External Steel Staircase Calculator — BS 5395 Compliant Design

Approved Document K 2013 Table 1.1 has three rows: private stair rise 150-220mm and going 220-300mm; utility stair rise 150-190mm and going 250-400mm; general access stair rise 150-170mm and going 250-400mm. The 2R + G figure of 550-700mm is described in the table as the normal relationship between rise and going, so treat it as a comfort check rather than a pass or fail. Pitch is measured per step as the inverse tangent of rise ÷ going, and AD K 2013 sets a maximum only for private stairs: 42 degrees, Table 1.1 note 1. There is no 38 degree or 33 degree limit anywhere in the document. The only other pitch figure is 35 degrees for gangways to spectator seating in assembly buildings (para 1.4a). AD K 2013 also sets no width for an external escape stair: escape widths come from Approved Document B (para 1.14), and the 1200mm of para 1.15a applies to stairs in buildings other than dwellings.

AD K 2013 para 1.34c requires a handrail on both sides if the stairs are 1000mm or wider, in all buildings, and para 1.34a puts the top of the handrail 900mm to 1000mm from the pitch line or floor. In buildings other than dwellings, and in common access areas in flats without passenger lifts, para 1.35 asks for continuous handrails on both sides of the flights and the landings, and Diagram 1.12 shows 900mm to 1100mm at the landing. Escape stair provisions come from Approved Document B (para 1.14). Handrails should be continuous and grippable: para 1.36g and Diagram 1.13 give a 32mm to 50mm circular profile for buildings other than dwellings, so 48.3mm CHS tube suits.

Yes, hot-dip galvanising to BS EN ISO 1461 is essential for external steel stairs in the UK. A typical 85μm zinc coating provides 25-40 years protection depending on the environment. For coastal or industrial locations, a duplex system (galvanising + powder coat) is recommended. Never rely on paint alone for external steelwork.

Open mesh grating (Weldlok/Webforge) is the most common for industrial and commercial external stairs — it is self-draining, non-slip, and allows light through. Durbar checker plate with drainage holes is another option. Steel treads should meet the strength and slip-resistance requirements of BS 4592 (industrial gratings and treads); a slip-resistant nosing at least 25mm wide is recommended, with a wet pendulum test value (PTV) of 36 or above for the trafficked surface. Avoid smooth surfaces that become slippery when wet.

External fire escape stairs within 1.8m of an unprotected opening in the building facade need 30-minute fire protection. However, if the stair is far enough from the building face (over 1.8m), or the adjacent wall has fire-resistant construction, no fire protection is needed. Galvanised steel without fire protection is the typical solution for detached external stairs.