How to protect structural steel from fire

Structural steel is widely used in modern construction because it offers strength, long spans and efficient installation. Its performance in fire, however, changes as temperature rises, which is why fire protection forms such an important part of building design, specification and compliance.

The purpose of structural steel fire protection is to help beams, columns and other load-bearing elements continue performing for the fire resistance period required by the building’s fire strategy and the relevant regulations. This generally means applying a tested passive fire protection system, with the most suitable method depending on the steel section, how exposed it is, and the fire resistance period needed.

What happens to steel in a fire?

Steel does not combust, but its structural performance is still affected by heat. As temperatures rise, it begins to lose strength and stiffness. It also expands as it heats up, which can create additional stresses where members are restrained.

This has direct implications for the effect of fire on steel structures. Beams can deflect more as stiffness reduces, especially when supporting floors or roofs over longer spans. Columns face a different risk because they are carrying compression loads and contributing to the stability of the frame. If unprotected steel heats up too quickly, the structure may no longer perform as intended for the period needed to support evacuation and fire service intervention.

What is steel fireproofing?

Steel fireproofing refers to systems that slow the rate at which heat passes into structural steel during a fire. The purpose is not to make the steel immune to fire, but to delay temperature rise so the steel member can continue performing for the required period, whether that is 30, 60, 90 or 120 minutes.

The specification is based on more than the fire resistance period alone. Guidance also takes account of the section factor, which reflects how quickly a member is likely to heat up based on its surface area relative to its mass. Smaller and more exposed sections tend to heat faster than heavier ones, so they may require different thicknesses of protective material. This is why fireproofing requirements are usually tied to the actual steel member and the performance of the protection system, rather than a generic thickness applied across an entire frame.

Main fire protection methods for steel structures

There are several recognised methods, but for many commercial and industrial projects, cementitious systems and fire protection boards are two of the most established approaches.

Cementitious spray fireproofing is a non-reactive system used to form an insulating layer around structural steel. Depending on the product, substrate and tested design, it may be spray applied directly to the steel, although some situations also require preparatory measures such as lath, mesh or spatter coats to help achieve the required build-up and fire performance. These systems are often used where efficient coverage across larger areas is needed, particularly in back-of-house areas, plant zones and service spaces where a more utilitarian finish is acceptable. Their suitability still depends on the tested scope of the product, the steel section, the required fire resistance period and the conditions on site. Sprayed cementitious systems can also offer programme advantages on some projects because they can be applied relatively quickly across sizeable areas.

Fire protection boards are also non-reactive and work by encasing the steel member within an insulating lining. Depending on the system, the boards may be fixed directly to the steel or installed as part of a boxed enclosure. This method can suit projects where a cleaner, more regular finish is preferred, or where the protected steel needs to align neatly with surrounding partitions, ceilings or service zones. As with sprayed systems, performance relies on the completely tested construction, including board type, thickness, joints, fixings and the profile and exposure of the steel member itself.

Intumescent coatings are another recognised option, particularly where exposed architectural steel is part of the finished design. For more details on how intumescent coatings work, where they are typically used and how they are specified, see Sharpfibre’s article on intumescent paint.

Choosing the right steel fire protection method

The right method depends on the building, the member and the environment in which the steel is used. Required fire resistance remains central, but other factors matter too, including section factor, whether the steel is exposed on three or four sides, the durability needed in service, access for installation, the desired finish and the evidence available from fire testing and assessment.

That is why structural steel fire protection tends to benefit from specialist input at an early stage. The most appropriate solution for a warehouse, for example, may differ from the best choice for a commercial office, a residential development or a plant enclosure. Cementitious spray may make sense in one setting, while fire boarding may offer better coordination or finish in another. That is where specialist contractors such as Sharpfibre can make a difference, helping ensure that specification, detailing and installation are aligned with the intended fire performance.

What building regulations apply to structural steel fire protection?

In England, the regulatory foundation sits within the Building Regulations 2010. Requirement B3 states that a building must be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period. Approved Document B supports this by giving guidance on fire resistance for elements of structure, including structural frames, beams, columns, load-bearing walls and floor structures. It also makes clear that where one element supports or stabilises another, the supporting element should have no less fire resistance than the element it supports.

BS 9999 is also an important reference point within the wider fire safety framework. BSI describes BS 9999:2017 as a code of practice covering the design, management and use of buildings to achieve reasonable standards of fire safety. In practice, it is often used alongside statutory guidance when shaping the broader fire strategy for a building.

How often should structural steel fire protection be inspected?

There is no single inspection interval that applies to every project. The right approach depends on the protection system used, the building environment, the likelihood of damage or alteration, and the wider fire safety management strategy.

What matters is that structural steel fire protection is reviewed as part of ongoing building maintenance, so that damage, gaps, moisture exposure or unauthorised changes can be identified and addressed before performance is affected.

Conclusion

Protecting structural steel from fire is therefore a matter of system performance, regulatory context and installation quality working together. Whether the project calls for cementitious spray fireproofing, fire protection boards or intumescent paint, the objective is the same: to slow the heating of the steel so the structure can continue to perform for the period the building requires.

Where structural steel fire protection needs to be carefully specified and delivered, Sharpfibre brings experience in the systems, details and installation standards that shape performance.