Intumescent coating is a paint that expands when it gets hot, forming a thick insulating char layer over the steel. That layer slows how fast the steel heats up, which delays the point at which it loses strength. It does not make steel fireproof — it buys a stated number of minutes, and only if the thickness and preparation were right.
What happens to steel in a fire
Steel keeps its design strength up to around 200°C. By roughly 550°C it has lost about half of it. Unprotected steel in a developed compartment fire can reach that in well under twenty minutes.
Nothing dramatic happens at that point. The section simply stops carrying what it was designed to carry, and deflection starts. That is the failure everyone is trying to delay.
How the coating actually works
At normal temperature it looks like paint, usually a millimetre or two thick. When it reaches its activation temperature it reacts: it foams and expands, many times its original thickness, into a light carbon char.
That char is a poor conductor of heat — which is exactly the point. The fire is now heating the char rather than the steel, and the steel's temperature rise slows down.
How long it holds is not a property of the tin. It comes from the thickness applied, matched to the steel section being protected. A thin section heats faster and needs more coating for the same rating than a heavy one.
The three things that decide success
In our experience auditing other people's work, coating failures almost never come from the product. They come from three things that happen on site.
- Surface preparation. Coating over rust, mill scale or an incompatible primer means it will eventually lift. Once it has lifted it protects nothing, and it will not lift visibly until it matters.
- Measured dry film thickness. The required thickness comes from the section factor and the rating. It is checked with a gauge at recorded points — not estimated from the number of coats.
- Damage repaired before it is hidden. Steel gets knocked by every trade that follows. A chip is a hole in the protection. The frame needs a final walk before ceilings and cladding close.
Dry film thickness readings, with the location of each reading. If a handover file contains photographs but no thickness record, nobody can say whether the rating was achieved — and after the ceiling closes, nobody can find out without removing finishes.
Where it is applied
| Element | Why it is coated | What to watch |
|---|---|---|
| Columns and beams | They hold the building up | Section factor drives the thickness — it varies across the frame |
| Bracing and connections | A failed connection fails the frame around it | Complex shapes get missed and under-coated at the corners |
| GI ducts | Carry heat across compartment lines | Coating plus correct fire stopping at the penetration itself |
| Cable trays and HT/LT runs | Cable sheathing spreads flame along the run | Barrier coating, as covered on our cable barrier page |
| Concrete where specified | Used to reach a required rating on an existing element | Needs the right primer for the substrate |
Scroll sideways on a phone.
What to ask for at handover
Four things, and they are all cheap to produce at the time and impossible to reconstruct later.
- The specification: which product, which system, and the target rating.
- The dry film thickness record, with locations.
- Photographs of the prepared steel before coating, not just the finished paint.
- A note of what was repaired after follow-on trades.
If a building you have inherited has none of these, that is not unusual and it is not a disaster. It is a reason to put the steel on the list for a passive fire audit, where thickness and condition can be checked against what the design required. Our coatings page covers how we apply and record it on new work.
