Fireproofing steel structures in new construction and renovations has become standard practice. In a fire, steel can quickly reach its yield point, leading to the collapse of the supporting structure. Fireproof paints, also known as intumescent paints, are designed to extend this time, giving people a chance to evacuate safely and allowing firefighters to intervene. During a fire, the intumescent paint layer swells, creating a protective foam on the structure.

Fire retardant paint is never applied to a structure as a standalone coating. A complete fire retardant system consists of three coatings, each of which fulfills a specific function.

  • primer paint – anti-corrosion protection of steel,
  • fire retardant paint – extending the time it takes for steel to reach its yield point,
  • Topcoat – protects the fire retardant paint coating from mechanical damage and moisture. In some cases, it may be omitted (depending on the specific intumescent paint, environmental corrosivity class, and exposure). Topcoat can be produced in a color selected by the customer from the RAL palette .

Both the primer and topcoat must be approved for use in a fire protection system. The Technical Approval/European Technical Assessment issued for the intumescent paint specifies which specific paints can be used in the fire protection system. These specific products have been tested in fire conditions. Using an inappropriate primer can result in the entire fire protection system becoming detached from the structure and failing to perform its intended function during a fire.

The fire protection system is applied to a clean steel or galvanized steel substrate. If the structure has old paint coatings of unknown origin, they should be removed. Some systems can also be applied to cast iron.

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Selection of fire retardant paint for steel structures

Selecting a fire protection system for steel structures is a complex process and depends on several factors. Each of the elements listed below is essential to selecting the optimal fire protection system.

Required fire resistance class

The fire resistance class indicates the additional time a fire-retardant paint coating provides before a steel element reaches its yield point. This class is designated as R30, for example, meaning 30 minutes of protection.

The fire resistance class for given building elements is determined on the basis of the table below, which can be found in the Regulation of the Minister of Infrastructure of 12 April 2002 on the technical conditions to be met by buildings and their location. 

fire resistance class of building elements

Steel section mass index

The steel cross-section mass index is the ratio of the perimeter of the cross-section exposed to fire to its cross-sectional area. The unit for the cross-section mass index is [  1  m ] . This means that the perimeter should be expressed in meters, and the cross-sectional area in square meters. These values ​​are usually found in tables for designing metal structures. Ready-made tables with mass indexes for various steel profiles are equally easy to find online. This value is individual for each cross-section, so for us to be able to prepare a quote for a fire protection system, it is necessary to send us a steel specification. Unfortunately, we cannot calculate the theoretical requirement for fire retardant paint based on the total surface area or weight of the structure.

Example calculations for a normal I-beam IPN240, assuming that it is exposed to fire on all sides (so-called four-sided exposure):

Cross-sectional area: A = 46.1 cm 2 = 0.00461 m 2
 Perimeter: U = 0.884 m

Sectional mass index:
U  A = 0.884 m   0.00461 m = 191.76 m If the same I -beam were to support a monolithic ceiling, for example, we would use the perimeter reduced by the width of the upper flange for calculations. In this case, the sectional mass index would be 168.76  m . We proceed similarly if only the flange protrudes from the ceiling (e.g., a Klein ceiling). As you can easily deduce, if we want a well-chosen system, we should provide detailed information at the inquiry stage. Additionally, the lower the sectional mass index, the thinner the intumescent paint layer, so this may turn out to be a cost-saving option.

If our structure consists of any unusual profiles, it's worth mentioning this. For example, two C-sections can be welded together, creating a closed profile. The thickness of the fire-retardant paint coating is then selected from a different table than for open profiles.

Design critical temperature

Critical temperature refers to the temperature at which our steel section reaches its yield point and loses its load-bearing capacity. The critical temperature should be specified by the designer.

Once we know the required fire resistance class, cross-sectional mass index, and design critical temperature, we select the appropriate coating thickness based on the Technical Approval or European Technical Assessment issued for the intumescent paint. Each of the aforementioned documents contains tables summarizing coating thicknesses.

Below is a table excerpt for Steelguard 564 , for open profiles, class R60, and a designed critical steel temperature of 550-700℃. Returning to the earlier example of the IPN240 standard I-beam, it's easy to see that depending on the degree of fire exposure and the cross-sectional mass index, the thickness of the fire retardant paint coating varies significantly. The coating thickness is expressed in millimeters and for a critical temperature of 550℃, it ranges from 0.22mm to 1.62mm, or 220 to 1620 microns dry.

steelguard561 table fragment

It may happen that a given profile cannot be protected to the required class. For example, class R60 was adopted, and the designer used closed rectangular cross-sections with thin walls. In such a case, unfortunately, other profiles must be selected.

Environmental corrosion class

The environmental corrosivity class is determined according to the PN-EN ISO 12944-2:2001 standard . A detailed description of environmental corrosivity classes can be found in our article "Selection of anti-corrosion paint systems ." For office/commercial buildings, the corrosivity class typically does not exceed C2. The corrosivity class is primarily used to determine the thickness of the primer and topcoat layers.

To sum up, in order to best select a fire protection system for a given steel structure, we will need the following information:

  • list of steel structures (how many linear or square meters of individual profiles are there)
  • critical temperature specified by the designer (if not specified, we assume a temperature of 550℃, but please remember that the responsibility always rests with the customer)
  • environmental corrosivity class
  • information whether it is black steel or galvanized steel
  • required topcoat color

FAQ – fire retardant paints for steel structures

1.What are fire retardant (intumescent) paints and how do they work?

Fire retardant paints – also known as intumescent paints – are special coatings that swell in the event of a fire, creating an insulating layer of foam around a steel structure. This slows the heating of the steel, extending the time the structure maintains its load-bearing capacity. Without this protection, steel reaches a critical temperature within minutes in the event of a fire, where it begins to collapse, potentially causing the entire structure to collapse. 

2. Is fire retardant paint enough on its own - or do I also need a primer and topcoat?

No – fire retardant paint is never used as the sole layer. A complete fire protection system typically consists of three coats: an anti-corrosion primer, an intumescent coat, and optionally a topcoat/protective coat. The wrong primer can prevent the system from working properly. Typically, an epoxy anti-corrosion paint, such as Telpox P100 , is used as the primer, followed by an intumescent coat, such as Flame Stal or Steelguard Telpur T300, as the topcoat .

3. How is it determined how many minutes of fire protection a given paint will provide?

Fire-retardant coatings are assigned a "fire resistance class"—e.g., R30, R60, R90, etc.—which indicates how many minutes the steel will maintain its load-bearing capacity after a fire. The class chosen depends on the building design and standard requirements. R30 indicates 30 minutes, R60 60 minutes, and so on. The thickness of the primer, intumescent paint, and topcoat are each selected to meet the specific requirements of the project. These may vary depending on the product selected—each product undergoes separate tests and meets the same standards at different consumption rates.

4. Can any steel profile be protected with fire retardant paint?

Not everyone. The thickness and effectiveness of the intumescent layer depend, among other things, on the cross-sectional area ratio (the ratio of the perimeter to the cross-sectional area). In some cases – for example, with thin walls or closed cross-sections – the paint may not provide the required protection. It's always worth submitting a query to a technical and sales advisor, who will determine whether application is possible based on the ETA , as well as the required thicknesses of the individual layers of the fire protection system.

5. Do fire retardant paints also work on galvanized steel structures?

Yes, fire protection systems can be applied to both black and galvanized steel. It's important that the substrate is properly prepared and clean. Using a special primer may also be important. 

6. Does fire retardant paint protect steel against corrosion? 

Not always – a fire retardant coating primarily serves a fire-retardant function. Therefore, the system includes an anti-corrosion primer, which protects the steel from rusting. Therefore, the anti-corrosion primer used in the fire retardant system provides corrosion protection, while the intumescent paint protects the steel from overheating in the event of a fire.

7. Can fire retardant paint be used on structures with existing paint or coating?

Generally speaking, no – old coatings, especially those of unknown origin, should be removed. The fire protection system requires a clean steel substrate – otherwise, proper adhesion and effectiveness in the event of a fire cannot be guaranteed. 

8. How to prepare steel before painting with a fire retardant system?

The surface must be cleaned of rust, old coatings, grease, and other contaminants. Sandblasting to bare metal is recommended as an effective method. Next, apply a suitable anti-corrosion primer approved for the given system, e.g., Telpox P100 , followed by a coat of intumescent paint, e.g., Flame Stal or Steelguard , and finally, a polyurethane topcoat, e.g., Telpur T300 .

9. Are fire retardant paints effective outdoors - e.g. steel structures exposed to weather conditions?

Yes, but a topcoat is necessary to protect the intumescent coating from moisture, UV radiation, and mechanical damage. The choice of system depends on the corrosiveness of the environment and the exposure of the structure.

10. How to check if fire retardant paint actually meets fire resistance standards? 

Check if the product has a Technical Approval or European Technical Assessment (ETA) – these documents contain tables with coating thicknesses for different resistance classes (R30, R60, etc.). Knowledge of the steel profile, mass index, and exposure conditions is crucial to selecting the right system.

If you have any questions, please contact us, we will advise you and help you choose the right system!