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 but increases the load on each post and sleeper.

Standard Post Spacing by Wall Height and Soil Type

The following is a general guide for standard residential retaining walls. 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, upgrade from C-channel to H-beam posts, and confirm spacing with a structural engineer. See our guide on retaining walls for driveways.

Post Spacing and Sleeper Bending

Post spacing also affects the bending moment on sleepers spanning between posts. As a general guide: at 2.0m post spacing, 75mm sleepers are generally adequate for walls under 1.0m in standard soil. For heavy clay or surcharge loads, 100mm sleepers are recommended regardless of post spacing. 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. In some cases, upgrading from C-channel to H-beam allows you to maintain wider post 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. See our full comparison: H-Beam vs C-Channel — Which Do You Need?

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. Add corner posts, end posts, and joiner posts as required. See our specialist post range and our quantity calculation guide.

When to Confirm Post Spacing with an Engineer

• 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

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 sleeper. At some point, wider spacing requires upgrading to heavier posts or thicker sleepers, which can offset the cost saving.

Does post spacing affect how many sleepers I need?

No. Sleeper quantity depends on wall length and height, not post spacing. See our quantity calculation guide.

What post spacing for a 1.2m retaining wall?

For a 1.2m wall, confirm spacing with a structural engineer. As a starting point, 1.5–1.8m centres with H-beam posts is a common specification. 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.

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