Post spacing is one of the most important specification decisions for a concrete sleeper retaining wall. Too wide and the posts are overloaded; too close and you're spending money on posts you don't need. Getting it right depends on wall height, soil conditions, post type, and whether there are surcharge loads above the wall.
How Post Spacing Affects Wall Performance
In a concrete sleeper retaining wall, the posts carry the lateral load from the retained soil. The sleepers span between posts and transfer the soil pressure to the posts. The wider the post spacing, the greater the bending moment on each post — and the greater the bending moment on the sleepers spanning between posts.
Increasing post spacing reduces the number of posts required (and therefore the cost), but increases the load on each post and each sleeper. At some point, wider spacing requires upgrading from C-channel to H-beam posts, or from 75mm to 100mm sleepers, to handle the increased load. The cost saving from fewer posts can be offset by the cost of heavier posts.
Standard Post Spacing by Wall Height and Soil Type
The following is a general guide for standard residential retaining walls. These are starting points, not engineered specifications. For walls over 1.0m, confirm post spacing with a structural engineer.
Walls Under 600mm
- Sandy or well-drained soils: 2.0–2.4m centres with C-channel posts may be adequate
- Clay soils: 1.8–2.0m centres with C-channel posts
- No surcharge loads
Walls 600mm–1.0m
- Sandy or well-drained soils: 1.8–2.0m centres with C-channel posts
- Light clay soils: 1.8m centres with C-channel posts
- Heavy clay soils: 1.5–1.8m centres with C-channel or H-beam posts
- No surcharge loads
Walls 1.0m–1.2m
- Sandy or well-drained soils: 1.8m centres with C-channel or H-beam posts
- Clay soils: 1.5–1.8m centres with H-beam posts
- Engineering sign-off recommended for all walls in this range
Walls Over 1.2m
- Post spacing must be confirmed by a structural engineer
- H-beam posts are the standard specification
- Council approval is typically required
How Surcharge Loads Affect Post Spacing
Surcharge loads (driveways, vehicles, structures above the wall) increase the bending moment on posts significantly. When surcharge loads are present:
- Reduce post spacing from the standard residential guide above
- Upgrade from C-channel to H-beam posts
- Confirm post spacing with a structural engineer
For driveway retaining walls, post spacing should always be confirmed by an engineer. See our guide on retaining walls for driveways.
Post Spacing and Sleeper Bending
Post spacing also affects the bending moment on the sleepers spanning between posts. Wider post spacing increases sleeper bending. As a general guide:
- At 2.0m post spacing: 75mm sleepers are generally adequate for walls under 1.0m in standard soil
- At 1.8m post spacing: 75mm sleepers suit a slightly wider range of conditions
- For walls in heavy clay or with surcharge loads: 100mm sleepers are recommended regardless of post spacing
Post spacing and sleeper thickness decisions should be made together. See our guide on choosing the right sleeper thickness.
C-Channel vs H-Beam — How Post Type Affects Spacing Options
H-beam posts have significantly greater bending strength than C-channel posts. This means:
- H-beam posts can be used at wider spacing than C-channel posts for the same wall height and soil conditions
- In some cases, upgrading from C-channel to H-beam posts allows you to maintain 2.0m spacing where C-channel would require 1.8m spacing — reducing the total number of posts
- For walls over 1.0m or in heavy clay, H-beam posts are required regardless of spacing
Browse our C-channel posts and H-beam posts.
Calculating the Number of Posts
Once you've determined your post spacing, calculate the number of posts using:
Number of posts = (Wall length ÷ Post spacing) + 1
Round up to the nearest whole number. Add corner posts, end posts, and joiner posts as required for your wall layout. See our specialist post range and our quantity calculation guide.
When to Confirm Post Spacing with an Engineer
Post spacing should be confirmed by a licensed structural engineer when:
- Wall height exceeds 1.0m
- Surcharge loads are present
- Soil is heavy clay, reactive, or unstable
- Wall is near a boundary or structure
- Council approval requires engineering certification
- You are unsure about any aspect of the specification
Frequently Asked Questions
What is the standard post spacing for a retaining wall?
For most standard residential retaining walls under 1.0m in normal soil conditions, 1.8–2.0m centres is the standard post spacing. Reduce spacing for taller walls, heavy clay soils, or surcharge loads. For walls over 1.0m, confirm spacing with a structural engineer.
Can I use wider post spacing to reduce costs?
Wider post spacing reduces the number of posts but increases the load on each post and each sleeper. At some point, wider spacing requires upgrading to heavier posts or thicker sleepers, which can offset the cost saving. For walls over 1.0m, post spacing should be confirmed by an engineer rather than adjusted to reduce costs.
Does post spacing affect how many sleepers I need?
No. The number of sleepers depends on wall length and wall height, not post spacing. Each sleeper is 2000mm long and spans one bay between posts, regardless of whether the posts are at 1.8m or 2.0m spacing. See our quantity calculation guide.
What post spacing should I use for a 1.2m retaining wall?
For a 1.2m wall, post spacing should be confirmed by a structural engineer. As a starting point, 1.5–1.8m centres with H-beam posts is a common specification for 1.2m walls in standard soil conditions. In heavy clay or with surcharge loads, closer spacing or larger posts may be required.
Ready to Order?
Browse our C-channel posts, H-beam posts, and specialist posts, or contact our team to confirm the right post spacing and type for your project before you order.
