Choosing the wrong post size is one of the most common — and costly — mistakes in retaining wall and fencing projects. Go too light and you risk wall failure. Go heavier than you need and you blow the budget unnecessarily.

This guide breaks down the real differences between the 150×150 UC post and the standard 100 UC and 100 PFC posts, so you can spec the right product for your project the first time.

What Is a UC Post?

UC stands for Universal Column — a hot-rolled steel section with wide, parallel flanges designed to carry compressive and bending loads. Unlike a PFC (Parallel Flange Channel), which is open on one side, a UC is a symmetrical I-section that resists load equally from both directions.

The number in the name refers to the nominal depth of the section:

• 100 UC — 100mm nominal depth, commonly used for standard retaining walls and fencing
• 150 UC — 150mm nominal depth, a significantly heavier and stiffer section for high-load applications

Both are available hot-dip galvanised for long-term corrosion resistance in outdoor and in-ground applications.

The Standard Posts: 100 UC and 100 PFC

The 100 UC and 100 PFC are the workhorses of the concrete sleeper retaining wall industry in Australia. They're the go-to choice for the vast majority of residential retaining walls and fencing projects.

100 UC (H Post) is used where the post needs to resist bending in two directions — common in fencing applications where lateral loads can come from either side, or in corner and end post positions on retaining walls.

100 PFC (C Post) is the standard inline post for concrete sleeper retaining walls. The channel shape allows sleepers to slot in from the side during installation, making it the most practical choice for straight wall runs.

Both sections are available from 900mm to 3600mm, covering wall heights from around 600mm up to approximately 2400mm retained height (depending on soil conditions and engineering requirements).

When the Standard Posts Are the Right Choice

For most residential retaining walls and fencing projects, the 100 UC and 100 PFC are entirely adequate:

• Wall heights up to approximately 1.2m retained height in average soil conditions
• Standard residential fencing — Colorbond, timber paling, pool fencing
• Garden retaining walls with normal surcharge loads (no vehicles, no structures nearby)
• Projects where post spacing is 1.8m or less

If your project fits these parameters, the standard posts will do the job at a lower cost per metre.

The 150×150 UC Post: When You Need More

The 150×150 UC is a substantially heavier section. Compared to the 100 UC, it has a larger cross-section, greater moment of inertia, and significantly higher bending resistance. In plain terms: it handles much greater loads without deflecting or failing.

At FPM Building Supplies, the 150 UC is available in 13 sizes from 1800mm to 6000mm — the longer lengths alone tell you something about the applications it's designed for.

Situations That Call for a 150 UC Post

High retaining walls. Once you're retaining more than 1.2–1.5m of soil, the bending moment on each post increases dramatically. A 150 UC provides the section modulus needed to handle these loads safely. For walls retaining 1.8m or more, a 150 UC is typically the minimum requirement — and an engineer should be involved.

Surcharge loads. If there's a driveway, vehicle access, a structure, or a pool near the top of the wall, the lateral pressure on the posts increases significantly beyond what standard soil pressure calculations assume. The 150 UC handles these additional loads where a 100 UC would be marginal or inadequate.

Long post spacings. Wider post spacing means each post carries a greater tributary load. If your design calls for post centres greater than 1.8m, moving to a 150 UC is often the right call to maintain adequate safety margins.

Deep embedment requirements. Taller walls require deeper post embedment to resist overturning. The 150 UC's longer available lengths — up to 6000mm — mean you can achieve the embedment depth you need without welding or splicing posts on site.

Gate posts and structural anchor points. Gate posts carry dynamic loads from swinging gates, wind loads, and the weight of the gate itself. A 150 UC provides the rigidity and strength to handle these loads without movement or deflection over time.

Commercial and civil applications. Car parks, commercial landscaping, civil retaining structures, and any application with engineered design requirements will often specify a 150 UC as the minimum section.

Side-by-Side Comparison

Feature	100 UC / 100 PFC	150×150 UC
Typical wall height	Up to ~1.2m retained	1.2m+ retained
Available lengths	900mm – 3600mm	1800mm – 6000mm
Best for	Residential walls, fencing	High walls, surcharge, structural
Post spacing	Up to 1.8m centres	1.8m+ centres possible
Gate posts	Light gates only	Heavy gates, all gate types
Engineering required	Often not for standard residential	Recommended for walls over 1.5m

A Note on Engineering

For any retaining wall over 1m in height — and certainly for anything over 1.5m — we strongly recommend involving a structural engineer. Post selection is just one part of the equation. Soil type, drainage, footing depth, surcharge loads, and local council requirements all affect what's appropriate for your specific site.

Getting an engineer's sign-off not only ensures the wall is safe — it protects you legally and can be required for council permits in many Victorian municipalities.

Pricing at $84.70/m inc. GST

The 150 UC is priced at $84.70 per metre including GST at FPM Building Supplies, with sizes available from 1800mm ($152.46) through to 6000mm ($508.20). All prices include GST unless otherwise stated.

For comparison, the 100 UC runs at $38.50/m and the 100 PFC at $29.70/m including GST — so the 150 UC carries a premium that reflects its significantly greater material weight and load capacity.

For most residential projects, the standard posts represent excellent value. But for the applications where a 150 UC is the right call, the cost difference is trivial compared to the cost of a wall failure or a rebuild.

Talk to the Team

Not sure which post is right for your project? Our team at FPM Building Supplies in Epping can help you work through the requirements. We stock the full range of 100 UC, 100 PFC, and 150×150 UC posts, along with concrete sleepers, plinths, and everything else you need to get the job done right.

Call us on +61 431 235 919 or visit us at 32 Fullarton Drive, Epping VIC 3076. We're open Monday to Friday 7am–5pm and Saturday 7am–3pm.