Post spacing is one of the most important variables in retaining wall design — and one of the most commonly misunderstood. Too wide and the posts are overloaded; too close and you're spending money on posts you don't need. Getting the spacing right requires understanding how post spacing interacts with wall height, soil conditions, post type, and sleeper thickness.

Why Post Spacing Matters

In a concrete sleeper retaining wall, the posts carry the lateral load from the retained soil. The sleepers span horizontally between the posts, transferring 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 each sleeper span.

If post spacing is too wide for the wall height and soil conditions, the posts will be overloaded and may rotate or fail. The sleepers may also be overloaded and crack or deflect excessively between posts. Post spacing must be matched to the structural capacity of the posts and sleepers for the specific wall conditions.

Standard Post Spacing for Residential Retaining Walls

The following spacings are general guides for standard residential conditions. Always confirm with a structural engineer for walls over 1.0m or in demanding soil conditions.

C-Channel PFC Posts

• Walls up to 600mm: 2.0m centres in standard soil conditions
• Walls 600mm–1.0m: 1.8–2.0m centres in sandy/loamy soils; 1.5–1.8m in clay soils
• Walls over 1.0m: Confirm with structural engineer — C-channel may not be adequate

H-Beam (UC) Posts

• Walls up to 1.0m: 2.0–2.4m centres in standard soil conditions
• Walls 1.0m–1.5m: 1.8–2.0m centres in standard conditions; 1.5–1.8m in clay soils
• Walls over 1.5m: Confirm with structural engineer

See our guide on C-channel vs H-beam posts to understand which post type is right for your wall.

Factors That Require Closer Post Spacing

• Heavy clay soils — clay exerts significantly more lateral pressure than sandy soils. Reduce post spacing by 200–400mm in heavy clay compared to sandy soil recommendations. See our guide on how soil type affects your wall.
• Surcharge loads — driveways, vehicle parking, or structures above the wall increase the load on each post. Closer spacing is required. Engineering sign-off is mandatory.
• Taller walls — as wall height increases, the bending moment on each post increases. Spacing must be reduced as height increases.
• 75mm sleepers — thinner sleepers have less bending capacity than 100mm sleepers. If using 75mm sleepers, post spacing should be at the closer end of the range. See our guide on sleeper thickness.
• Waterlogged or unstable soils — reduce spacing and get engineering sign-off.

The Relationship Between Post Spacing and Post Type

Post spacing and post type are interdependent. If you want to use wider post spacing to reduce the number of posts, you may need to upgrade from C-channel to H-beam posts to handle the increased bending moment. Conversely, if you're using C-channel posts, you may need to reduce spacing compared to H-beam posts at the same wall height.

Do not increase post spacing beyond the recommended range for your post type and wall conditions to save on post costs. The cost of additional posts is small compared to the cost of rebuilding a wall that has failed due to overloaded posts.

Post Spacing and Sleeper Span

Each sleeper spans the distance between posts. Standard concrete sleepers are 2000mm long. At 2.0m post spacing (centre-to-centre), the sleeper spans the full 2000mm between posts with minimal overhang. At 1.8m spacing, the sleeper has a 100mm overhang on each side — which is acceptable and common in practice.

Do not use post spacing greater than 2.0m with standard 2000mm sleepers. At spacings greater than 2.0m, the sleeper cannot span between posts without cutting, and the structural assumptions change significantly.

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

Always round up. Add specialist posts (corner, end, joiner) as required. See our full guide on how to calculate sleepers and posts.

Post Spacing at Corners and Ends

At corners, the post spacing on each wall run is measured from the corner post. The corner post itself replaces one standard post on each run. At wall ends, an end post or the last standard post terminates the wall. See our corner, end and joiner post range.

Frequently Asked Questions

Can I use 2.4m post spacing to reduce the number of posts?

2.4m spacing is only appropriate for H-beam posts in low walls (under 1.0m) in standard soil conditions. At 2.4m spacing, standard 2000mm sleepers cannot span between posts without cutting. Do not use 2.4m spacing with standard sleepers unless you have engineering confirmation that the post and sleeper specification is adequate.

What post spacing should I use in clay soil?

In heavy clay soil, reduce post spacing by 200–400mm compared to sandy soil recommendations. For a wall that would use 2.0m spacing in sandy soil, use 1.6–1.8m in heavy clay. For walls over 800mm in heavy clay, get engineering sign-off on the post spacing.

Does post spacing affect how many sleepers I need?

Post spacing affects the number of posts, not the number of sleepers. Sleeper quantity is determined by wall length and wall height (number of courses). See our sleeper and post calculator guide.

Ready to Order?

Browse our C-channel posts, H-beam posts, and specialist posts, or contact our team to confirm the right post spacing for your wall before ordering.