In short
Intumescent paint swells in a fire into an insulating char that keeps steel below the temperature at which it weakens. In a house it is mainly used on beams and columns left on show. Where the steel is hidden, fire-rated plasterboard does the same job with less fuss.
Why steel needs protecting at all
Steel does not burn, but it loses strength as it heats. At around 550°C a beam has roughly 60% of the strength it had cold, and a loaded beam can begin to sag. A room fire reaches that temperature within minutes.
Fire protection buys time. Its job is to keep the steel cool enough, for long enough, that people get out and the fire service can work inside.
When a beam in a house needs it
The guidance for England is in Approved Document B. For an ordinary house whose top floor is no more than 5 m above the ground, structural elements need 30 minutes of fire resistance. Taller houses, including many with a loft conversion, need more for some elements.
In practice:
- A beam holding up a floor or a wall above needs protection.
- A column supporting that beam needs the same.
- A beam that only holds up a roof, in a single-storey extension, often needs none. There are exceptions, such as a frame close to the boundary or one that also steadies a protected wall.
Your structural engineer or building control officer will mark on the drawings which members need protection and for how long. Scotland, Wales and Northern Ireland have their own technical guidance with similar aims.
How intumescent paint works
The coating looks like ordinary paint, about a millimetre thick or less. When it reaches roughly 200°C it reacts and expands to many times its thickness, forming a soft carbon char full of gas bubbles. That foam insulates the steel beneath.
It is a system, not a single tin:
- A primer compatible with the intumescent coat.
- The intumescent basecoat, applied to a measured thickness.
- A decorative topcoat, which also seals the basecoat against moisture.
Thickness is everything
The film thickness needed depends on three things: the fire resistance required, the shape of the section, and how much steel there is to heat up. A slim, light section heats quickly and needs a thicker coat than a chunky one.
Manufacturers publish loading tables that turn those inputs into a dry film thickness in microns. The applicator should work from the table, measure the wet film as they go, and leave you a record. Building control may ask for the product data sheet and that record. A coat brushed on by eye, with no measurement, proves nothing.
Paint or plasterboard?
For steel that will be boxed in, plasterboard wins. Two layers of 12.5 mm board with staggered joints, or one layer of a fire-rated board fixed as its maker specifies, gives 30 minutes on a typical domestic beam. It is cheap, any plasterer can fit it, and the inspector can see at a glance that it is there.
Paint earns its place when the steel is part of the look: an exposed beam across a kitchen, slender columns beside sliding doors, a frame visible through glass. It keeps the crisp outline of the section, which boarding buries.
There is a third option. Some fabricators apply the intumescent coat in the workshop, under controlled conditions, and deliver the steel finished. The coating is more even than site work, and only the bolted connections need touching up afterwards. If your steel frame extension has steel on show, ask whether this is included in the price.
Applying it on site
If the paint is going on at home, a few rules decide whether it works:
- The steel must be clean, dry and primed with a compatible primer. Mill scale, rust and old paint of unknown type all have to go.
- Water-based products need a warm, dry building. They are for interiors only.
- Several thin coats beat one thick one, with the drying time between coats that the data sheet gives.
- Do not skip the topcoat. Unsealed basecoat absorbs moisture and softens.
Protected steel should not be drilled, clad tight or wrapped in insulation afterwards without checking, because the coating needs room to expand.
What to keep
File the product data sheet, the thickness record and a few photographs with the building control completion certificate. A future buyer’s surveyor, faced with an exposed steel beam, will ask how it is protected. Paperwork answers that in a minute. Without it, the only proof is to bore a sample out of the coating.