Approved Document K: Protection from Falling, Collision & Impact

Summary

Approved Document K (Protection from falling, collision, and impact) is the building regulation most relevant to carpenters, joiners, and builders who construct or alter stairs, balconies, landings, and guarding systems. Falling is one of the leading causes of serious injury in UK homes, and Part K is designed to reduce these risks through prescriptive dimensional requirements for stairs and barriers.

The current edition is Part K 2013 (with subsequent amendments), which consolidated previous guidance and introduced specific requirements for glazing in critical locations and for guarding in domestic buildings. Part K must be read alongside Part M (Access and use) for accessible design, and Part B (Fire safety) for fire escape staircases.

For tradespeople, Part K is most commonly triggered by: building a new staircase, replacing or repairing a domestic staircase, fitting a new balustrade or guarding to a balcony or landing, and designing raised platforms or mezzanine floors.

Key Facts

• Stair dimensions (domestic):
  • Maximum rise: 220mm
  • Minimum going: 220mm
  • The relationship 2R + G must be between 550mm and 700mm (where R = rise, G = going)
  • Maximum pitch: 42°
  • Minimum headroom: 2000mm (2m) measured vertically from the pitch line
• Stair width: No minimum width specified in ADK for domestic stairs. However, for means of escape (Part B), minimum 750mm clear width between handrails for a single-storey dwelling
• Handrails: Required on at least one side for stairs with a rise of more than 600mm. For stairs wider than 1000mm, handrails on both sides. Height: 900mm minimum above the pitch line to the top of the rail. Stairs with more than 36 risers in a flight require a landing
• Guarding height:
  • Stairs (alongside the stairwell): 900mm minimum above the pitch line
  • Landings and floors with a change of level >600mm: 1100mm minimum above the floor level
  • Balconies and flat roofs accessible to public: 1100mm minimum
• Baluster spacing: Must prevent a 100mm sphere from passing through any opening. The 100mm sphere test applies to the entire guarding — including the gap between the floor and the bottom rail
• No horizontal rails: Horizontal rails in a balustrade that could be used as a climbing frame (especially in properties with children) should be avoided. Vertical balusters or other non-climbable infill preferred
• Glazing in critical locations: Glass in doors, glazed panels adjacent to doors, glazing in staircase areas, and any glazing below 800mm from floor level must be safety glazing (BS EN 12600 Class C or better — laminated or toughened)
• Part K and conservation areas / listed buildings: ADK applies in principle, but exemptions may be granted where achieving dimensional compliance would harm historic fabric. Discuss with Building Control and the LPA/Historic England before any stair work in listed buildings

Quick Reference Table

Detailed Guidance

Stair Geometry Calculations

The key formula for stair design is: 2R + G = 550–700mm, where R is the rise (vertical height of each step) and G is the going (horizontal depth of each step, measured from nosing to nosing).

Examples:

Finding rise and going from floor-to-floor height:

1. Measure the floor-to-floor height (e.g., 2700mm)
2. Divide by maximum rise (220mm): 2700 ÷ 220 = 12.27 → 13 risers minimum
3. Calculate actual rise: 2700 ÷ 13 = 207.7mm (round to 208mm)
4. Calculate going from formula: G = 600 − 2 × 208 = 184mm — too small. Try 14 risers
5. 14 risers: rise = 2700 ÷ 14 = 192.9mm (193mm). G = 600 − (2 × 193) = 214mm — marginal
6. Use G = 250mm for comfort: 2R + G = 386 + 250 = 636mm — compliant and comfortable

All risers must be the same height (tolerance ±5mm between any two risers in the same flight). Tapered risers at the bottom (common in historic staircases) are addressed in the non-standard stair guidance.

All goings must be the same depth — except for winding stairs (see below).

Winding Stairs (Tapered Treads)

Quarter-turn stairs and spiral stairs use tapered treads that are wider at the outer edge than the inner. Part K allows this with conditions:

• The minimum going measured at the narrowest point (the inner edge, or 'narrow end') must be at least 50mm
• The going measured at 270mm from the narrow end (the 'walking line' for a domestic stair) must meet the minimum going requirement (220mm domestic, 250mm common stair)
• The angle of taper must not exceed 30° from the stair axis

Winding stairs reduce the effective going on the inside of the turn and must be laid out carefully. A full-size setting-out board or CAD layout is recommended before cutting strings and treads.

Handrails

A handrail provides a grippable support surface along the stair. Requirements:

• Profile: Must be grippable. A rounded section (42–50mm diameter circular rail, or an equivalent oval/D-section) is best. Flat plate handrails are not grippable and do not comply
• Height: 900mm above the pitch line (measured vertically from the front nosing of each tread to the top of the handrail)
• Continuity: Must be continuous along the full length of the stair flight, with no interruptions. Returns at top and bottom (turned to the newel post or wall) prevent clothing from catching
• Projection: Where a handrail projects into a circulation area (at the bottom of a stair landing), it must not project more than 100mm beyond the newel post without a safety provision

Material: Timber, steel, chrome, stainless steel — any structural material. Chrome tubular steel (42mm OD) is very common for open-tread stairs. Timber handrails must be a profile that is grippable — a standard architrave moulding is not compliant.

Guarding (Balustrades)

Guarding is required at any open edge with a change of level of more than 600mm. The purpose is to prevent people from falling.

Height requirements:

• Alongside a stair flight: minimum 900mm above the pitch line (not 900mm above the tread — above the line connecting the stair nosings)
• On a landing, balcony, or floor: minimum 1100mm above the floor surface
• On a roof terrace or flat roof accessible to occupants: 1100mm minimum

Baluster spacing (100mm sphere rule): No opening in the guarding (including the gap between the bottom rail and the floor, and between any balusters, spindles, or infill panels) may allow a 100mm sphere to pass through. In practice:

• Vertical balusters: maximum 100mm clear gap (typically 95–98mm centres to be safe)
• Toughened glass panels: the glass itself is a continuous barrier — check the gap at the fixing point between panels
• Open balustrade with horizontal rails: check that no 100mm sphere can pass between rails at any height. A small child could squeeze between horizontal rails — a common legacy issue in pre-2013 installations

Climbability: Part K recommends that balustrades in dwellings likely to be occupied by children should be designed to avoid a climbing-frame effect. Vertical balusters are preferred over horizontal rails. Check with the client whether this is a concern.

Structural load: Guarding must be designed to resist a lateral load of 0.36 kN/m (applied as a line load at the top of the guarding) for domestic dwellings. Balcony guarding in public buildings: 0.74 kN/m. Most standard baluster systems are designed to meet these loads — confirm with the manufacturer's structural data.

Glazed Balustrades

Structural glass balustrades (toughened or laminated glass infill panels, or bolted glass panels with no top rail) are increasingly common in contemporary domestic settings. Requirements:

• Glass must be laminated toughened safety glass (minimum 17.52mm laminate for domestic use — typically 8.8.4 or 10.10.4 laminate)
• Fixings must be engineered for the load — bolt fixings must be designed by a structural engineer
• The glass supplier must provide a structural certificate confirming compliance with BS 6180 (guarding) and the specific load case
• All glass edges must be polished (no sharp edges)
• An obscure silicone bead at the base of each panel must not be the primary structural fixing — the glass must be clamped or bolted

Frameless glass balustrades are notifiable as structural glazing — Building Control will typically require a structural engineer's calculation.

Vehicle Barriers

Part K also covers barriers to prevent vehicles from falling off the edges of car parks, ramps, and elevated driveways. For domestic use, this is most relevant for driveways that slope down to a lower level or basement garages. Vehicle barriers must resist the specific impact load from a car at rolling speed — typically a 25 kN horizontal impact load applied at 375mm above the floor. This almost always requires a structural engineer's design for the barrier and its fixings.

Frequently Asked Questions

My customer's existing stair has a rise of 240mm. It was built legally in the 1970s. Does it need to be replaced?

No. Part K does not require the retrospective upgrading of staircases that were compliant with the regulations at the time of construction. The 240mm rise was compliant under pre-1991 standards. However, if the staircase is being materially replaced (new treads, strings, and newel posts — effectively a new stair), the replacement must comply with current Part K. If only repairs are being made (replacing a couple of treads), like-for-like replacement is acceptable.

How do I achieve 1100mm guarding on a balcony where the existing newel post is only 900mm?

Options: (a) Replace the newel post with a taller one and extend the balusters; (b) Add an extension post or cap fitting to the existing newel; (c) Install a top rail extension system above the existing rail. Ensure structural continuity — the guarding must still resist the design lateral load at its full 1100mm height. Adding a thin timber cap on top of an existing post that is not structurally connected is not compliant.

Can I use glass in a balustrade that's next to a stairs?

Yes — safety glazing (toughened or laminated to BS EN 12600) is required for any glass in a critical location, which includes glazing alongside stairs, in doors, and below 800mm from floor level. Toughened glass (single pane) breaks into small granules and is acceptable. Laminated glass (stays in place when broken) is preferred for balustrades because it continues to provide a barrier even after breakage.

Regulations & Standards

• Approved Document K (2013) — Protection from falling, collision, and impact

• BS 6180:2011 — Barriers in and about buildings: code of practice (guarding design)

• BS 5395-1:2010 — Stairs: code of practice for the design of stairs with straight flights

• BS EN 12600 — Glass in building: pendulum test and classification (safety glazing)

• BS 8300:2018 — Design of accessible and inclusive built environments (complements Part M/K for accessible stairs)

• GOV.UK — Approved Document K — Full text of the 2013 edition

• TRADA Timber Stair Design Guidance — Stair geometry and structural guidance for timber stairs

• NHBC Standards Chapter 6.9 — Guarding — NHBC interpretation of Part K for new-build houses

• staircase construction — Staircase construction sequence and joint types

• part m access — Access and use requirements for accessible stairs and handrails

• part b fire — Fire escape staircase requirements

• structural engineer — When to commission a structural engineer for balustrades and platforms