Calculating the number of sleepers for a retaining wall is straightforward once you know your wall's dimensions and your chosen sleeper size. The basic formula is simple: divide the wall height by the sleeper thickness to get the number of courses, then multiply by the number of sleepers needed per course based on your wall length. Read on for a step-by-step breakdown, plus important NZ-specific rules you need to know before you start digging.
Step 1 – Measure Your Wall Length and Height
Start with two key measurements:
- Wall length – Measure the total horizontal run of your retaining wall in metres.
- Retained height – Measure how high the wall needs to retain soil. This is the vertical distance from the base of the wall to the top of the retained earth, not just the visible face height.
Be precise. Even a small error multiplied across many courses adds up quickly.
Step 2 – Choose Your Sleeper Size
In New Zealand, treated timber sleepers are the most common DIY choice. Typical sizes include:
| Sleeper Size | Thickness | Common Use |
|---|---|---|
| 200 × 75 mm | 75 mm | Low walls, garden edging |
| 200 × 100 mm | 100 mm | Medium retaining walls |
| 250 × 125 mm | 125 mm | Heavier-duty walls |
Always use H5 treated timber (hazard class 5) for ground contact in retaining wall applications — this is the NZ standard for timber in direct contact with soil. You'll find H5 sleepers at suppliers like Bunnings, Mitre 10, and PlaceMakers.
Step 3 – Calculate the Number of Courses
Divide your retained height by the sleeper thickness:
Number of courses = Retained height (mm) ÷ Sleeper thickness (mm)
Example: A 600 mm retained height using 100 mm sleepers = 6 courses.
Round up to the nearest whole number — you never want to be short on your final course.
Step 4 – Calculate Sleepers Per Course
Divide your wall length by the sleeper length:
Sleepers per course = Wall length (mm) ÷ Sleeper length (mm)
Standard sleeper lengths are typically 2400 mm or 3000 mm. Again, round up to the nearest whole number.
Example: A 6-metre wall using 2400 mm sleepers = 6000 ÷ 2400 = 2.5, rounded up to 3 sleepers per course.
Step 5 – Calculate Your Total Sleepers
Total sleepers = Number of courses × Sleepers per course
Using the examples above: 6 courses × 3 sleepers = 18 sleepers for the face of the wall.
Step 6 – Add Deadmen (Anchor Sleepers)
For walls over about 600 mm in retained height, you'll typically need deadmen — horizontal sleepers buried back into the bank at right angles to the wall face to anchor it against soil pressure. These are a critical structural element.
As a rough guide, deadmen are spaced every 1.5–2 metres along the wall and every second or third course in height. The exact spacing depends on your soil type, wall height, and design — check with a qualified retaining wall builder or structural engineer for your specific situation.
Add the number of deadmen sleepers to your total count.
Step 7 – Allow for Waste and Cutting
Always add a 10–15% waste allowance to your final total to account for:
- End cuts and trimming to fit
- Any damaged or warped sleepers
- Mistakes during construction
Final order quantity = Total sleepers × 1.10 to 1.15
NZ Building Consent Rules — Critical to Check
This is where many homeowners get caught out. In New Zealand, retaining walls are subject to the NZ Building Code and may require a building consent depending on their height and proximity to boundaries or other structures.
Under Schedule 1 of the Building Act, some low retaining walls may be exempt from consent — but the rules are specific and depend on factors like retained height, surcharge loading (e.g. driveways or buildings above), and site conditions. Always confirm with your local council before starting work. You can find general guidance at MBIE Building Performance.
For walls over 1.5 metres in retained height, or walls in difficult ground conditions, you will almost certainly need consent and likely an engineer's design.
When to Call a Professional
While a simple, low garden retaining wall can be a satisfying DIY project, there are situations where you should engage a qualified retaining wall builder or structural engineer:
- The retained height exceeds 1 metre (especially near boundaries, driveways, or structures)
- Your site has unstable, wet, or clay-heavy soils
- The wall is on or near a slope or embankment
- You're unsure about drainage requirements — poor drainage is the number one cause of retaining wall failure
- A building consent is required — a Licensed Building Practitioner (LBP) may need to be involved in or supervise the work
You can find qualified local retaining wall builders on tradedirectory.co.nz. For LBP information, visit Licensed Building Practitioners.
Licensed Building Practitioner (LBP)
