Walk into any building supplies yard and you'll find a range of galvanised steel posts — H posts, C posts, SHS posts, and more. If you're not sure which one you need, you're not alone. This guide breaks down the key differences and helps you choose the right post for your project.

The Basics: What Are Steel Posts Used For?

Galvanised steel posts are used as the structural backbone of retaining walls and fences. They're set into the ground (usually in concrete) and the wall panels — concrete sleepers, timber sleepers, or fence panels — slot into the channels or flanges of the post.

The post carries the lateral load from the retained soil or the wind load on a fence, so choosing the right type and size is critical.

C Posts (Channel Posts / PFC Posts)

A C post (also called a channel post or PFC — Parallel Flange Channel) has a C-shaped cross-section. The open channel faces inward toward the wall, and the sleepers or fence panels slide into the channel from the side.

Best for:

• Retaining walls where sleepers are inserted from the end (not dropped in from the top)
• Situations where you need to add or remove sleepers after installation
• Fencing applications with timber or concrete panels

Common sizes at FPM:

• 100 PFC — suits standard 2.4m concrete sleepers for walls up to 1.2m exposed height

Advantages:

• Easy to install sleepers — slide in from the end
• Good for staged construction
• Widely available and cost-effective

Limitations:

• Lower moment capacity than equivalent H post
• Not ideal for very tall walls or heavy soil loads

H Posts (Universal Column Posts / UC Posts)

An H post (also called a UC post — Universal Column) has an H-shaped cross-section with two flanges. Sleepers or panels are dropped in from the top between the flanges.

Best for:

• Taller retaining walls (over 1m exposed height)
• Heavy soil loads or surcharge loads (driveways, structures near the wall)
• Situations where maximum structural performance is needed

Common sizes at FPM:

• 100 UC — general purpose, suits walls up to 1.5m
• 150 UC — heavier duty, suits walls 1.5–2.5m
• 200 UC — engineered walls, commercial applications

Advantages:

• Higher moment capacity than equivalent C post
• Better suited to taller walls and difficult soil conditions
• Sleepers drop in from the top — no need to slide from the end

Limitations:

• Sleepers must be installed from the top — can't be added later without removing upper courses
• Slightly higher cost than C posts

SHS Posts (Square Hollow Section)

SHS posts are square hollow section steel posts, typically used for fencing rather than retaining walls. They offer a clean, modern aesthetic and are commonly used with Colorbond or aluminium fence panels.

Best for:

• Colorbond fencing
• Pool fencing
• Decorative boundary fencing

How to Choose

Here's a simple decision guide:

Situation	Recommended Post
Retaining wall under 1m	100 PFC C Post
Retaining wall 1–1.5m	100 UC H Post
Retaining wall 1.5–2.5m	150 UC H Post
Retaining wall over 2.5m	Engineer required
Colorbond or panel fencing	SHS Post
Under-fence concrete plinth	100 PFC C Post

When in doubt, go bigger. The cost difference between a 100 UC and a 150 UC is modest compared to the cost of a wall failure.

Galvanising: Why It Matters

All steel posts used in ground contact should be hot-dip galvanised to AS/NZS 4680. This gives a zinc coating of at least 600 g/m² — far more durable than paint or cold galv spray. In Melbourne's clay soils, ungalvanised posts will rust through within 10–15 years.

FPM stocks only hot-dip galvanised posts. If you're buying elsewhere, always check the galvanising specification.

Post Depth: The Rule of Thumb

For retaining walls, embed the post at least as deep as the exposed height — so a 1.2m wall needs 1.2m of post in the ground. In soft or sandy soils, go deeper. In rock or very stiff clay, you may be able to reduce depth slightly, but always err on the side of caution.

Need Help Choosing?

Our team at FPM Building Supplies can help you select the right post for your project. Bring your wall dimensions and soil type and we'll point you in the right direction. We stock the full range of C posts, H posts and SHS posts from our Epping warehouse.