Steel H posts are the structural backbone of any concrete sleeper retaining wall. Get the sizing right and your wall will stand firm for decades. Get it wrong and you'll be dealing with a leaning, cracking wall long before its time.

This guide covers everything you need to know about choosing the right H post for your retaining wall project.

What Is a Steel H Post?

A steel H post (also called a UC post — Universal Column) is a hot-dip galvanised steel section with an H-shaped cross-section. The two flanges of the H create channels that concrete sleepers slide into, holding them in place without any bolts or fixings.

The post is set in a concrete footing below ground, with the above-ground section holding the sleepers. The depth of the footing is critical — it's what gives the wall its resistance to the lateral pressure of the retained soil.

The Key Variables

Post sizing depends on four main factors:

1. Wall height — the taller the wall, the more leverage the soil exerts on the post
2. Post spacing — closer spacing reduces the load on each post
3. Soil type — clay soils exert more pressure than sandy or gravelly soils
4. Sleeper thickness — thicker sleepers are heavier and require a wider flange

Standard Post Sizing Guide

Here's a practical guide to post selection for typical residential retaining walls using 75mm or 100mm concrete sleepers:

Walls Up to 600mm High

• Post size: 75 x 75mm UC or equivalent
• Post spacing: Up to 2.0m
• Footing depth: Minimum 600mm
• Notes: Suitable for garden edging and low boundary walls

Walls 600mm to 1.0m High

• Post size: 100 x 100mm UC
• Post spacing: 1.8m maximum
• Footing depth: Minimum 800mm
• Notes: Most common residential retaining wall height

Walls 1.0m to 1.5m High

• Post size: 100 x 100mm UC (heavy section) or 150 x 150mm UC
• Post spacing: 1.5m–1.8m
• Footing depth: Minimum 1.0m
• Notes: Engineering advice recommended; permit likely required in Victoria

Walls Over 1.5m High

• Post size: 150 x 150mm UC or larger — engineer to specify
• Post spacing: As per engineering design
• Footing depth: As per engineering design
• Notes: Building permit required; engineering certificate mandatory

FPM stocks engineering certificates for our concrete sleeper and H post systems — ask our team for details.

H Post vs C Post: What's the Difference?

Both H posts and C posts are used in retaining wall construction, but they work differently:

H posts (UC posts):

• The sleepers slide into the flanges of the H section
• Suitable for walls where sleepers are installed from above (standard installation)
• The most common choice for new retaining walls

C posts (channel posts):

• The sleepers slide into a C-shaped channel
• Often used for fencing applications
• Can be used for retaining walls in specific configurations

For most concrete sleeper retaining walls, H posts are the standard choice.

The Importance of Footing Depth

The footing depth is just as important as the post size. A rule of thumb used by experienced installers is:

Footing depth = 1/3 of total post length

So for a wall that requires 1.5m of post above ground, you'd want at least 750mm of post in the ground — meaning you'd order a 2.4m post.

In practice, many builders go deeper than this minimum, especially in clay soils or on slopes where water pressure is a factor.

Hot-Dip Galvanising: Why It Matters

All steel H posts used in retaining walls should be hot-dip galvanised, not just painted or zinc-plated. Hot-dip galvanising provides:

• A zinc coating that bonds metallurgically to the steel
• Protection against corrosion for 30–50 years in typical soil conditions
• Resistance to the alkaline environment created by concrete footings

FPM's H posts are all hot-dip galvanised to Australian standards. We also stock 45-degree corner posts and 90-degree corner posts for walls that change direction.

Corner Posts and Special Configurations

Standard H posts work for straight wall runs, but you'll need special posts for corners:

• 45-degree corner post: For walls that turn at a 45-degree angle
• 90-degree corner post: For walls that turn at a right angle
• End posts: Some installers use a standard H post at the end of a wall run, with the sleepers butting up to the flange

FPM stocks all these configurations in galvanised steel.

Practical Tips From the Trade

• Always order posts slightly longer than you think you need — you can cut them down, but you can't add length
• Check your post spacing before ordering sleepers — sleeper lengths need to match your post centres
• Use a post hole digger or auger for clean, straight holes — hand digging rarely produces a hole straight enough for a plumb post
• Brace posts while the concrete cures — even a slight lean at this stage will be amplified as the wall goes up

Get the Right Post for Your Project

FPM Building Supplies stocks a full range of hot-dip galvanised steel H posts, C posts, and corner posts. Our team can help you select the right size for your wall height and soil conditions.

Visit us at 32 Fullarton Dr, Epping VIC 3076, or call +61 431 235 919 to discuss your project. We also have engineering certificates available for our sleeper and post systems if your project requires council approval.