Post spacing is the most critical structural decision in any concrete sleeper retaining wall. Too far apart and the sleepers will bow or crack under soil pressure. Too close and you're wasting money. Get it right and your wall will stand for decades.

This guide covers everything you need to know about steel post spacing for concrete sleeper retaining walls — including H posts, C posts, embedment depths and the relationship between post length and maximum wall height.

Why Post Spacing Matters

Concrete sleepers span between posts like a horizontal beam. The soil behind the wall exerts lateral pressure on those sleepers, which transfer the load into the posts. The posts then transfer the load into the ground through their embedded section.

If posts are spaced too far apart, the sleepers are spanning too great a distance and will experience excessive bending stress — leading to cracking or failure. If the posts are too short (insufficient embedment), they'll rotate forward under load.

Standard Post Spacing Rules

For standard FPM concrete sleepers (2.4m length), the recommended post spacing is:

Wall Height	Post Spacing
Up to 600mm	2.4m centres
600mm–1200mm	1.8m–2.0m centres
1200mm–1800mm	1.5m–1.8m centres
Over 1800mm	Engineer to specify

These are general guidelines for standard soil conditions. Heavy clay soils, saturated ground or surcharge loading (driveways, structures) above the wall will require closer spacing or larger posts.

H Post vs C Post: Which Do You Need?

H Posts (100UC or 150UC) are the most common choice for concrete sleeper retaining walls. The H-section (universal column) provides equal strength in both directions and the sleepers slot into the flanges on either side.

• 100UC H Post — suitable for walls up to approximately 1.2m
• 150UC H Post — suitable for walls up to approximately 1.8m
• 150UC with closer spacing — for walls approaching 2.1m

C Posts (channel section) are used where the wall terminates — at ends, corners and gate openings. The C-section provides a clean finish and allows the sleeper to butt up against the post face rather than slotting through.

For most residential retaining walls, you'll use H posts along the run with C posts at each end.

Post Embedment Depth

The embedment depth (how deep the post is set in the ground) is just as important as the spacing. The general rule is:

Embedment depth = exposed wall height × 1.0 to 1.5

So for a 1.2m wall, you need at least 1.2m of post in the ground — ideally 1.5m for a factor of safety. This means a 1.2m wall requires a post at least 2.4m long (1.2m exposed + 1.2m embedded).

For walls in soft or sandy soils, increase the embedment ratio to 1.5 or consult an engineer.

Post Length Selection Guide

Exposed Wall Height	Minimum Post Length	Recommended Post Length
600mm	1.5m	1.8m
900mm	2.1m	2.4m
1200mm	2.4m	3.0m
1500mm	3.0m	3.6m
1800mm	3.6m	4.2m

FPM stocks galvanised H posts in lengths from 1.8m to 4.8m. Our Engineering & Specifications page has the full wall height tables for both 75mm and 100mm sleeper systems.

Corner and End Posts

At corners, you have two options:

1. 45-degree corner post — a fabricated post that accepts sleepers from two directions at 45°. Ideal for clean, angled corners.
2. 90-degree corner post — accepts sleepers at a right angle. Used for square corners.
3. Two C posts back-to-back — a site-built solution using standard C posts.

FPM stocks both 45° and 90° galvanised corner posts to suit our sleeper systems.

Setting Posts in Concrete

All posts should be set in concrete — never just compacted soil. Use a minimum N20 concrete mix and allow at least 48 hours curing time before loading the sleepers.

Post hole diameter should be at least 3× the post flange width. For a 100UC post (100mm flange), dig a 300mm diameter hole. For a 150UC post, dig a 400mm hole.

Brace posts during curing to keep them plumb and in line. Even small deviations compound as you build up the wall.

Surcharge Loading

If there's a driveway, vehicle path or structure within 1× the wall height of the top of the wall, you have surcharge loading. This significantly increases the lateral pressure on the wall and requires:

• Closer post spacing (typically 1.2–1.5m)
• Larger post sections (150UC minimum)
• Deeper embedment (1.5× wall height minimum)
• Engineering sign-off

Never assume standard spacing is adequate when there's surcharge loading — this is a common cause of wall failure.

When to Call an Engineer

For walls over 1.0m retaining height, a building permit is generally required in Victoria, which means engineering computations are mandatory. Even for walls under 1.0m, it's worth getting engineering advice if:

• Soil conditions are poor (soft clay, fill, saturated ground)
• There's surcharge loading
• The wall is near a boundary or structure
• The wall is tiered (multiple walls on a slope)

FPM's engineering computations (available on our Engineering page) cover standard soil conditions for our sleeper systems. For non-standard conditions, engage a structural engineer.

Summary

The key rules for steel post spacing:

• Standard spacing: 1.8m–2.4m centres for walls up to 1.2m
• Embedment depth: minimum 1× exposed wall height, ideally 1.5×
• Use 100UC for walls to 1.2m, 150UC for walls to 1.8m
• Always set posts in concrete
• Reduce spacing for surcharge loading or poor soils
• Get engineering sign-off for walls over 1.0m

FPM Building Supplies stocks the full range of galvanised steel posts, concrete sleepers and accessories from our Epping warehouse. Call 0431 235 919 for trade pricing and project advice.