Do commercial rooflights need fire ratings?

Distance to the boundary sets the first answer

The external fire performance of a roof matters because of fire spreading from a neighbouring building, or to one. Approved Document B sets limits on roof coverings by their classification and their distance from the relevant boundary, and rooflights are treated as part of the roof covering for that purpose. Close to a boundary the permitted options narrow sharply and the areas of individual plastic rooflights are limited.

On a large industrial or distribution roof, the units in the field are typically well clear of any boundary and the constraint disappears. On a terraced commercial building in an Oxford side street, where the roof runs up to a party wall, it is the controlling factor. Measure before you specify.

Broof(t4) and what an external classification covers

Roof coverings are classified for external fire exposure under BS EN 13501-5, and the designation you will see quoted is Broof(t4), which is the highest of the four European test methods in common use in the United Kingdom. The older national classification from BS 476-3 used a two-letter designation such as AA, AB or AC, and older specifications and existing building records often still refer to it.

What matters practically is that this classification is about fire attacking the roof from outside: burning brands landing on it, or radiant heat from next door. It says nothing about how the unit behaves in a fire inside the building, which is a separate question with separate tests.

TP(a) and TP(b): the classification that limits a plastic dome

Thermoplastic materials used in rooflights are classified as TP(a) rigid, TP(a) flexible or TP(b), based on how they behave in a small-scale test. Polycarbonate multiwall is commonly TP(a) rigid; some PVC and thinner acrylic sheets fall to TP(b). The classification then drives limits in Approved Document B on the size of each rooflight, the total area of the space they can occupy, and the spacing between them, with tighter limits on TP(b) and in circulation spaces.

This is where a like-for-like replacement can quietly go wrong. Two domes that look identical from the ground can carry different classifications, and swapping to a cheaper sheet can put a room outside what the regulations permit without anything visibly changing.

Rooflights over an escape route or a protected stair

Circulation spaces and escape routes carry the tightest limits, because a rooflight that softens, drips or drops out in a fire is directly over the route people are using to leave. Approved Document B restricts the use of TP(b) materials in protected stairways and applies stricter area and spacing rules in corridors and other escape routes.

Where a rooflight sits in a compartment floor or over a protected shaft, the requirement moves further again and becomes a fire resistance question, measured in minutes of integrity and insulation, rather than a surface classification. That is a specialist glazed product, not a standard rooflight, and the specification for it comes from the fire engineer or the designer.

Fire performance and smoke ventilation are separate specifications

These get confused constantly. A smoke vent opens in a fire to clear smoke and heat from a space or a stairwell, and it is a life safety system with its own standards, actuation and testing regime. A fire rating describes how a construction behaves when fire reaches it. A unit can be one, or the other, or both, and it will be priced accordingly.

Asking for a fire-rated rooflight when what the building actually needs is an automatic opening vent will produce the wrong product at the wrong price. The ventilation side is covered on our page about smoke ventilation rooflights and AOVs.

What glass contributes, and what it does not

Glass does not burn, which puts glazed rooflights in a comfortable position for external fire classification, and glass units are widely accepted close to boundaries where plastic would be restricted. Toughened outer panes and laminated inner panes are standard on commercial glazed rooflights for overhead safety reasons anyway, so there is often no cost in specifying them.

What glass does not automatically give you is fire resistance. An ordinary insulating glass unit will lose integrity in a fire in short order. Fire-resisting glazing uses specific interlayers or intumescent laminates in a tested frame, and the rating only holds if the whole assembly, frame and fixings included, matches the tested arrangement. Substituting a frame invalidates the rating.

Who determines the requirement on your building

The fire strategy for a commercial building is the responsibility of the designer, the fire engineer or the building control body, and for an existing occupied building the responsible person under the Regulatory Reform (Fire Safety) Order carries the duty to keep the risk assessment current. We install to that specification. We do not write it, and we will not tell you that a particular rooflight satisfies a fire strategy we have not seen.

What we will do is put the manufacturer’s classification data in front of whoever is making the decision, before anything is ordered. That is a five-minute exercise at the front of a job and a very expensive one after installation.

Replacing existing units without losing compliance

Replacement work is where the risk sits, because the assumption is that swapping like for like cannot make anything worse. It can. The original units may have been TP(a) and the proposed ones TP(b). A single large replacement can exceed the permitted individual area near a boundary where three smaller ones did not. And a change of use below, from storage to office or from workshop to a space with sleeping accommodation nearby, can move the requirement without any roof work at all.

Where the building has drawings or an original specification, we want to see them at survey. Where it does not, we record what is up there and what is proposed, and hand that to your building control body to confirm.

What we need before ordering

Three things move a fire question forward quickly. The distance from the rooflight positions to the relevant boundary. The use of the space directly below, including whether it forms part of an escape route. And any existing fire strategy document or building control record for the premises.

With those, the unit selection is usually straightforward and the data sheets settle it. We install and replace rooflights across Oxford and twenty-five miles by road, fitting VELUX, Fakro, Keylite and Roto units, with a ten-year workmanship guarantee behind the installation and fifteen years in the trade behind that. Call 01865 704245.