R, E and I
Fire resistance is not a single property. Under BS EN 13501-2 an element is classified against separate criteria, each with its own period in minutes:
- R: Résistance (loadbearing capacity)
- The element continues to carry its load. Applies to anything structural: columns, beams, floors, loadbearing walls.
- E: Étanchéité (integrity)
- The element does not allow flames or hot gases to pass through it. Failure is a crack, a gap, or a joint opening up.
- I: Isolation (insulation)
- The unexposed face does not get hot enough to ignite what is against it. Conventionally a mean rise of 140 K, or 180 K at any point.
So REI 60 is a loadbearing compartment floor that keeps carrying load, keeps flames out, and stays cool enough on the far side, all for 60 minutes. EI 30 is a non-loadbearing partition doing the last two for 30 minutes. R 90 is a column that only has to stay standing.
Additional letters appear for particular elements: W for radiation control, S for smoke leakage, C for self-closing, M for mechanical impact resistance.
The one that gets missed. Specifying "60 minutes fire resistance" without saying which criteria is ambiguous, and the ambiguity is expensive. A partition tested to EI 60 does not satisfy a requirement for REI 60. Write the full classification in the strategy and on the drawing.
How the required period is set
Approved Document B Table B2 sets minimum periods by building use, by height, and by whether the storey is a basement or above ground. Three principles run through it:
- Taller means longer. Evacuation takes longer, firefighting access takes longer, and collapse consequences are greater.
- Basements mean longer than the equivalent above-ground storey. Heat cannot escape upwards, access is harder, and firefighters attack downwards into the fire.
- Sprinklers earn a reduction. In several categories a sprinklered building attracts a lower period, which is one of the few places prescriptive guidance gives credit for an active system.
Typical residential periods run 30 minutes for low-rise, 60 minutes in the middle band, 90 minutes for taller buildings and 120 minutes at the top of the range. Use the table rather than the pattern: the bands differ by building use. Table B2 has seven columns but only five of them are height bands: up to 5 m, up to 11 m, up to 18 m, up to 30 m and more than 30 m. The other two are basement depth bands ("more than 10 m" and "up to 10 m"), measured to the lowest basement, not upward from the ground. Counting the columns as though they were all height bands is a common way to land in the wrong one. The height band that catches designers out is up to 11 m, which sits between "up to 5 m" and "up to 18 m"; miss it and every band above it shifts by one.
Above 11 m, blocks of flats have no unsprinklered column. Note 2 to Table B2 requires sprinklers in blocks of flats with a top storey more than 11 m above ground, so the non-sprinklered rows above that height read not permitted rather than giving a period. If a tool or a spreadsheet hands you a fire resistance period for an unsprinklered block of flats at 21 m, it has read the table wrong. The answer is that the design is not one Approved Document B supports.
Compartmentation is what the periods are for
Fire resistance periods only make sense as part of a compartmentation strategy. Compartmentation divides a building so that a fire in one part does not become a fire in all of it, and it is what a stay-put strategy in a block of flats depends on entirely.
Compartment walls and floors need the period from Table B2, and, critically, the compartmentation has to be continuous. The performance of a compartment line is the performance of its weakest point, which in practice is never the wall:
- Penetrations. Every service passing through needs fire stopping appropriate to the service and the substrate. This is the most common defect found in existing buildings, by a wide margin.
- Junctions. Wall-to-floor, wall-to-external-wall, wall-to-roof. A compartment wall that stops at a suspended ceiling is not a compartment wall.
- Cavities. A continuous cavity behind a compartment line defeats it. Cavity barriers close them.
- Openings. Doors, dampers, glazing: each rated, each installed as tested, each maintained.
Fire doors
Door ratings are written as FD followed by the integrity period, with S added where smoke control is required: FD30, FD30S, FD60, FD60S.
Door periods come from Table C1, "Provisions for fire doorsets", in Appendix C of Approved Document B (the same table, with the same rows, in both Volume 1 and Volume 2). The table sets them by the position of the door, not by a single formula.
The default is that a door in a wall is rated at the same period as the wall it is fitted in. Table C1 item 2e, the catch-all for any door in a compartment wall not covered by a more specific row, reads "same as for the wall in which it is fitted". A door in a compartment wall separating buildings (item 1) is also the same as the wall, with a floor of 60 minutes, and a door in a compartment floor is the same as the floor.
"Half the wall's period" is the exception, not the rule. Table C1 gives "half the period of fire resistance of the wall in which it is fitted, but 30 minutes minimum" for exactly two positions: a door enclosing a protected shaft forming a stairway or evacuation shaft other than those in item 2b (item 2c, which also takes the S suffix), and a door enclosing a protected shaft forming a lift or service shaft (item 2d). Applying the half rule to an ordinary compartment wall door under-specifies it: a 60-minute compartment wall needs FD60 under item 2e, not FD30.
Several positions are fixed periods rather than a fraction of anything: a door between a flat and a space in common use is FD30S (item 2a), a door in the enclosure of a protected stairway is FD30S (item 4a), and a door in any other protected corridor is FD20S (item 5b). Doors onto protected escape routes and into protected stairways carry the S suffix because cold smoke will incapacitate people long before integrity failure matters.
A door is an assembly, not a leaf. The rating belongs to the tested assembly: leaf, frame, intumescent and smoke seals, hinges, closer, latch, and the gap it was tested at. Change the closer, pack the frame differently, or let the gap grow, and the tested performance no longer applies. Most fire door failures found on inspection are installation and maintenance failures, not product failures.
Evidence: test, classification, assessment
Fire resistance is demonstrated by evidence, and the hierarchy matters when someone challenges it.
- Test evidence: the product tested to the relevant standard, with a classification report. The strongest position, and the narrowest: it applies to what was tested, in the configuration tested.
- Extended application: a formal extension of test evidence to a defined range of variations, produced under the relevant standard.
- Assessment: a competent third party's opinion that an untested variation would perform. Legitimate, widely used, and the area of most variable quality. Read the scope and the caveats before relying on one.
Under the Gateway regime, the expectation is that the evidence for what was actually installed goes into the golden thread, not the evidence for what was specified.
Last reviewed 25 July 2026 against the editions named above. Standards are revised; check the current published edition before relying on anything here in a design.