Intumescent coatings explained simply

What building control is actually asking for, how the coating works in a fire, and what you need on file afterwards.

If building control has asked for “60 minutes fire protection to the steelwork” and you are holding a quote you cannot make sense of, this is the plain-English version.

How the coating works

Bare structural steel loses strength as it heats up. Around 550°C it can no longer carry its design load, and in an unprotected fire that temperature is reached in about fifteen minutes. An intumescent coating looks like ordinary thick paint until it gets hot, then it swells to many times its thickness into a stiff carbon char. That char insulates the steel and buys the time the building regulations demand.

What the numbers on the spec mean

• 30, 60, 90 or 120 minutes — the fire resistance period required by your approved inspector.
• Section factor (A/V or Hp/A) — how quickly a particular beam or column heats up. Thin sections need more coating than chunky ones.
• Dry film thickness (DFT) — the actual thickness in microns needed to achieve that period on that section. This is the number that matters and it is measured, not guessed.
• Loading condition — whether the steel is a beam or column, and exposed on three or four sides.

A proper quote lists each steel member, its section factor and its required DFT. If a quote just says “fire paint to steels”, it is not a specification and building control will not sign it off.

How it is applied

Steel is cleaned and degreased, any mill scale or loose rust removed, then a compatible primer goes on — existing primers are checked because not all of them work under intumescent products. The basecoat is applied in passes, with wet film thickness checked as we go, and each member is measured with a DFT gauge once cured. Where the steel is visible, a topcoat seals and colours it.

Thickness is everything. The same product at 70% of the specified thickness does not give you 70% of the protection.

What you should get on file

• The product data sheet and its third-party certification for your required period.
• A member-by-member schedule showing specified and achieved DFT readings.
• Photographs of the completed works.
• A signed installation certificate for the building control file and your insurer.

The usual mistakes

Coating over an incompatible primer, spraying in cold or damp conditions so the film never cures properly, missing the back of connections and cleats, and applying the topcoat before the basecoat has cured. All of them are avoidable, and all of them are why we measure and record rather than eyeball it.

We handle intumescent work on structural steel, fire-rated cladding and soffit spraying across the West Midlands, and we hand the paperwork over as standard.

On-site or off-site application?

Coating steel in a controlled shop before it is erected gives better conditions and fewer access problems, but connections, cleats and any damage from handling still need touching in on site. Coating on site suits refurbishments and exposed steel in existing buildings, where the steel is already up. Either way the same rules apply: clean substrate, compatible primer, measured thickness, recorded results.

Exposed steel that also has to look right

In restaurants, showrooms and open-plan extensions the steel is part of the design. Intumescent basecoats are chalky and uneven by nature, so where appearance matters we apply a decorative topcoat in the specified colour and sheen over the fire-rated build. That topcoat is part of the certified system, not an afterthought — using the wrong one can affect how the basecoat performs.

Other fire protection work

• Fire-rated cladding — installation and coating to meet the required reaction-to-fire class.
• Soffit and ceiling spraying — protecting steel decks and beams above car parks and plant rooms.
• Timber and board treatments — intumescent systems for exposed structural timber where a spec calls for it.
• Remedial surveys — checking existing coatings, measuring thickness and reporting on what is missing.

Getting it signed off first time

The projects that go smoothly are the ones where the fire engineer’s schedule reaches us before we price, not after the steel is up. Send us the drawings and the required periods and we will come back with a member-by-member specification, a price and a realistic programme.