Steel Staircase Calculator — Stringer, Treads & Landings

Building Regulations Approved Document K (2013, England) specifies: private stairs — rise max 220mm, going min 220mm, pitch max 42°. Utility stairs — rise max 190mm, going min 250mm. General access stairs — rise max 170mm, going min 250mm. The 2R + G rule (twice the rise plus the going) should fall between 550mm and 700mm. A flight should also have no more than 16 risers before an intermediate landing is required (per AD K and BS 5395-1). A typical commercial steel staircase uses a 175mm rise and 250mm going (2R+G = 600mm).

Parallel Flange Channels (PFC) are the most common stringer section for steel stairs as they provide a flat inner face for tread bracket welding. Typical sizes are 150x75 PFC for short flights up to 3m and 200x90 PFC for spans up to about 5m, with heavier 230x90 or 260x90 PFC (or UBs) for longer/heavier flights. The section suggested here is an indicative pre-sizing guide for two stringers sharing a stair imposed load (around 4.0 kN/m² for general/commercial use under BS EN 1991-1-1 UK NA); the final section must be checked for strength and deflection (commonly a span/360 limit) by a structural engineer to BS EN 1993-1-1. Section masses used in the weight estimate are the published values from the Steel for Life Blue Book (SCI P363).

Durbar (checker plate) is the standard tread material for industrial and commercial steel stairs — typically 6mm thick. For public buildings, concrete-filled treads (using folded plate pans) are common. Open mesh treads (Weldlok/Webforge) are used where drainage or ventilation is needed. Treads should have a slip-resistant nosing with a visually contrasting strip (at least 30 points difference in light reflectance value) per BS 8300-2:2018 for inclusive access.

Approved Document K does not set a general minimum stair width — it only requires that a flight more than 1800mm wide is divided into sections at least 1000mm wide. Width is instead driven by use: means-of-escape widths come from Approved Document B and BS 9999:2017 (escape stairs are sized to the occupancy capacity they serve, with a minimum of around 1000mm and 1100mm for firefighting stairs), and accessible/inclusive widths come from Approved Document M and BS 8300-2:2018. BS 5588 — sometimes quoted for the old 1050mm / 250-person rule — was withdrawn and its escape guidance is now in BS 9999. Clear width is measured between handrails; allow extra for any handrail standoff from the wall.

Protected escape stairs in commercial buildings require 30 or 60 minutes fire resistance. Steel stairs can be protected with intumescent paint (thin film for 30 min, thick film for 60 min), fire-rated boarding, or spray-applied protection. Concrete-filled treads provide inherent fire resistance. Building Control will specify the required fire rating.