Steel Staircase London: Commercial Interior Fit-Out Project

The Challenge

Commercial staircases in the UK operate under a different regulatory framework than residential ones, and the differences are significant. Where a domestic stair in a private house follows the relatively relaxed requirements of Approved Document K for dwellings, a staircase in a commercial fit-out must satisfy Approved Document K for buildings other than dwellings, Approved Document B (fire safety), and the structural loading requirements of BS 6399-1 — all enforced by Building Control and, in many London boroughs, the District Surveyor.

For this London project, the key regulatory requirements were:

• Stair geometry (AD K, Table 1.2): For buildings other than dwellings, the maximum rise is 190 mm and minimum going is 250 mm. This is more restrictive than domestic stairs (220/220 mm) and directly affects the stair footprint — commercial stairs need more floor area for the same floor-to-floor height.
• Guardrail height (AD K): 1,100 mm minimum for stairs and landings in buildings other than dwellings (versus 900 mm on domestic stairs). This increases the visual impact of the railing system and requires stiffer guard construction to maintain rigidity at the greater height.
• Imposed loading (BS 6399-1): This building was classified as Category C (congregation areas with fixed seating), requiring 4.0 kN/m² imposed load on the stair — roughly four times the domestic requirement of 1.5 kN/m². The stringer sections, tread brackets, and landing connections all needed to be sized for this substantially higher load case.
• Handrail (AD K): Handrails required on both sides of stairs wider than 1,000 mm. Height between 900 mm and 1,000 mm from the pitch line, with 300 mm extensions beyond the top and bottom nosings.

The practical challenge was equally demanding: the building was in central London with no dedicated loading bay, narrow corridors, and a goods lift with a 2.4 m × 1.2 m car. Every stair component had to be sized to fit through these access constraints — ruling out the large pre-assembled sections that simplify installation on suburban or greenfield sites.

Project Gallery

The hero image shows the completed staircase in its commercial setting. The twin-stringer construction is visible — two parallel C-channel stringers provide substantially more torsional rigidity than a mono-stringer, which matters in a commercial application where the stair may carry asymmetric loads (people passing on one side) and must maintain deflection limits under continuous use. The RAL 9005 Jet Black powder coat gives the steel a matte industrial finish that reads as intentional in a commercial interior without the visual fragility of polished or brushed metal. Our straight staircase systems are available in twin-stringer and mono-stringer configurations, with the twin-stringer typically specified for commercial applications where loading and rigidity requirements exceed residential levels.

The detail images show the checker-plate tread surface and the railing connection to the stringer. Checker plate (also called durbar plate or diamond plate) provides inherent slip resistance through its raised pattern — a practical requirement in commercial stairs where Approved Document K mandates slip-resistant nosings and the HSE (Health and Safety Executive) guidelines recommend a minimum 36 PTV (Pendulum Test Value) for stair surfaces.

Our Solution

We designed the staircase as a bolted twin-stringer steel assembly that could be broken into sections small enough to pass through the building’s access constraints, then bolted together on site to form a rigid structure. This segmented approach trades some installation speed for the flexibility to work within tight urban buildings — a common requirement in London’s commercial property market, where many buildings predate modern loading-bay standards.

Structural Design

• Stringers: Twin 250 × 90 × 6 mm parallel flange channels (PFC) in S275JR steel, spanning floor-to-floor with an intermediate landing at mid-height. The PFC section was selected over rectangular hollow section (RHS) because it provides a flat inner face for tread bracket welding and simplifies the connection to the landing beams. Stringer sections were limited to 2.3 m maximum length to fit the goods lift.
• Treads: 5mm checker-plate steel, welded to 75 × 50 × 5 mm angle brackets cantilevered from both stringers. Tread width was 1,050 mm clear between stringers — exceeding the 1,000 mm minimum recommended by AD K for escape stairs. Each tread was load-tested in the factory to 3.0 kN concentrated (150% of the 2.0 kN point load specified by BS 6399-1 for this category), with measured deflection under 1 mm at mid-span.
• Landing: 200 × 100 × 6 mm RHS beams forming a half-landing platform, with checker-plate deck welded to the beams. The landing connects the upper and lower stringer sections through bolted splice plates (M16 Grade 8.8 bolts, torqued to 190 Nm).

Railing System

• Balusters: 40 × 10 mm flat bar, welded to the outer stringer face at 120 mm centers — well within the 100 mm sphere-passage limit required by AD K.
• Handrail: 42.4 mm diameter circular hollow section (CHS) continuously welded to the baluster tops. The 42.4 mm diameter provides a comfortable graspable profile within the AD K recommended range. Handrails extend 300 mm horizontally beyond the top and bottom nosings as required by AD K Section 1.34.
• Guard height: 1,100 mm measured vertically from the pitch line to the top of the handrail — the AD K minimum for buildings other than dwellings.

Surface Finish

The entire assembly received a factory-applied powder coat in RAL 9005 Jet Black. The specification was a polyester powder at 80–100 μm dry film thickness (DFT) over a shot-blasted surface prepared to SA 2.5 (near-white metal). This preparation standard removes all mill scale and provides the mechanical key needed for powder adhesion. In a commercial environment with daily foot traffic, the higher DFT provides measurably better chip and abrasion resistance than the 60 μm typical of residential specifications.

Delivery & Installation

London commercial fit-outs impose logistics constraints that do not exist in suburban or rural projects. For this installation, the key access limitations were:

• No loading bay: The building had street-level access only, requiring delivery during a timed slot (typically 07:00–09:00 before business hours) with a parking suspension arranged through the local council.
• Goods lift constraint: Maximum component size 2.3 m × 1.1 m × 0.4 m, maximum weight 500 kg per lift load. All stair sections were designed to fit within these dimensions.
• No site welding: The fit-out contract prohibited hot works (welding, cutting) inside the building due to fire risk and adjacent occupied floors. All connections were bolted using pre-drilled splice plates.

Component Breakdown

The staircase was shipped as 14 individual components in two crated pallets:

• 4 × stringer sections (2 per flight, each under 2.3 m)
• 2 × landing beam assemblies
• 1 × landing deck plate
• 4 × handrail sections (pre-welded with balusters, bolted at splice points)
• 2 × base plates and anchor bolt sets
• 1 × fastener kit (all M16 bolts, washers, and nuts, individually bagged by connection point)

The installation team worked from a numbered assembly drawing that mapped each component to its position. Bolted connections allowed two fitters to erect the complete staircase in approximately 1.5 days — significantly faster than a welded installation would have required, even without the hot-works restriction. For contractors coordinating steel stair assembly with railing and tread installation, our stair and railing coordination reference includes erection sequences optimized for restricted-access commercial buildings.

The Result

The completed staircase delivers the commercial-grade performance the project required without the overbuilt appearance that sometimes characterizes industrial steel stairs. The twin-stringer construction provides the structural rigidity needed for the 4.0 kN/m² commercial loading, while the checker-plate treads ensure slip resistance under daily foot traffic without requiring applied nosing strips that can degrade and delaminate over time.

The RAL 9005 powder coat presents a consistent matte black finish across all components — stringers, treads, balusters, and handrail — creating a monolithic visual effect that reads as a single designed element rather than an assembly of parts. At 80–100 μm DFT over SA 2.5 preparation, the finish is specified for the 15–20 year maintenance cycle typical of commercial interior fit-outs in the London market.

From a compliance perspective, the staircase satisfies all applicable requirements: Approved Document K geometry and guardrail provisions, BS 6399-1 structural loading for Category C occupancy, and the building’s specific access and hot-works restrictions. The bolted construction also means the staircase can be disassembled and relocated if the tenant fit-out changes in future — an increasingly relevant consideration in London’s flexible commercial leasing environment.

Related Guides & Resources

• 10 Costly Staircase & Railing Mistakes to Avoid (2026)
• How to Choose the Right Staircase: 2026 Decision Guide
• ADA vs IBC Handrail Codes (2026): Key Differences
• Contact Double Building Materials