Intumescent Coating for Steel: How Fire Paint Protects an Exposed Beam

You leave the new steel beam exposed because the raw industrial look suits the kitchen-diner, and your builder paints it the same grey as the walls. Six months later building control asks for the fire protection details and the dry film thickness records. There are none, because what is on the beam is decoration, not protection. Now the beam has to be stripped, primed, and coated with a tested intumescent system, all after the room is finished. That is the difference between a coating that holds steel up in a fire and paint that just looks the part.

What it is and what it does

Intumescent coating is a paint-like material applied to structural steel that swells into a thick insulating char when it gets hot. The char is a foam-like crust, many times thicker than the original coat, and it shields the steel from the heat of a fire for a set period. Bare steel left unprotected loses strength fast above about 550 degrees Celsius and starts to buckle and fail. The coating buys time before the steel reaches that point, so the structure stays standing long enough for people to get out and for the fire service to act.

The reason this matters at all is that UK Building Regulations require load-bearing structure to keep doing its job for a minimum period in a fire. Approved Document B (fire safety) sets that period, written as an "R" figure in minutes. R30 means the element must carry its load for 30 minutes in standard fire conditions; R60 means 60 minutes. A bare steel beam holding up a wall, a floor, or a roof has no fire resistance on its own, so it has to be given that rating somehow. Intumescent coating is one way. Boxing the steel in fire-rated plasterboard is the other, and the two are covered side by side further down.

This page is about the coating that goes on structural steel. It is not the same product as an intumescent strip, which is the thin plastic-cased seal fitted around a fire door to close the gap when it heats up. Both swell under heat and both are called intumescent, but a door strip seals a gap and a steel coating insulates a beam. If you came here looking for fire door seals, that is a different material and a different job.

When a domestic steel needs fire protection

Most steel in an extension gets hidden inside the structure, wrapped in plasterboard as the ceiling or wall is finished. That boxing-in delivers the fire rating without anyone thinking about it as "fire protection". The question of intumescent coating only comes up when you want to see the steel: an exposed beam across a kitchen-diner, a visible column at the corner of a bifold opening, an industrial-style frame left deliberately bare.

The required rating comes from what the steel is holding up and how the house is arranged. Your structural engineer and building control set it, not the builder and not you. As a rough guide for domestic work:

• A single-storey extension with nothing occupied above the beam usually needs R30 (30 minutes).
• A beam carrying an upper floor, a loft conversion, or a third storey, or any steel in a house with a basement, typically needs R60 (60 minutes).

Do not treat those as the decision. The engineer reads the actual requirement off Approved Document B against the building's height, storey count, and use, and states the rating on the structural drawing or in the fire strategy. If the drawing says R60, the coating has to deliver R60, and a coating system tested only to R30 will not do.

How the coating works

At normal temperatures the coating is just a slightly thick paint film sitting on the steel. When a fire raises the surface temperature past the activation point, the chemistry in the coating reacts: it releases gas and softens, and the film blows up into a carbonaceous char (a black, foam-like crust mostly made of carbon). That char can be anything from 5 to 50 times thicker than the original paint film, depending on the product.

The char is a poor conductor of heat, so it acts as a blanket. The steel underneath heats up far more slowly than it would bare, which is what keeps it below its failure temperature for the rated time. The thicker the char and the better it insulates, the longer the protection lasts. That is why the applied thickness is the number everything hinges on.

The coating system: three parts, not one tin

Intumescent protection is a system of layers, not a single product, and the layers have to be compatible with each other and with the steel. Treating it as one tin of paint is where peeling and failures start.

Primer. The steel arrives from the fabricator with a shop primer already on it, applied to stop it rusting in the yard. The intumescent basecoat has to be compatible with that primer, or be applied over a fresh primer the coating manufacturer approves. Put the basecoat straight onto bare, oily, or incompatible steel and it will not bond. It then peels, blisters, or sheets off, and a coating that has lifted off the steel protects nothing.

Intumescent basecoat. This is the active layer, the part that swells into char. It is built up to a specified dry film thickness (DFT), which is the thickness of the dried coating measured in microns (thousandths of a millimetre). The DFT is not a fixed number. It is calculated for the particular beam and the required rating, and getting it right is the whole job, covered in the next two sections.

Topcoat or sealer. A decorative and protective sealer over the basecoat. It gives the colour and finish you actually see, and protects the intumescent layer from knocks, moisture, and wear. It must be a topcoat the coating manufacturer approves, because the wrong sealer can stop the basecoat expanding when it needs to.

The whole system is one tested assembly. You cannot mix a basecoat from one maker with a primer and topcoat from another and assume the rating holds. Buy the system as specified and follow the manufacturer's data sheet for each layer.

Dry film thickness and the section factor

This is the part homeowners find odd: two beams of the same length, both needing R60, can need completely different thicknesses of coating. The reason is the section factor, written Hp/A.

Section factor is the heated perimeter of the steel (Hp, the length of the surface exposed to fire, in metres) divided by the cross-sectional area of the steel (A, in square metres). In plain terms, it compares how much surface the fire can attack against how much solid steel there is to heat up. A thin, light section has a lot of surface and not much metal, so it heats quickly and has a high section factor. A chunky, heavy section has more metal for its surface, heats more slowly, and has a low section factor.

The practical upshot: a chunkier beam needs less coating for the same rating, and a light, slim section needs more. A heavy column might reach R60 on a relatively thin film, while a slender beam of the same length might need a much thicker build to hit the same 60 minutes. The number of faces exposed to fire matters too. A beam exposed on three sides (the underside and both sides, with a floor sealing the top) heats differently from one exposed on all four, and the section factor is worked out for the actual exposure.

How the required thickness is set

Nobody guesses the thickness. Every tested intumescent product comes with a loading chart (sometimes called a thickness table or assessment) that has been produced by burning the coating on real steel sections in a furnace under BS EN 13381-8, the European standard for testing reactive fire protection on steel. The Association for Specialist Fire Protection publishes the recognised methodology in its "Yellow Book", which the industry treats as the reference for applying these results.

To find the thickness for your beam, the applicator or fire engineer takes three inputs:

The output is a specific number of microns of basecoat. The applicator then builds the coating up to that thickness, measures it with a DFT gauge as it dries, and records the readings. Those records are what building control wants to see: proof that the right thickness of a tested product went on the right steel.

On-site versus shop-applied

There are two points in the build where the coating can go on.

Shop-applied (off-site). The fabricator or a coating shop applies the system to the steel before it is delivered, under controlled conditions with good spray equipment and proper thickness measurement. The finish is more even, the DFT is easier to control, and it does not hold up the site. The catch is handling: the coating can be chipped or scraped during transport and installation, and any damage has to be made good on site before sign-off. For an exposed feature beam where finish quality matters, shop application usually gives the cleaner result.

On-site. The coating is brushed, rolled, or sprayed onto the steel after it is installed. This suits work that has to be touched up or coated in place, and it lets the steel be welded and bolted up first without worrying about damaging the coating. The downsides are real: site conditions (dust, damp, cold, poor access up at beam level) make it harder to get an even film and an accurate thickness, and brush or roller work takes many more coats to build up the same DFT than spray does. Either way, the surface has to be clean, dry, and properly primed first, and each coat has to cure before the next.

Whichever route, a specialist coating applicator should do the work, not the general builder with a tin and a roller. The thickness has to be measured and recorded, and that is a trade skill with the right gauges, not a decorating job.

The alternative: boxing in with plasterboard

For a beam that does not need to be seen, coating is usually not the cheapest answer. Boxing the steel in fire-rated plasterboard is.

Boxing in means cladding the beam in two layers of fire-rated plasterboard, fixed to the beam soffit and sides with staggered joints, built exactly to the board manufacturer's tested specification. Two layers of 12.5mm board to the right detail gives R30; two layers of 15mm fire-rated board gives R60. The board does the insulating job that the char would have done, and it leaves a clean boxed-in shape ready to plaster and paint like any other ceiling or bulkhead.

The table below compares the two approaches on the things that decide which you pick. Prices are current 2026 UK figures including VAT, for the coating materials at retail and a typical applied or supply cost for a single domestic beam. Treat them as ballpark: the real cost depends on the section size, the rating, and access.

The choice is mostly about the look. If the design calls for an exposed steel beam, coating is how you keep it visible and still meet the regs. If the beam is going to be hidden in a ceiling or bulkhead anyway, boxing in is simpler and usually cheaper, and most extension beams are boxed in for exactly that reason. The retail tin prices in the table are for guidance against a quote: a single 5 litre tin of a recognised water-based intumescent basecoat sits in the lower band, with higher-build solvent or specialist products at the top of the range.

Standards, who specifies, and who applies

The coating's performance is established under BS EN 13381-8, the test standard for reactive fire protection applied to steel members. The ASFP Yellow Book sets out how those test results are turned into the loading charts applicators use. A coating without test evidence behind it is not a fire protection product, whatever the tin says.

Three roles keep this honest. The structural engineer (or a fire engineer) specifies the required rating, the limiting temperature, and confirms the section factor for the steel. Building control checks that the specified protection is in place and signs it off. A specialist coating applicator prepares the steel, applies the system to the calculated DFT, measures it, and records the readings. The general builder installs the steel and may box in hidden beams, but applying a tested intumescent system to an exposed beam is specialist work.

Inspection and sign-off

Building control treats fire protection as a hold point. Do not let an exposed coated beam be decorated over or signed off until the protection is confirmed. The applicator should hand over a record of the product used, the system (primer, basecoat, topcoat), and the measured dry film thickness along the beam, ideally with the inspection readings. That paperwork is the evidence the coating actually delivers the rating on the drawing.

Common mistakes

Treating intumescent paint as decoration. The single biggest error. A beam painted with ordinary metal paint, or with a thin single coat of fire paint and no thickness calculation, has no rating. The protection only exists when a tested system goes on to the calculated DFT and is recorded.

Wrong primer or no primer. The intumescent basecoat has to bond to a compatible primer. Applied over bare, oily, or incompatible steel it peels and blisters, and a coating that has lifted protects nothing. Confirm the shop primer matches the chosen coating system before the steel is delivered, or have it re-primed correctly.

Under-thickness DFT. Building up too few coats, or spreading the product too thin, leaves the film below the microns the loading chart calls for. It looks finished but underperforms. This is why the thickness is gauged and recorded, not eyeballed.

Painting over the coating with ordinary emulsion. A decorator coming through later and rolling household emulsion over the beam can stop the basecoat expanding when it heats, killing the protection. Any topcoat over an intumescent system has to be one the coating manufacturer approves. Tell the decorator the beam is fire-coated and must not be repainted with anything else.

Assuming "fire paint" gives a rating without a calculation. No product gives a rating by default. The same tin gives different ratings at different thicknesses on different sections. Without the section factor, the rating, and the loading chart, a coat of fire paint is just paint that happens to char.

Confusing the coating with door strips. Intumescent coating protects structural steel. Intumescent strips seal fire doors. They are different products for different jobs, and one cannot stand in for the other.

Where you'll need this

• Steels and lintels - an exposed steel beam or column left visible as a feature gets its fire rating from an intumescent coating rather than being boxed in with plasterboard

Intumescent coating comes up during the structure phase of any extension, loft conversion, or knock-through where a steel beam or column is deliberately left exposed instead of hidden. Hidden steels are almost always boxed in with fire-rated board, which is cheaper; the coating is the route you take when the design calls for visible steel and that steel still has to meet its R30 or R60 rating. Specify it with the engineer from the start, use a tested system to the calculated thickness, and keep the records for building control.