Intumescent Paint Explained

What is intumescent paint? Fire protection paint explained.

When a fire breaks out it can take just minutes to spread and the effects can be devastating. Intumescent paint slows the effects of fire on steel structures, creating an expanding insulation layer when activated by extreme heat. In a fire every second counts. Heat, fuel, and oxygen are all it takes for a blaze to ignite and once fire takes hold it grows fiercer by the minute, threatening to destroy anything in its path. In fact, fire can reach around 500 degrees Celsius in just five minutes – a temperature hot enough to weaken structural steel – one of the most commonly used construction materials.

That's where intumescent paint comes in.

Intumescent paint is an industrial paint specially formulated to react to heat. When applied to steel it forms a protective coating that keeps the steel sections below their critical failure temperature, preventing them from bowing in the heat of a fire.

In Britain, intumescent paint is now a legal requirement when using steel in the construction of commercial and industrial buildings. Its application is highly technical and, as such, the process requires an intumescent spraying contractor who can apply the coating to the correct specifications to ensure compliance.  

intumescent paint
Protecting steel from heat damage

If steel is susceptible to heat damage, why build with it?

No construction material is immune to the effects of fire. Under extreme heat, concrete will crack, clay bricks will melt, and we all know how wood reacts to flame. Steel, which responds to heat at a much higher temperature, has one of the best strength to weight ratios of any construction material, which is why it’s so widely used in commercial buildings like airports, shopping centres, and office blocks.

As well as its strength, steel has many other inherent qualities that make it ideal for large infrastructure projects. It’s highly durable, can be assembled quickly, requires less excavation and is efficient in all seasons. In earthquake-prone territories, steel structures are highly resilient. Steel can also be melted down and recycled at the end of its life making it more environmentally friendly than other construction materials.

What’s intumescent paint made from?

At first glance, intumescent paint may look and behave much like any other commercial paint on the market, with water-based and solvent-based options being available. Generally, only produced in white, a compatible top seal can also be applied to provide additional durability and colour.  

it’s the make-up of its unique chemical composition that sets it apart though as Intumescent paints contain chemicals that react when exposed to heat and impede heat transfer. It’s for this reason that intumescent paints are sometimes referred to as fire-resistant paints or fire protection paints.

Intumescent paint made from fire resistant materials
Intumescent paints are activated by heat

How does intumescent paint work?

Intumescent paints are essentially dormant until they’re activated by heat. This reaction usually occurs at temperatures exceeding 200 degrees Celsius.

When the paint heats up it expands to form a protective char layer that completely surrounds the steel beam. This layer can grow up to 50 times the original thickness of the paint layer applied and protect the steel for up to 120 minutes. During this time the area around the steel is starved of oxygen and the char layer insulates the steel, preventing it from reaching its critical failure temperature.

This buys valuable time for anyone in the building to escape and for the fire to be brought under control. All the while keeping the steel structure at a temperature that doesn’t compromise its integrity.

How is intumescent paint applied?

Intumescent paint is applied as a spray coating directly to the steel beam or column. In most cases, especially with galvanised steel, the surface is cleaned and a primer first applied before application of the intumescent basecoat. Once the intumescent paint is dry a compatible topcoat may also be added. This doesn’t compromise the integrity of the product in any way.

Generally, it’s the job of a professional spraying contractor to apply the intumescent coating to the necessary specification and loadings. It is recommended that the application takes place on-site once the beams are in situ, thus ensuring an even finish that includes all bolts, or the steel can be pre-sprayed off-site. The former achieves a better aesthetic finish.

Commercial construction requiring intumescent coating
Curtain wall spraying

How much does intumescent spraying cost?

There is no ‘one size fits all’ rule when it comes to intumescent spraying. Each project is unique and as the practice is highly regulated in the UK compliance is vital.

The cost of intumescent spraying will be determined by a number of factors:

  • The specified duration of fire protection (30, 60, 90, 120 minutes)
  • The size and thickness of each steel section
  • The paint thickness / loading required to achieve the specified fire protection
  • Access, masking, and location


Using this information the intumescent paint manufacturer will calculate the required quantities and loadings for each steel section. It is the job of the applicator to ensure that the paint thickness achieved on-site corresponds to that detailed in the report. A certificate can then be issued on completion of the project.

If you are embarking on a commercial or industrial steel construction requiring intumescent coating the best thing to do is get in touch for a quote.

At Sprayzone we specialise in on-site spraying for commercial and industrial application and our operatives work nationwide. Our team is highly experienced in applying intumescent paint according to UK building regulations so if you’re looking for a trusted intumescent spaying contractor, get in touch.

Call our office on 023 8070 4238 or email and we’ll be happy to discuss your project in more detail. 

Commercial construction requiring intumescent coating

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