Working out the correct rise and run for deck stairs is straightforward once you know the rules. In New Zealand, stair dimensions are governed by NZS 3604 (Timber-framed buildings) and the NZ Building Code, so getting them right isn't just about comfort — it's a legal requirement. Here's how to calculate safe, code-compliant steps for your deck.
Why Rise and Run Matter
The rise is the vertical height of each step, and the run (also called the going) is the horizontal depth of the tread. Get these wrong and your stairs become a trip hazard. The goal is a consistent, comfortable rhythm that feels natural to walk up and down — especially important when carrying a chilly bin or a tray of snags.
NZ Building Code Requirements
For residential deck stairs, MBIE Building Performance sets out the following general limits under Clause D1 (Access routes):
| Dimension | Minimum | Maximum |
|---|---|---|
| Rise (riser height) | 150 mm | 190 mm |
| Going (tread depth) | 240 mm | 355 mm |
| Stair width | 860 mm | — |
Important: These figures apply to private stairways in residential buildings. Always confirm requirements with your local council, as interpretations can vary and your specific situation may have additional requirements.
All risers in a single flight must be equal height (within a 5 mm tolerance), and all treads must be equal depth. Inconsistent steps are a leading cause of falls.
Step 1 — Measure Your Total Rise
The total rise is the vertical distance from the finished ground level (or lower landing) to the finished deck surface. Measure this carefully with a tape measure and a spirit level or a long straight edge.
For example: if your deck surface sits 900 mm above the ground, your total rise is 900 mm.
Step 2 — Choose Your Riser Height
Pick a riser height within the 150–190 mm range. A comfortable middle ground for most people is around 175 mm.
Divide your total rise by your chosen riser height to find the number of steps:
Number of risers = Total rise ÷ Riser height
Using our example: 900 ÷ 175 = 5.14 risers
Since you can't have a fraction of a step, round to the nearest whole number — in this case, 5 risers.
Now recalculate the exact riser height:
Actual riser height = Total rise ÷ Number of risers
900 ÷ 5 = 180 mm per riser ✓ (within the 150–190 mm range)
Step 3 — Calculate the Going (Tread Depth)
Use the classic rise + going formula as a comfort check:
Rise + Going = 420–450 mm (this is a widely used rule of thumb for comfortable stairs)
Using our 180 mm riser: 450 − 180 = 270 mm going ✓ (within the 240–355 mm range)
So each tread needs to be at least 270 mm deep. If you're using standard 90 × 290 dressed timber, that gives you a 290 mm going — perfectly comfortable.
Don't Forget the Nosing
If your treads have a nosing (an overhang beyond the riser face), this is typically 15–25 mm and is not counted as part of the going measurement. The going is measured from nosing to nosing.
Step 4 — Calculate the Total Run
The total run is the horizontal distance your staircase will occupy on the ground:
Total run = Number of treads × Going
Note: the number of treads is always one less than the number of risers (the deck surface acts as the top tread).
Using our example: 4 treads × 270 mm = 1,080 mm (just over a metre of horizontal space needed)
This tells you how far out from your deck your stair stringer will land — useful for planning your lower landing or paving.
Step 5 — Check Your Headroom
If your stairs pass under any structure (like a deck above), you need a minimum 2,000 mm of headroom measured vertically above the nosing line. Confirm this with your council if applicable.
Do You Need a Building Consent?
Deck stairs are often part of a larger deck project. Under Schedule 1 of the Building Act, some low decks are exempt from consent, but this depends on height, area, and other factors. MBIE Building Performance has guidance on exemptions, but always check with your local council before starting work — don't assume your project is exempt.
For decks over 1.5 m above ground, a building consent is almost certainly required, and the stairs must comply fully with the Building Code. You can find licensed deck builders through Licensed Building Practitioners or on tradedirectory.co.nz.
Practical Tips for NZ Conditions
- Use treated timber rated appropriately for its exposure — generally H3.2 or above for ground contact and H3.2 for above-ground exterior use, as required by BRANZ guidelines for NZ's variable climate.
- Non-slip surfaces are essential — NZ's wet winters make untreated smooth timber treads dangerous. Consider grooved decking boards or non-slip strips.
- Handrails are required when there are two or more risers. Check your council's requirements for balustrade height and spacing.
When to Call a Professional
If your deck is more than one metre above ground, involves complex geometry, or requires a building consent, it's well worth engaging a qualified deck builder or Licensed Building Practitioner (LBP). Structural work on elevated decks can be restricted work under the Building Act, meaning it must be carried out or supervised by an LBP. You can search for qualified local deck builders on tradedirectory.co.nz.
Safety note: Always work safely at height. Use scaffolding or a stable platform when measuring and building elevated decks, and follow WorkSafe NZ guidelines.
Licensed Building Practitioner (LBP)
