If you have had a wall knocked through or an extension built, your building control officer may have asked how the new steel beam is fire protected. Intumescent coating is one answer, but it is not the only one, and in plenty of domestic jobs it is not even the cheapest. Here is what the regulations actually require and how to meet them without overspending.
Building Regulations in England do not name intumescent paint anywhere. Approved Document B simply requires that structural elements, including steel beams, achieve a minimum period of fire resistance so the building stays standing long enough for people to escape. For a typical two-storey house the requirement is 30 minutes. Add a third storey, such as a loft conversion, and it usually rises to 60 minutes for the elements supporting that floor.
Bare structural steel fails quickly in a fire. It loses roughly half its load-bearing strength at around 550 degrees Celsius, a temperature a house fire can reach in well under 30 minutes. So an exposed RSJ over a new kitchen opening will not pass building control on its own. Something has to protect it, but you get to choose what.
Intumescent coating is a paint-like product that stays thin in normal use, typically 0.3 to 2 millimetres, then swells into an insulating char up to 50 times its thickness when heated. It is the sensible option when you want the steel visible, for example an exposed beam in an open-plan kitchen or a barn-style conversion, or where there is no room to box the beam in.
For 30 minutes of protection on a standard domestic beam, expect a professional application to cost somewhere in the region of £250 to £600 per beam, depending on beam size, access and how much surface preparation the steel needs. Sixty-minute ratings need a thicker build and cost more. The exact loading is calculated from the beam's section factor, so quotes should always be based on the actual steel size, not a flat rate.
If the beam is going to be hidden anyway, plasterboard is usually cheaper. Two layers of 12.5 millimetre board, or a single layer of 15 millimetre fire-rated board fixed to a suitable frame, will typically achieve 30 minutes and cost less than a coating on a straightforward beam. Cementitious sprays and mineral fibre boards exist too, though they are more common on commercial jobs than in homes.
One point people often miss: a beam that sits fully within a fire-protected floor zone, encased above a plasterboard ceiling that itself achieves the required rating, may already be compliant with no extra work. This is exactly the sort of question worth putting to your building control officer before you spend anything.
Whether you use Manchester City Council building control or a private approved inspector, they will want evidence, not just a painted beam. For intumescent work that means the product data sheet, confirmation of the dry film thickness applied, and ideally a certificate of application from the contractor. Photographs taken before the beam is boxed in or decorated help too.
Be wary of a general decorator offering to 'fire paint' a beam with whatever is on the van. Intumescent products are tested to specific standards such as BS 476 Part 21 or EN 13381-8, and the thickness must match the manufacturer's certified data for your beam size. Applied too thin, it simply will not deliver the rating, and that only comes to light at sign-off or, far worse, in a fire.
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