Geoff Wilkinson demystifies the task of providing fire protection for ceilings
Manufacturers and suppliers to our industry use the terms ‘fire-proof’, or ‘complies with Building Regulations’ all too often, so much so that it’s hardly surprising architects are sometimes confused by the various claims that are made.
As the Grenfell Tower fire tragedy proved, even reliance on British Board of Agrément (BBA) certificates can be misleading, as a ceiling, for instance, can have multiple requirements that it has to comply with – acoustics, fire, and hygiene being common examples. I can’t cover all the details in a short article, so let’s focus on some of the fire safety requirements for residential projects.
The first regulation to look at is Requirement B2 (internal fire spread), which says that a ceiling’s lining must have a surface spread of flame rating showing its resistance to flames spreading over its surface to another part of the room. This is summarised in table 4.1 (reproduced below) and immediately we can see that different rules apply, depending on the location of the ceiling. Therefore in your specification you will need to establish whether the ceiling is in a small room, a large room or a circulation space and/or a common escape route, as the requirements increase incrementally in each case.
Bear in mind that a ceiling can also include things that aren’t obviously a ceiling, such as glazed surfaces, part of a wall at 70 degrees or less to the horizontal, the underside of a gallery, or even the underside of a roof exposed to the room below. All of these are considered to be ceilings for the purposes of the regulations. In some cases the ceiling is purely decorative and only needs to comply with the requirements for surface spread of flame in B2.
However, often the ceiling does the important work of providing fire protection to the structure above it, usually a floor. In this case, Requirement B3 (structural fire protection) also applies and a surface spread of flame rating is no longer enough. Instead, ‘fire resistance’ is required – this is the measurement of the ability of a material or system to resist fire and is measured in minutes of integrity and, crucially, radiated thermal transmission, which is key for specifying compartmentation.
The ceiling may be attached directly to a structural member, or suspended from it, or be self-supporting. As a general rule, it must be installed as described in the fire test report, as its contribution to fire resistance is valid only if the system is installed exactly as in the fire tests, including the hanging (suspension system), junctions between membrane and walls or edge panels, joints and jointing materials. Any changes at all could invalidate the test certificate and potentially lead to fire spread.
In particular, check that the height of the cavity (the space between the upper surface of the ceiling and the underside of any floor) is at least as high as the one tested. It’s possible to increase it, but not decrease it. That’s because the radiated heat could be enough to ignite an insulation product that’s within the cavity if it’s too close to the surface exposed to the fire. Therefore, no combustible or insulating material should ever be added to the cavity, unless it was included in the fire test.
Putting lights, speakers, extract fans and so on through penetrations to the ceiling will also compromise the fire performance without additional measures such as fire safety hoods to prevent the fire breaching the surface.
Another thing to check is that the total area occupied by fixtures and fittings relative to the area of the ceiling is not increased from the maximum that was tested. In other words, if a ceiling is tested with one fire-rated recessed downlighter every square meter, you cannot simply increase that number and assume the ceiling will still achieve the required standard. Therefore sometimes it is better to provide a directly applied fire protection layer to the structure itself, and then install a second suspended ceiling below that which can house the lights, speakers, additional acoustic insulation etc without compromising on fire safety.
As there’s a lot to understand, you should always check with the manufacturers before you confirm the specification and refer to them any changes to make sure they still guarantee their product in that specific situation.
Geoff Wilkinson is managing director of Wilkinson Construction Consultants. www.thebuildinginspector.org