Why the location and type of steel affects intumescent choice

Why the location and type of steel affects intumescent choice

We’ve said it before, and we will say it again – steel is a wonderful construction material because it is versatile, sustainable and flexible. Most often used in-situ with concrete, it creates brilliant and long-lasting buildings. It is also known to be a very budget-friendly material and where other materials are unsuitable, its high strength-to-weight ratio can frequently provide a definitive solution.

Although steel is generally considered to be inflammable, the truth is its structural strength can be profoundly reduced when exposed to high temperatures (in excess of 550°C) over extended periods, with loses in loadbearing capacity experienced as a result. Given general office and home fires regularly reach these temperatures within minutes, measures need to be taken to help protect the structure as long as possible.
Government and regulatory bodies have indeed increased regulations around steel frame structures and their structural steel protection, but given architects are increasingly looking for greater design capacity, using concrete encasement is not always a viable solution, albeit it can be effective.
As a result, innovative companies such as Nullifire and Sherwin Williams, have, over the years, invested heavily in extensive and innovative research which has led to the development of pioneering and intumescent fireproofing coatings for structural steel, designed to prevent major calamities in steel frame structures around the world.

As regular readers and those exposed to the industry will know, intumescent paint and coatings are reactive coatings used to minimise the effects of a fire – not as a suppressant, but as a passive fire resistance fire measure. When they are exposed to high temperatures, such as in a fire, they expand significantly.  Applied to structural steel, intumescent products are designed to expand several times their original thickness when exposed to temperatures above 250ºC to form a layer of inorganic carbonaceous char which is a poor thermal conductor and less prone to burning. The result in this increase in density insulates the steel from the heat, enabling it to maintain its load-bearing capacity for longer periods. This potentially saves lives because it buys more time for evacuation of the building and emergency response.

So, now you know that intumescents are essential, but when you’re selecting an intumescent coating system for steel, do you know what consideration to give, or what factors to consider?


Internal vs external

Like a normal paint, consideration needs to be given to where the intumescent paint will be applied.

  • Is the steel internal or external? Think of aspects such as temperature, weather, and even exposure to traffic?
  • Are there harsh chemical fumes or do you anticipate splash and spills of chemicals?
  • Will the coating be insulated against the elements?
  • Is there any anticipated thermal cycling/shock?
  • IS mechanical abrasion likely?
  • What is the existing condition of the steel to be coated?

Size of steel beam


To guarantee safety as well as consistency with construction standards and building codes, the product specification (this should be done during the design phase) should clearly stipulate the size of the steel to be coated, the number of sides exposed and the clearance around the steel. Small, lightweight steel segments need a higher film thickness to accomplish the desired protection, which contrasts with bigger, heavier assemblies.
The thickness is most precisely dependent on the steel weight (per lineal foot)/fire exposed steel perimeter (W/D) ratio of the steel section (i.e. the ratio of the section’s weight [W] to the total square area in contact with fire [D]). The value of D is a significant variable, as some steel sections are completely exposed to fire, while others are protected on at least one or more faces because of their orientation and mounting. For further information on the role of different steel sections, check out our previous blog – The role of steel sections in the use of intumescents.


Clearance rate of the surrounding materials

As we alluded to above, it is important you consider the clearance rate the materials you are looking to use require, because of the swelling nature of intumescent in the event of fire. As part of this, it is also imperative you consider the space required for a full intumescent coating system that includes:

  • Primer: this mainly provides corrosion resistance, but also ensures coating adhesion.
  • The coating: In the case of a thin-film intumescent, coatings may be water-borne (single pack), solvent-based (single pack) or epoxy (two-pack). The type selected mainly depends on the kind of environment. Similarly, there are other considerations like abrasion resistance or aesthetic appeal.
  • Where the selection is an alternative (EG concrete, vermiculite or fireboard) calculations need to be made to the space needed.



No one likes to design them, and no one wants to work/live in an ugly building. Thus, aesthetics are an integral factor when considering how to protect exposed steel which forms part of the building’s overall look. Where steel is exposed, thin-film intumescent coatings are a great solution. In this instance, a fire rated coating which can be finished with an architectural top-coat  is important. Nullifire products are not only extremely efficacious, but a wide scope of decorative topcoats can be used over the top to protect steel from moisture and enhance the appearance. However, in this instance, it is essential a Nullifire manufacturer approved paint-system is used, and that again, you take into consideration the extra thickness and how it may affect clearance rates.
Where a non-compatible system is applied to a thin-film coating, application of topcoats can compromise the intumescent properties of the system.

Type of fire

Perhaps most importantly, context is everything. What you are protecting and where it is is essential to consider. If you are protecting a chemical plant structure, then you will need different solution to an office structure. In other words, the type of fire risk is a strong factor when choosing an intumescent paint for steel fire protection. The two most common types of fire are

  • Cellulosic Fire -This fire occurs where the fuel is primarily of a cellulose material: for example, paper or wood or office furniture. A fire like this spreads gradually and slowly compared with other types. Ordinarily, this kind of fire occurs in private and business structures burning up to 500°C in as little as 5 minutes and can escalate to 1100°C with a radiation value measured at 50 kW/m2 (Kilowatt/square meter).
  • Hydrocarbon Fire – These sorts of fires are brought about by flammable liquid, such as oil and gas. These kinds of flames pose some problems with suppression mainly because they do not respond well with water. Petroleum, kerosine and synthetic substances fall in this category and can heat up to 1000°C in 5 minutes, and rapidly escalate. The radiation value is 160 kW/m2 in comparison to a cellulosic fire.



Correlated to thickness and steel weight, the load exerted on steel members should be considered when choosing intumescent. However, as a fire-engineer will be able to advise, the impact of the chosen fire rating material which will be applied to the steel needs to be factored into calculations. Vermiculite is much heavier on a structure being of a cementitious nature, thus it will require extra steel to support it.


Likelihood of damage once installed

From our experience we know that after erection of a steel structure, damage often occurs on steel columns and beams due to transport, assembly and accidental knocks.
It is vital you consider the type of product you are using, where it is going, the likelihood of damage and whether it is easily repaired without compromising the performance of the product.

If you need any help: either to explain the above, or in selecting the right material, or even with understanding your project specifications, give us a call – we are happy to help!

Get in Touch

Speak to the leaders in passive fire protection

You are about to download a file from the Permax site. Please note All technical advisory notes generated by Permax are based on research papers, indicative fire tests and any other existing evidence. These documents should not be used as an official evidence as design engineers should review the information and determine the reliability of the documents.

Permax constantly update the documentations based on the new fire testing outcomes and change of standards and regulations. To ensure the documents you read are up-to-date, please contact the Permax technical team.