Intumescent paint is a reactive coating that swells when it gets hot, forming an insulating char layer that slows the rate at which structural steel heats up in a fire. It buys time. That’s the whole job.
Unprotected steel starts losing strength at around 400C and is down to roughly half its capacity by 550C. In a real fire that happens in minutes. Intumescent coatings push that back to 30, 60, 90 or 120 minutes depending on the specification, which is long enough for people to get out and for the fire service to arrive.
How Does Intumescent Paint Actually Work?
At normal temperatures it looks like ordinary paint. A steel column with 1.5mm of intumescent on it looks like a painted steel column.
Somewhere around 200 to 250C the chemistry kicks in. The coating softens, releases gas and expands to somewhere between 20 and 50 times its original thickness, turning into a thick carbon foam. That foam is a poor conductor, so the steel underneath heats up far more slowly than it otherwise would.
It’s a one-shot product. Once it’s reacted, it’s spent. After any fire the affected steelwork needs stripping and recoating.
When Do You Need It?
Approved Document B sets fire resistance periods for structural elements based on building use, height and whether there’s a basement. Once the fire engineer has set the period, someone has to deliver it, and on a steel frame that means either boarding, spray-applied cementitious protection or intumescent coating.
In practice you’ll see intumescent specified when the steel is visible and the client wants it to stay looking like steel. Exposed frames in retail, leisure, education and commercial fit-outs are the usual candidates. Board it and you get a boxed column. Coat it and you keep the profile.
It also gets used where access is tight. Boarding needs room to work around a section. A spray gun doesn’t.
On warehouses and distribution sheds the picture is different. Plenty of single-storey industrial buildings need no protection at all on the frame, because the fire resistance requirement is nil once you’re outside the boundary distances. That’s worth checking before anyone prices a coating, because it’s a large sum to spend on something the regulations don’t ask for.
How Thick Does It Need to Be?
Thickness comes from the section factor of the steel, the required fire resistance period and the manufacturer’s test data. It isn’t a judgement call.
Section factor is the ratio of the heated perimeter to the cross-sectional area, written as Hp/A. A chunky column has a low section factor and heats slowly, so it needs less coating. A slender beam has a high section factor, heats fast, and needs more.
The numbers move a long way across that range. Sixty minutes on a heavy column might be a few hundred microns. A hundred and twenty minutes on a light beam can run to several millimetres, applied in multiple passes with drying time between each.
Every project should have a loading schedule setting out the dry film thickness required for each individual member. If a contractor is pricing off a blanket thickness for the whole frame, they’re either overcoating half of it or undercoating the rest.
Can It Be Applied on Site or Does It Need to Be Off Site?
Both work, and both have trade-offs.
Off-site application in a controlled shop gives you better conditions, faster drying and no weather delays. Steel arrives coated. The downside is transport and erection damage, and every bolt, weld and cut end needs touching up on site anyway.
On-site application deals with all of that at once but depends on the building being weathertight, warm enough and dry enough. Most water-based intumescents want temperatures above about 5C and relative humidity under roughly 80 per cent, which in a British winter is not a given.
Programme usually decides it. If the frame is on the critical path, off site wins.
What Goes Wrong?
Thickness is the big one. Wet film thickness gets measured as it goes on and dry film thickness gets checked afterwards, and if nobody is checking, you find out at handover when the inspecting officer asks for the readings.
The wrong primer is next. Intumescents are fussy about what’s underneath them. Put one over an incompatible primer and it can lose adhesion years later, usually somewhere nobody can reach.
Then there’s the topcoat. Decorative sealer coats over intumescent have to be from an approved system, because the wrong one can restrict expansion and stop the coating doing its job.
Damage during follow-on trades is the quiet one. Services get fixed to protected steel, brackets get drilled through the coating, and nobody patches it. Every penetration is a gap in the protection.
What Documentation Should You End Up With?
A certificate is not enough on its own. You want the loading schedule showing required thickness per member, the recorded dry film thickness readings, the product datasheets and third-party certification, and the applicator’s approval status for that system.
Third-party accredited applicators under schemes like FIRAS or Certifire are the standard ask on anything of size, and building control will normally want to see it.
Work with Anglia Decor intumescent coatings or another accredited applicator and that paperwork comes as part of the job rather than as a scramble at handover.
The Short Answer
Intumescent paint is fire protection that looks like paint. You need it when a steel frame has a fire resistance period to meet and the client wants the steel visible.
Get the fire engineer’s requirement first, get a loading schedule second, and make sure someone is measuring thickness while it goes on rather than arguing about it afterwards.
