Fire Protection Steel Calculator — Hp/A Factor & Protection Thickness

Hp/A is the ratio of the heated perimeter (Hp) to the cross-sectional area (A) of a steel section, measured in m⁻¹. It indicates how quickly a section heats up in a fire — higher Hp/A means faster heating and more protection needed. Light sections (e.g., 203x102x23 UB, profile 3-sided Hp/A = 234) need more protection than heavy sections (e.g., 305x305x97 UC, Hp/A = 121). The values here are the profile (3-sided, slab on top flange) factors from the Tata Steel Blue Book; for a beam fully boxed in or exposed on all four sides the section factor differs, so always use the factor that matches the actual protection arrangement.

Fire-resistance periods follow Building Regulations Approved Document B and depend mainly on building height and use. For offices and similar buildings the typical periods are: up to 18m tall = 60 minutes; 18–30m = 90 minutes; over 30m = 120 minutes plus a mandatory life-safety sprinkler system. Basements are 60 minutes where less than 10m deep and 90 minutes where deeper. Single-storey buildings often need no protection, and 30 minutes applies to many low-rise cases. Always confirm the exact period with Building Control or a fire engineer for your specific building.

Intumescent paint is a thin coating (typically 0.3–5mm dry film thickness) that swells to form an insulating char layer when exposed to fire. It is used where the steel must remain visible (architectural steelwork). Modern thin-film (water- or solvent-borne) intumescents are certified to BS EN 13381-8 for up to 90 and 120 minutes depending on the product and section factor (e.g. some Interchar grades reach 120 minutes); thick-film epoxy intumescents are reserved for very high section factors, hydrocarbon (jet/pool) fires and long durations. Intumescent costs more per m² than board or spray but gives the cleanest finish. The required film thickness must come from the product’s certified loading table, not a rule of thumb.

Board protection (e.g., Promat, Supalux) uses rigid boards mechanically fixed around the steel. It gives a neat boxed appearance and can be plastered/painted. Spray protection (e.g., vermiculite/cement mix) is sprayed directly onto the steel surface. Spray is cheaper per m² and faster to apply but gives a rough finish. Board is used in visible areas; spray in hidden voids.

Yes, heavier steel sections have lower Hp/A values, meaning they heat up more slowly and need less protection (or may achieve some fire resistance unprotected). In some cases, specifying a heavier beam can eliminate the need for applied fire protection at 30 minutes. This is called 'fire engineering' and should be assessed by a fire engineer.