Fire Protection for Structural Steelwork

Steel loses significant strength at elevated temperatures. A bare steel beam that is adequate in ambient conditions can fail structurally at temperatures reached within minutes in a building fire. Where Building Regulations require a specific fire resistance period — typically 30, 60, or 90 minutes in domestic construction — the steel must be protected.

Who Specifies Fire Protection?

The architect specifies fire protection as part of the Building Regulations package. This is not PorthouseDean’s scope — PorthouseDean designs the steel section for structural load-carrying, not fire resistance. The required fire resistance period is determined by the building’s use, size, and construction type under Part B of the Building Regulations.

Common Fire Protection Methods

Intumescent Paint

The most widely used method in domestic construction. Intumescent paint is applied to the steel surface at the required dry film thickness (DFT) — typically between 0.5 mm and 3 mm depending on the fire resistance period and the section factor (the surface-area-to-volume ratio of the steel section).

Under fire conditions, the paint swells to many times its original thickness, forming an insulating char layer that slows heat transfer to the steel.

Board or Spray Systems

For higher fire resistance periods or where appearance is less critical, mineral fibre spray, calcium silicate boards, or gypsum boards encasing the steel are used. These are more common in commercial construction.

Concrete Encasement

Historical method, less commonly specified in new domestic construction.

Critical Compatibility Requirement

When intumescent paint is used, it must be chemically compatible with the corrosion protection system already applied to the steel. If the primer or intermediate coat is incompatible with the intumescent coating, adhesion fails, the paint peels, and fire protection is compromised.

The architect should specify:

1. The corrosion protection system (primer type and any intermediate coat)
2. The intumescent coating system (manufacturer, product, DFT)
3. Confirmation of compatibility between the two systems — usually by way of the intumescent manufacturer’s published compatibility guidance

Do not allow a painter to apply intumescent over an unknown or untested primer without confirming compatibility first. Most intumescent manufacturers publish lists of compatible primer systems.

Practical Notes

– Fire protection is typically applied after steelwork is erected and before plastering begins
– Ensure Building Control can inspect the steel before fire protection is applied — they may wish to check section references and corrosion protection condition before it is covered
– Intumescent paint requires periodic inspection in occupied buildings — record the product and DFT at time of application for future reference

Contact us if you have questions about structural steel specification — and speak to your architect about your fire protection requirements.