Stairs under D1/AS1: geometry, handrails & landings

NZBC D1/AS1 is the Acceptable Solution that governs stair geometry — riser height, tread depth, pitch, landings and handrails. It matters on site because getting the geometry wrong at framing time means cutting in the entire stringer to fix it, so measure twice.

Stairway types and their limits

D1/AS1 Table 1 sorts stairs into types by where they’re used, and each type carries its own minimum tread, maximum riser, maximum pitch and minimum width. The looser the use, the steeper and narrower you’re allowed to go. The four types are:

• Main private — inside a single house, between habitable floors. Min tread 280mm, max riser 190mm, max pitch 37°, min width 800mm.
• Secondary private — inside a house to an attic or non-habitable space. Min tread 240mm, max riser 220mm, max pitch 43°, min width 800mm.
• Common — apartment building common areas and business. Min tread 280mm, max riser 190mm, max pitch 37°, min width 1000mm.
• Accessible — where step-free access is required (D1 plus NZS 4121). Min tread 310mm, max riser 180mm, max pitch 32°, min width 1200mm.

Getting the geometry right

A comfortable stair follows the rule 2×riser + tread = 600–650mm; D1/AS1 enforces this through its pitch and minimum-tread limits. Beyond the basic dimensions, watch these details:

• Open risers — only allowed on main private stairs, and they cannot let a 100mm sphere through.
• Winders (turning treads) — the narrow end must be at least 125mm wide, and tread depth at the centreline still has to meet the type minimum.
• Nosings — 12–25mm projection over the riser below; add a contrast strip of at least 75mm for accessibility.
• Headroom — a minimum 2m clear above the pitch line.
• Slip resistance — at least 0.4 when wet under D1/AS1: broom-finish concrete, grooved timber or an anti-slip strip.

Handrails

Handrails run parallel to the stair slope and have their own rules under D1/AS1:

• Height — 900–1000mm above the pitch line.
• Which sides — one side is fine if the stair is up to 2.0m wide; two sides are required above 2.0m; add an intermediate handrail at the centre if wider than 4.0m.
• Continuous — the full length of the stair with no breaks, starting no further than the second riser from the lower end.
• Graspable — a circular section 32–50mm diameter is ideal; oval or rectangular is fine if it gives an equivalent grip within that 32–50mm range.
• Extensions — a 150mm horizontal extension past the top and bottom is recommended to help when stepping off.

Landings

Landings break up long flights and sit at door openings. The key requirements:

• Required after a maximum of 16–18 risers.
• Minimum dimension equal to the stair width (for example, 800mm).
• A top landing at every door opening onto the stair.
• External landings (door to deck) must shed water — fall to the outside, not back into the building.

Worked example — 2.7m floor-to-floor

For a main private stair (max 190mm riser) over a 2700mm floor-to-floor height:

1. Divide the height by the max riser: 2700 ÷ 190 = 14.21.
2. Round up to 15 risers — rounding up makes the risers smaller, not larger.
3. Recalculate the riser: 2700 ÷ 15 = 180mm.
4. Work out the run with the 280mm minimum tread: 14 × 280 = 3920mm, plus landings.