boundaries, decks and classifications in design.
- Plan compartment lines – Include 1.5 m B_ROOF(t4) strips and full fire stops.
- Differentiate balconies from roofs – A balcony = A2-s1,d0. A terrace = B_ROOF(t4).
- Design fire breaks into green roofs – 500 mm at penetrations, 1 m across long spans.
- Consider solar/fire interaction – Plan detailing with insurers and competent installers.
- Know your legal duties – Fire Safety Order applies to most non-domestic roofs.
Looking Ahead
ADB will continue evolving through 2026 and 2029 updates, with expected new guidance on high-rise design and product testing.
With longer liability periods and rising scrutiny, the best approach is always to: – Specify tested systems – Maintain thorough records – Document boundary, wall and fire detailing
Resources
– Approved Document B – gov.uk
– BS EN 13501-5 Summary – BSI Group
– BS 8579 Balcony/Terrace Guide – bsigroup.com
– 2013 Green Roof Fire Report – gov.uk
– Fire Safety Act & Building Safety Act – legislation.gov.uk