Complete Guide

Stairs

Design and build stairs correctly — with standard building code dimensions, the going-line formula and correct balustrade requirements. Applies to internal and external stairs in residential buildings.

Building codeRiseGoingBalustradeString / StringerGoing line

Building Code Requirements for Residential Stairs

Parameter	Min	Max	Recommended
Rise (riser height, residential)	150 mm	200 mm	170–185 mm
Going (tread depth, residential)	250 mm	—	270–290 mm
Clear width, private stair	800 mm	—	min 900 mm recommended
Clear width, common stair (shared)	900 mm	—	min 1,100 mm for multiple dwellings
Headroom above stair	2,000 mm	—	2,100 mm+
Balustrade / guard height	900 mm	—	1,000 mm recommended
Max baluster spacing	—	100 mm	89 mm where children present

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For public buildings, building code requirements are stricter: max 150 mm rise, min 300 mm going, min 1,500 mm clear width. Always check the applicable code for your project type and jurisdiction.

The Going-Line Formula

A comfortable staircase follows the ergonomic formula where one step (two risers plus one going) equals approximately one natural walking pace of about 630 mm:

2 × R + G = 620 mm (±20 mm)

Valid range: 600–640 mm · R = rise · G = going

For winding stairsⓘwinding stairsStairs that change direction using tapered (winder) treads instead of a landing. The going line is measured at 450 mm from the inner (narrow) edge of the winder., the going is measured along the going line: 450 mm from the inner edge of the tread for winders, or at the centre of the stair width for straight flights.

Rise	Going (formula)	Character
160 mm	300 mm	Very shallow — good for elderly users
170 mm	280 mm	Comfortable — recommended
175 mm	270 mm	Standard residential
185 mm	250 mm	Steep — building code minimum going
200 mm	≥250 mm*	Maximum permitted rise — formula gives 220 mm but min 250 mm going applies

How to Calculate the Number of Steps

Example: Floor-to-floor height 2,800 mm

1. Target rise ≈ 175 mm

2. n = 2,800 / 175 = 16.0 → 16 risers

3. Exact rise = 2,800 / 16 = 175 mm

4. Going: 620 − 2×175 = 270 mm ✓ (within 600–640)

5. Horizontal run (treads = risers − 1): 15 × 270 = 4,050 mm

6. String length: √(2,800² + 4,050²) ≈ 4,893 mm

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The number of treads (goings) is always one less than the number of risers in a straight-flight stair — the upper floor landing serves as the top tread. Horizontal run = (risers − 1) × going.

Stair Types

Straight flight

Single straight direction. Simplest to build and calculate. Requires the greatest horizontal floor plan length.

Half-turn stair (U-stair)

180° turn, often with a half-landing. Saves floor plan length. Common in houses.

Quarter-turn stair (L-stair)

90° turn with a quarter landing or winder treads. Good compromise between space and comfort.

Spiral stair

Full 360°. Smallest footprint. Going line measured at 450 mm from the centre column. Not suitable for accessible design.

Balustrade & Handrail — Building Code Requirements

Handrail

Required when there are 3 or more risers
Both sides where stair width is ≥ 1,800 mm
Height: 800–900 mm measured from the nosing of the tread
Diameter: 40–50 mm for a comfortable grip
Must be continuous along the full length of the stair flight

Balustrade / Guard

Min 900 mm height (residential stair)
Min 1,000 mm recommended (balcony / deck edge)
Min 1,200 mm where there is a large drop
Max 100 mm between balusters (child safety)
Avoid horizontal rails — children can use them as a ladder

⚠️

Horizontal balustrade members are not prohibited, but building code guidance strongly discourages them because children can climb and fall. Use vertical balusters.

External Stairs

Frost & Drainage

Foundations below the local frost depth (varies by region — see fence guide for reference depths)
Tread cross-fall: min 1:50 (2%) outward for water run-off
Timber decking boards: 5–8 mm gap between boards for drainage
Concrete mix: air-entrainment 4–5% for frost resistance
Anti-slip surface: R11/R12 classification for outdoor use

External Materials

Pressure-treated softwood: economical, requires regular maintenance
Hardwood (e.g. bangkirai, ipe, oak): very durable, low maintenance
Concrete: best for high-traffic and exposed locations
Steel with open-bar grating: excellent drainage, suited to contemporary design

Step by Step

Measure the exact floor-to-floor height (finished floor to finished floor)
Calculate the number of risers and verify with the going-line formula
Calculate the horizontal run (n × going) — confirm there is adequate space
Determine whether planning or building control approval is required (structural alterations typically require approval)

Use a stair square or adjustable bevel to mark the rise and going on the string board
Cut the notches with a circular saw — stop 10 mm short of the corner and complete with a jigsaw to avoid weakening the string
Check that the first and last step compensate for the finished floor covering thickness
Use the first string as a template for the second — matching strings are critical

Fix strings with frame anchors or expansion bolts into the wall and / or against the trimmer joist
Fit the treads (goings) first — use adhesive plus screws from below
Risers (vertical faces): optional but increase rigidity and reduce dust passage
Check each tread is level with a spirit level

Set newel posts at the bottom and top of the flight — use heavy-duty expansion bolts
Fit balusters at max 100 mm spacing
Handrail at 900 mm height measured vertically from the tread nosing
Test stability by applying full body weight — the balustrade must not deflect

Sand all timber through 80 → 120 → 180 grit
Hard wax oil gives a durable and attractive finish for interior stairs
External: preservative stain or paint — two coats, with extra coats on end grain
Anti-slip nosing strips are recommended for external stairs and loft access stairs

Common mistakes

✗Uneven riser heights — even one step 10 mm taller than the rest is enough to cause a trip; measure and calculate carefully
✗Too steep a pitch without adjusting the tread depth — use the formula 2R + T = 630 mm as a check
✗Going line not checked — comfortable pitch is measured along the going line, not the centreline of the staircase
✗Insufficient headroom clearance — minimum 2200 mm above the going line (check your local building code)
✗Handrail too low — minimum 900 mm for interior stairs, 1000 mm for external stairs and balconies over 0.5 m
✗Strings not properly secured to the floor structure — stairs without rigid top anchorage flex and creak
✗Skipping anti-slip strips on external stairs — wet timber is extremely slippery in winter

Calculate Stair Materials

Enter the floor-to-floor height and target rise — the calculator finds the number of steps, string length, timber quantities and balustrade requirements.

Open Stair Calculator

Frequently asked questions

What are the IRC requirements for stair rise and run?

IRC requires: max 7¾" rise, min 10" run (tread depth), and the formula 2×rise + run should equal 24–25". All steps in a flight must be within ⅜" of each other.

What is the minimum handrail height?

Handrails must be 34–38" measured vertically from the stair nosing. A graspable handrail is required on stairs with 4 or more risers.

What is the walking line in stair design?

The walking line is an imaginary line 12–18" from the narrow side of the tread (inside of a curved stair). Tread width is measured along the walking line.

References

→ IRC 2021 R311 — stairway dimensions: rise, run, headroom and handrail requirements
→ TRADA — timber stair design and specification guidance
→ Local building authority — national stair code dimensions, balustrade requirements and permit thresholds for your jurisdiction

Rise, run and stringer calculations are indicative. Always verify against the building code applicable in your jurisdiction before construction. Last reviewed: May 2026