Part K Protection from Falling: Stair Geometry, Guarding Heights, Balustrade Spacing Requirements

Summary

Falls from stairs and from height within buildings account for a significant proportion of the 6,000+ fatal accidents in UK homes each year. Part K of the Building Regulations addresses this through requirements for stair geometry, guarding (balustrading and handrails), and protection from falling on balconies, landings, and raised floors.

For builders, joiners and architects, the stair geometry requirements in AD K are the daily reference. The intersection of rise, going, and pitch determines whether a proposed stair complies — and there is surprisingly little flexibility within the permitted range. Errors at design stage lead to stairs that cannot be built to comply, or that require rebuilding at significant cost.

Equally important is guarding specification. Many builders fit guardrails and balustrading with spacings that allow a child to pass through, or at insufficient height for an adult leaning against them. The 100mm sphere rule and the height requirements are not advisory — they are minimum standards that apply to all new construction.

Key Facts

• Approved Document K — Protection from falling, collision and impact; current edition 2013
• Private stair — Stairs within a single dwelling; maximum 42° pitch
• Common stair — Stairs serving more than one dwelling; maximum 38° pitch
• Maximum rise — 220mm (private stair); 190mm (common stair)
• Minimum going — 220mm (private stair); 250mm (common stair)
• Pitch limit — Calculated as: rise ÷ going × 45° ratio; maximum 42° (private) or 38° (common)
• 2R + G formula — 2 × Rise + Going should fall between 550mm and 700mm for comfortable stairs
• Guarding height (stair flights) — Minimum 900mm measured vertically from nosing
• Guarding height (landings, external) — Minimum 1,100mm
• Guarding height (dwellings, landings) — 900mm within single dwelling; 1,100mm on external balconies/terraces
• 100mm sphere rule — Baluster or infill spacing must not allow a 100mm sphere to pass (child safety)
• Climbability — Guarding must not incorporate elements that can be used as a ladder to climb over
• Handrail height — 900mm–1,000mm measured vertically from stair nosing
• Minimum headroom — 2,000mm measured vertically from stair nosing; 1,900mm on loft stair (AD K Table 1.1)
• Width (private stair) — No minimum specified; AD K recommends min 600mm; Building Regs Approved Inspector may have comments
• Open risers — Permitted only where a 100mm sphere cannot pass between tread and riser above

Quick Reference Table

Detailed Guidance

Stair Geometry: Calculating Compliance

Every stair must have consistent geometry — all rises equal, all goings equal — throughout a flight. Variable geometry is not permitted (except where tapered treads form a curve).

Rise and going relationship:

• Use the 2R + G formula: 2 × Rise + Going must be between 550mm and 700mm
• Example: Rise 180mm, Going 260mm: 2(180) + 260 = 620mm — compliant ✓
• Example: Rise 220mm, Going 220mm: 2(220) + 220 = 660mm — compliant ✓
• Example: Rise 220mm, Going 210mm: 2(220) + 210 = 650mm, but Going 210mm < 220mm min — non-compliant ✗

Pitch calculation:

• Pitch = arctan(rise / going)
• Maximum private stair: arctan(220/220) = 45° — exceeds 42° limit; this combination fails
• Correct maximum: for 220mm rise: going = 220/tan(42°) = 220/0.9004 = 244mm minimum
• Practical combination at maximum rise: 220mm rise, 245mm going = arctan(220/245) = 41.9° — just compliant

Landings:

• Landing required at top and bottom of every flight
• Minimum landing dimension: width of stair
• Landing within a flight (e.g. quarter landing): clear width in same direction of travel must equal stair width
• Doors must not swing onto a stair flight (must have a landing of at least stair width before door swing)

Tapered treads (spiral and winders):

• Tapered treads must have minimum going of 50mm at the narrow end; minimum 220mm at a point 270mm from the narrow end
• All tapered treads in a flight should be of uniform design

Guarding Requirements

Guarding prevents falls from the edges of stairs, landings, balconies, and other raised areas. Two distinct situations:

On a stair flight:

• Required where the drop is ≥600mm (a standard domestic stair always qualifies)
• Minimum height: 900mm measured vertically from the stair nosing
• Guarding forms the handrail at 900mm; the space below 900mm filled by balusters

On a landing, floor, or balcony:

• Required where the drop is ≥600mm
• Minimum height: 900mm for internal landings within a single dwelling
• Minimum height: 1,100mm for external balconies, terraces, and areas accessible from the outside
• Note: a first-floor landing inside a house needs only 900mm guarding; the balcony outside needs 1,100mm

AD K exceptions:

• Low-use industrial or storage areas: different heights may be agreed with Building Control
• Loft conversions with restricted space: 800mm minimum in certain circumstances [verify with your local BCO]

Balustrade Spacing: The 100mm Sphere Rule

The 100mm sphere rule is designed to prevent children getting their heads through balustrade openings:

Basic rule: No opening in guarding must allow a 100mm sphere to pass through. This applies to:

• Gaps between vertical balusters
• Horizontal rail spacings
• Infill panel openings
• Gaps between top rail and ceiling/floor above

Common spacings that comply:

• Vertical balusters at 90mm centres (edge-to-edge, not centre-to-centre) → 90mm gap → 100mm sphere cannot pass ✓
• Horizontal rails: if rails are horizontal with ≤100mm gap between them, a child can use them as a ladder — this also fails the "climbability" test

Climbability requirement: AD K Diagram 5 notes that guarding must not incorporate horizontal elements between 150mm and 760mm above the stair nosing that would facilitate climbing. This typically rules out horizontal rails and grid patterns in this zone for stairs serving areas accessible by children.

Openwork balustrades: Any decorative infill (circular, square, irregular openings) must be checked: can a 100mm sphere pass through any opening in any orientation? Some decorative panels have diagonal openings that are larger than they appear.

Handrails

A handrail serves a different function from guarding: it provides a gripping surface for stability.

Requirements:

• Required on at least one side of any stair flight with ≥2 risers
• Required on both sides of stairs more than 1m wide
• Height: 900–1,000mm measured vertically from stair nosing
• Must be continuous from top to bottom of flight (no gaps except for newel posts at change of direction)
• Must extend 300mm horizontally beyond top riser and at foot of stair (aids users approaching top and bottom)

Handrail form: Should be graspable — circular profile 40–50mm diameter; oval or D-shape acceptable. Flat top rail that cannot be gripped is not a compliant handrail. The rail must be separated from the wall by minimum 50mm for safe gripping.

Glazed Guarding

Where glass is used as balustrade infill or as the guarding itself (structural glass balustrade):

Requirements:

• Must be toughened safety glass or laminated safety glass
• Must comply with BS 6180 (barriers in and around buildings) for loads
• Structural glass balustrades: design by engineer required; must resist 0.74 kN/m horizontal load at top of guarding
• Frameless glass systems: typically 17.5–21.5mm thick laminated toughened glass; check with structural engineer

Thermal movement: Glass panels in guarding must accommodate thermal movement. Fixed frames without clearance can cause glass cracking — particularly in roof terraces and external balconies.

Vehicle Barriers

AD K also covers vehicle barriers in car parks:

• Barriers to edges of floors and ramps open to vehicles
• Minimum height: 375mm for passenger car parks
• Force resistance: 150 kN (or BS EN 1337 as appropriate)
• Separate from pedestrian guarding which must still be provided

This is less relevant for residential work but important for mixed-use or commercial buildings with car parking.

Frequently Asked Questions

Can I fit a spiral staircase to a loft conversion and still comply with Part K?

Yes, in some cases. Spiral stairs with tapered treads can be used for a loft conversion serving a single room. The minimum going at 270mm from the narrow end must be 220mm; headroom must be 1,900mm (reduced from 2,000mm for loft stairs per AD K). However, a spiral stair may not comply with Part B (fire escape) requirements — if the loft conversion is the only means of escape, the stair may need to be enclosed in fire-resisting construction. Check both Part K and Part B simultaneously.

Our customer wants open riser stairs — is this allowed?

Yes, for domestic stairs, but only where the open riser gap cannot allow a 100mm sphere to pass through between the back of one tread and the front of the tread above. Typically this means the gap between treads must be less than 100mm. Open riser stairs are also not suitable where the stairs serve areas accessible by small children, as they present a falling-through risk. If the stairs serve an area with children (bedrooms), open risers may not comply with Part K in spirit even if technically 100mm sphere compliant.

Do regulations apply to replacement stairs in an existing house?

Yes, if you are replacing the whole stair (a material alteration). Building Regulations apply when carrying out building work, including replacing a staircase. However, if you are like-for-like replacing treads or handrails as maintenance, Building Regulations do not apply — though good practice is to improve to current standards where feasible.

What is the minimum width for a domestic stair?

AD K does not specify a minimum width for private stairs within a dwelling. However, most building inspectors will expect sufficient width for passage of furniture and emergency egress — typically minimum 600mm clear. AD K Table 1.1 note 3 states "there is no restriction on minimum width, but it should be wide enough for its intended use." In practice, less than 600mm is likely to raise questions.

Regulations & Standards

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

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

• BS EN 1337 — Structural bearings (reference for vehicle barrier loads)

• BS EN 12150 — Glass in building: thermally toughened soda lime safety glass

• BS EN ISO 12543 — Laminated glass and laminated safety glass

• GOV.UK Approved Document K — Full text of Part K

• TRADA Stair Design Guide — Timber research and development association stair geometry guidance

• part e sound — Sound insulation requirements for stairs between dwellings

• built in storage — Carpentry regulations context for joinery work on stairs