BROOF(t4) Explained: Fire Classification for Commercial Flat Roofs

BROOF(t4) turns up in specifications, manufacturer data sheets and building control comments, usually without an explanation. This guide explains what the classification is, where English building regulations use it for commercial buildings, and what evidence to ask for before you accept a roof system.

It covers England and Approved Document B, Volume 2 (buildings other than dwellings). Scotland, Wales and Northern Ireland have their own fire guidance.

What the classification is

Approved Document B says roof constructions are classified in the European system as BROOF(t4), CROOF(t4), DROOF(t4), EROOF(t4) or FROOF(t4) in accordance with BS EN 13501-5. BROOF(t4) is the highest performance and FROOF(t4) the lowest. The suffix (t4) means that Test 4 is the test used for the purposes of the document (Approved Document B Volume 2, Appendix B).

BS EN 13501-5 refers to four separate roof test methods (CEN/TS 1187). Test 4 is a two-stage test on a complete roof construction: a preliminary stage with burning brands alone, then a main stage with burning brands, wind and supplementary radiant heat combined (Measurlabs overview of EN 13501-5). The Scottish Technical Handbook gives the reason for the suffix: only test method 4 evaluates penetrability in a satisfactory way to meet the fire performance requirements in the United Kingdom.

A system, not just a covering

Approved Document B describes a roof covering as ‘one or more layers of material, but not the roof structure as a whole’ (paragraph 14.1). The test, though, is run on a whole construction. A 2024 research note for the Building Safety Regulator’s ‘Real Fires’ project (OFR Consultants, retrospective roof insulation fire testing) points out that the guidance refers to roof coverings while the test method applies to a system.

For a buyer the consequence is practical. A classification certificate belongs to the build-up that was tested: the deck, the insulation, the membrane and how they are fixed. Change the insulation, the fixing method or the surface finish and the certificate may no longer describe the roof you are getting.

Where Approved Document B uses it

Distance to the relevant boundary

Paragraph 14.3 and Table 14.1 set separation distances by the type of roof covering and the size and use of the building, measured from any point on the roof to the relevant boundary (paragraph 13.5). Between buildings on the same site, an assumed notional boundary can apply (Diagram 13.3).

The pattern in Table 14.1 is simple at the top end. BROOF(t4) and CROOF(t4) are acceptable at every distance band. For DROOF(t4), EROOF(t4) and FROOF(t4) the table adds minimum distances and footnoted conditions, and Note 2 excludes industrial, storage and other non-residential buildings (purpose groups 6 and 7) of any size, and any building over 1,500 m³, from the footnoted options. So on a commercial or industrial roof close to a boundary or a neighbouring building, BROOF(t4) or CROOF(t4) are the classes that need no distance argument. Read Table 14.1 and its notes for your building rather than relying on a summary.

Paragraph 14.3 also recognises roof covering products defined in Commission Decision 2000/553/EC as meeting the external fire performance requirement without testing, provided national design and execution provisions are met. If a product relies on that route, ask which entry it is listed under.

Roofs passing over a compartment wall

Paragraph 8.26 says that to reduce the risk of fire spreading over the roof from one compartment to another, a 1,500 mm wide zone of the roof either side of a compartment wall should have a covering classified BROOF(t4), on a substrate or deck of a material rated A2-s3, d2 or better (Diagram 8.2a). Thermoplastic rooflights counted as BROOF(t4) under paragraph 14.7 are not suitable in that zone.

Paragraph 8.28 asks for double-skinned insulated roof sheeting to incorporate a band of A2-s3, d2 material at least 300 mm wide, centred over the wall. Paragraph 8.29 allows the compartment wall to extend through the roof instead: by 375 mm above the adjoining roof covering where the height difference between roofs is under 375 mm, or by 200 mm where the height difference is 375 mm or more, or where the coverings either side are BROOF(t4).

Rooflights

Rooflights are specified in a similar way (paragraph 14.4). Polycarbonate or uPVC rooflights achieving at least class C-s3, d2 can be regarded as BROOF(t4) (14.7), and unwired glass at least 4 mm thick likewise (14.8). Plastic rooflights with a D-s3, d2 or TP(b) lower surface are subject to size and spacing limits (Diagram 14.1, Tables 14.2 and 14.3), including a 5 m² maximum area for a rooflight or group, and 3 m between rooflights in any direction.

What changed in 2025

The 2025 amendments to Approved Document B Volume 2 took effect on 2 March 2025 in England. The document lists among their changes the removal of national classes for reaction to fire and roofs. The table that transposed the old BS 476-3 roof classifications (AA, AB, AC and so on) to the European classes has gone, and the text now says that products or systems whose performance needs assessing should be assessed against the document’s recommendations as capable of meeting the European classification.

The practical point for an older roof: an old test report quoting a BS 476 class is no longer the evidence building control is directed to accept. Ask for a BS EN 13501-5 classification report.

A worked check for a commercial roof

This is the order in which a designer or building owner can check what a roof needs. It is a prompt for the conversation with your building control body, not a substitute for it.

1. Find the relevant boundary (paragraph 13.5), or the notional boundary between buildings on the same site (Diagram 13.3).
2. Measure the distance from the nearest point of the roof to that boundary.
3. Read Table 14.1 for the class you are considering against the building’s purpose group and size, including Note 2 for industrial, storage and other non-residential buildings and anything over 1,500 m³.
4. Check compartment wall crossings against paragraphs 8.26 to 8.29.
5. Check rooflights against paragraphs 14.4 to 14.8 and Tables 14.2 and 14.3.
6. Write the result into the specification as a required classification (‘roof build-up classified BROOF(t4) in accordance with BS EN 13501-5’) and make the classification report a required submittal before the system is ordered.

What to ask for before you accept a system

• A BS EN 13501-5 classification report that states BROOF(t4) and names the tested construction: deck type, insulation product and thickness, membrane, fixing method and any ballast or surfacing.
• A match between the report and your build-up. If the quoted insulation, fixings or finish differ from the tested set-up, ask the manufacturer or test house in writing whether the classification extends to it. Check, too, that the installer is approved for that system: the accreditations page sets out where BBA certificates and approved installers fit in.
• The route under Commission Decision 2000/553/EC, if the product claims no testing is needed.
• The boundary distances and building category from your Table 14.1 check, so the classification you need is written into the specification.
• Special zones: compartment wall crossings, rooflights and any roof edge within the separation distance.
• Evidence for an overlay. A new membrane over an old roof creates a build-up that was not tested as such. Ask what the classification covers when old layers remain.

Which roof systems this touches

Manufacturers publish classifications for specific tested build-ups across the main families, so the question is always ‘this build-up’, not ‘this family’. Start from the system pages to see the build-ups involved: single-ply membranes, built-up felt and reinforced bitumen, liquid-applied and GRP systems and green and blue roofs. The EPDM, TPO and PVC comparison sets out how the single-ply options differ before the fire evidence is requested.

If photovoltaics are planned, ask the roof manufacturer and the array supplier how the two systems have been assessed together; the solar-ready roof guide covers load and sequencing. Adding insulation to meet energy rules also changes the build-up, which is another reason to read the Part L guide alongside this one.

Common questions

What does BROOF(t4) mean?

It is the highest classification in the European system for a roof’s resistance to external fire, from BS EN 13501-5. The (t4) shows that Test 4 was used, which combines burning brands, wind and radiant heat on a complete roof construction.

Does a commercial flat roof need BROOF(t4)?

It depends on the distance to the relevant boundary, the building’s use and size, and any compartment wall crossings. BROOF(t4) is acceptable at every distance in Approved Document B Table 14.1, so it is the class that needs no separation-distance argument. Check Table 14.1 for your building.

Is an old BS 476 roof classification still accepted?

Approved Document B no longer carries the table transposing BS 476-3 classes to the European classes. Since the 2025 amendments, ask for a BS EN 13501-5 classification report instead.

Does the classification apply to the membrane alone?

No. The test is run on a complete roof construction, so a classification belongs to the tested build-up of deck, insulation, membrane and fixings.

Sources and last checked

Last checked on 20 October 2026 against the sources below.

• GOV.UK: Approved Document B, Volume 2 (2019 edition incorporating 2020 and 2022 amendments, collated with the 2025, 2026 and 2029 amendments): paragraphs 8.26 to 8.29, 13.5, 14.1 to 14.9, Table 14.1, Diagram 14.1, Appendix B
• OFR Consultants for the Building Safety Regulator: summary report, retrospective roof insulation fire testing (3 July 2024)
• Measurlabs: overview of the EN 13501-5 standard