19th August 2026Structural steel is widely used in modern construction because it provides strength, durability and flexibility when designing buildings. However, like other construction materials, steel has limitations when exposed to high temperatures.
During a fire, structural steel can heat up rapidly. As temperatures increase, the strength and load-bearing capacity of the steel can be significantly reduced. If the steel reaches a critical temperature, it may no longer be able to support the loads it was designed to carry.
This is where intumescent paint can play an important role.
Intumescent coatings are a form of passive fire protection designed to protect structural steel by creating an insulating layer when exposed to high temperatures. This can help slow the rate at which the steel heats up and provide valuable additional time for the building’s fire strategy to work.
But how exactly does intumescent paint work, where is it used and why is correct specification and application so important?
What Is Intumescent Paint?
Intumescent paint is a specialist fire protection coating applied to structural elements, particularly structural steel.
Unlike conventional paint, an intumescent coating is designed to react when exposed to high temperatures.
When a fire occurs and the temperature around the coated steel rises, the coating undergoes a chemical reaction. It expands significantly and forms a thick, insulating layer known as a char.
This char acts as a protective barrier between the fire and the steel.
The expanded coating has a much greater thickness than the original dry film and helps reduce the rate at which heat reaches the underlying steel.
The purpose is not to make structural steel completely fireproof. Instead, intumescent protection is designed to delay the steel reaching temperatures at which its structural performance could be significantly compromised.
How Does Intumescent Paint Work?
The process can be broadly understood in three stages.
1. The coating is exposed to increasing temperatures
Under normal conditions, the intumescent coating remains relatively unchanged.
When a fire develops, the temperature surrounding the coated steel begins to increase.
As the coating reaches its activation temperature, its components begin to react.
2. The coating expands
The intumescent coating starts to expand as the chemical reaction takes place.
The original coating can expand many times its dry film thickness, creating a thicker carbonaceous layer around the steel.
This is commonly referred to as the char layer.
3. The char insulates the steel
The resulting char acts as an insulating layer.
It slows down the transfer of heat from the fire to the steel, giving the structural element more time before reaching a temperature at which its load-bearing capacity may be significantly affected.
This additional time can be an important part of the building’s overall fire safety strategy.
Why Does Structural Steel Need Fire Protection?
Steel is a strong and widely used structural material, but its performance changes significantly when exposed to high temperatures.
As steel heats up, it progressively loses strength and stiffness. The exact behaviour depends on a range of factors, including the steel section, loading conditions and the temperature reached.
In a fire, the objective is therefore to control the rate at which the steel heats up.
Structural fire protection systems are designed to help achieve the required level of fire resistance for the building.
This can be particularly important in buildings where structural steel forms a significant part of the primary load-bearing structure.
Without appropriate protection, structural steel may reach critical temperatures more quickly during a fire.
Intumescent coatings provide one method of helping to delay this process.
Where Is Intumescent Paint Used?
Intumescent coatings can be used in a wide range of commercial, industrial and construction applications where structural steel requires fire protection.
Typical applications can include:
- Structural steel columns
- Steel beams
- Steel frames
- Trusses
- Other load-bearing steelwork
- Commercial buildings
- Industrial buildings
- Offices
- Retail developments
- Warehouses
- Multi-storey buildings
- New-build construction projects
- Refurbishment projects
The appropriate protection will depend on the specific building, structural design and required fire resistance.
Intumescent paint can also provide an alternative to some forms of encasement or boarding, particularly where maintaining the appearance of exposed structural steel is important.
Why Choose Intumescent Coatings for Exposed Steel?
One of the advantages of intumescent coatings is that they can provide fire protection without completely concealing the appearance of the structural steel.
In some architectural projects, exposed steel forms an important part of the building’s design.
Traditional fire protection methods can involve enclosing structural steel within boards or other protective materials. While these systems can be highly effective, they can change the appearance and dimensions of the structural element.
An appropriately specified intumescent coating can provide a comparatively slimline solution while allowing the underlying shape of the steelwork to remain visible.
This can make it particularly useful in commercial and architectural projects where both fire protection and aesthetics need to be considered.
How Thick Does Intumescent Paint Need to Be?
There is no single thickness that applies to every intumescent coating application.
The required dry film thickness depends on the specific system and the conditions in which the structural steel will be exposed during a fire.
Factors that can influence the required thickness include:
- The size and shape of the steel section
- The section factor
- The required fire resistance period
- The type of steelwork
- The design temperature
- The exposure conditions
- The particular intumescent coating system being used
- The manufacturer’s tested or assessed requirements
This is why simply applying a thicker layer of intumescent paint does not automatically mean that the steel has greater or more appropriate protection.
The coating needs to be correctly specified for the application.
Why Correct Application Matters
Correct application is just as important as selecting the right intumescent coating.
A high-quality product cannot provide the intended level of protection if it is incorrectly applied.
Application needs to follow the requirements of the relevant system, including surface preparation, environmental conditions, application method and coating thickness.
Problems can occur when:
- The steel surface has not been adequately prepared
- The coating is applied too thinly
- The coating is applied unevenly
- The wrong primer is used
- Incompatible products are applied
- Environmental conditions are unsuitable
- The specified number of coats is not followed
- The finished coating is damaged during construction
These issues can affect the performance and durability of the system.
For this reason, intumescent paint should be treated as a specialist passive fire protection system rather than simply another type of paint.
Surface Preparation Is an Important Part of the Process
Before intumescent paint can be applied, the structural steel needs to be appropriately prepared.
The preparation required will depend on the coating system and condition of the steel.
The surface needs to be suitable for the primer and intumescent system being used. Contamination, corrosion, dirt or other issues can affect adhesion and the performance of the coating.
If the surface preparation is inadequate, the coating may not bond correctly or may deteriorate prematurely.
A properly planned application therefore considers the condition of the steel before the intumescent system is installed.
The Importance of the Primer
Intumescent systems often involve more than one coating layer.
Depending on the specified system, a primer may be applied to the structural steel before the intumescent coating.
The primer can provide corrosion protection and create the appropriate surface for the intumescent coating to adhere to.
However, not every primer is compatible with every intumescent system.
Using an unsuitable primer can compromise the overall coating system.
This is another reason why products should be specified and applied as a complete tested or assessed system rather than selected individually.
What About the Topcoat?
In some applications, a topcoat may be required over the intumescent layer.
The topcoat can provide additional protection to the system and may also provide the desired decorative finish.
Again, compatibility is important.
The topcoat needs to be suitable for the specific intumescent system and the environment in which the steel is located.
The requirements for structural steel inside a dry commercial building may differ from those for steel exposed to more demanding environmental conditions.
Inspecting Intumescent Coatings
Inspection is an important part of any passive fire protection installation.
The finished coating should be checked to help confirm that it has been applied in accordance with the specified system.
One important consideration is coating thickness.
Dry film thickness measurements can be taken using appropriate equipment to assess whether the coating has been applied to the required thickness.
Visual inspection can also identify issues such as:
- Uneven application
- Missed areas
- Damage
- Cracking
- Contamination
- Poor finishing
- Areas requiring repair
Inspection should be carried out at appropriate stages rather than relying solely on a final check once the entire project has been completed.
What Happens If Intumescent Paint Is Damaged?
Construction sites can be busy environments, and structural steel may be exposed to other trades after the intumescent coating has been applied.
Impact, drilling, cutting, welding or other work can damage the protective coating.
If damage occurs, it should not simply be ignored.
The affected area should be assessed and repaired in accordance with the requirements of the specified system.
This is particularly important because even relatively small areas of damage can interrupt the continuity of the protective coating.
Good communication between trades and regular inspection can help identify damage early and prevent problems from being hidden by subsequent construction work.
Intumescent Paint and Refurbishment Projects
Intumescent coatings are not limited to new-build construction.
They can also be considered as part of refurbishment and remedial projects where existing structural steel requires additional or renewed fire protection.
Older buildings may contain structural steel that has been exposed, modified or protected using systems that no longer meet the requirements of the current project.
A survey can help establish the condition of existing protection and identify areas requiring remedial work.
Before a refurbishment project begins, it can therefore be useful to understand:
- What structural steel is present
- What fire protection is currently installed
- Whether the existing system is identifiable
- Whether the coating is damaged
- Whether alterations have affected the protection
- What level of fire resistance is required
This information can help inform the appropriate approach to any remedial work.
Intumescent Paint as Part of a Wider Fire Strategy
It is important to remember that intumescent paint is only one part of a building’s passive fire protection strategy.
A building’s fire safety arrangements can include a range of measures designed to help protect occupants and the structure during a fire.
These may include:
- Compartmentation
- Fire stopping
- Fire barriers
- Fire doors
- Structural fire protection
- Fire detection and alarm systems
- Emergency lighting
- Fire suppression systems
- Evacuation procedures
Intumescent coatings have a specific role: helping to protect structural steel from the effects of fire.
They should therefore be considered alongside the building’s wider fire strategy rather than viewed in isolation.
Choosing the Right Intumescent Paint System
There is no one-size-fits-all approach to structural steel fire protection.
The correct system needs to be determined based on the specific requirements of the project.
This can involve consideration of:
- The structural steelwork – The size and shape of the steel sections can affect the protection requirements.
- The required fire resistance – The system needs to achieve the specified fire performance for the application.
- The environment – Internal and external environments can present different challenges for coating systems.
- The existing construction – On refurbishment projects, existing coatings and substrates need to be assessed.
- The complete coating system – Primer, intumescent coating and topcoat, where applicable, need to be compatible.
- Installation requirements – Application needs to follow the specified system and manufacturer’s requirements.
Getting these details right at the beginning can help avoid problems later in the construction process.
Why Professional Intumescent Paint Application Matters
Structural fire protection is not an area where assumptions should be made.
The performance of an intumescent coating depends on the complete system, from specification and surface preparation through to application and inspection.
Using experienced professionals can help ensure that the appropriate system is selected and applied correctly.
It can also help project teams identify potential problems early, particularly where structural steel is being protected as part of a larger construction or refurbishment programme.
Good workmanship, appropriate inspection and accurate documentation all contribute to confidence in the finished installation.
Protecting Structural Steel From Fire
Intumescent paint provides an effective way of protecting structural steel while allowing the underlying steelwork to remain relatively unobtrusive.
When exposed to high temperatures, the coating expands and forms an insulating char layer, slowing the rate at which heat reaches the steel.
However, the effectiveness of the system depends on much more than simply applying a coat of specialist paint.
The correct system needs to be selected for the application, the steel needs to be appropriately prepared, the coating needs to be applied to the required specification and the completed work should be inspected and maintained appropriately.
For construction projects, considering these requirements early can help reduce the risk of delays, remedial work and problems later in the project.
Need Intumescent Paint for a Construction Project?
TBL Fire Protection provides specialist passive fire protection services for construction and commercial projects, including intumescent paint for structural steel.
Our experienced team can help with the specification, application and inspection of passive fire protection systems, helping project teams address their structural fire protection requirements.
Whether you are working on a new-build development, commercial property or refurbishment project, getting structural steel fire protection right is an important part of the wider fire strategy.
02476 404534
WhatsApp
