Under-Fence Plinth Sizes & Maximum Retaining Height

Getting the size right is the single biggest factor in whether an under-fence plinth does its job properly. Order too short a run and you're left with gaps. Underestimate the height and soil keeps pushing through underneath your fence anyway. This guide covers the standard dimensions used across the Australian market, how much soil a plinth can actually hold back, when stacking makes sense, and the point where you should stop looking at plinths altogether and start pricing a proper retaining wall.

Everything below reflects the dimensions and height limits used consistently across Australian concrete-plinth suppliers, cross-checked against current product listings — not just one brand's marketing copy. We've also flagged where figures vary between suppliers, so you know what to double-check before you order.

Standard Plinth Dimensions

Almost every concrete under-fence plinth sold in Australia follows the same general profile. That's not a coincidence — it's sized to match standard fence post spacing and standard fence panel widths, so a plinth run lines up neatly with the posts already in the ground.

Length

Most plinths run 2,300mm to 2,400mm long, with 2,340mm being the most common figure across suppliers. This lines up with the standard Australian fence panel span — the typical distance between fence posts — so one plinth generally spans one bay of fencing without needing to be cut.

• 2,300mm and 2,400mm are both common; check which your supplier stocks before you measure up.
• Post spacing that doesn't match a standard span (older fences, non-standard builds) may need a plinth trimmed to fit.
• Some suppliers also offer a 2,370mm option for slightly wider "good neighbour" panel styles.

Height

200mm is the standard single-plinth height. Some ranges also offer 100mm and 150mm options for smaller gaps, but 200mm is what you'll find as the default across most Australian suppliers, including RWD's own range.

Thickness

50mm is the near-universal thickness for this product. It's thin enough to sit flush under a fence line without stealing usable yard space on either side, but the concrete is graded and reinforced to handle the loads involved — typically 50–60MPa concrete with two steel reinforcing bars running the length of the plinth.

Dimension	Typical measurement	Why it matters
Length	2,300–2,400mm	Matches standard fence post spacing, avoiding cuts
Height (single)	200mm (100mm and 150mm also available)	Sets how much soil a single plinth retains
Thickness	50mm	Slim profile with enough steel reinforcing to carry soil load
Weight (typical)	~50–56kg per plinth	Manageable for two people to lift and position without machinery

Concrete Grade and Reinforcement

The dimensions get most of the attention, but the concrete specification is what actually determines how much load a plinth can take. Standard Australian under-fence plinths are cast in 50–60MPa concrete, complying with AS3600 (the Australian Standard for concrete structures), and reinforced with two steel bars — commonly N10 deformed bars — running the full length of the plinth. That reinforcing is what stops a thin 50mm slab from cracking under lateral soil pressure or the knock of a mower or whipper-snipper.

Two plinths that look identical can have different concrete grades or reinforcing behind them, which is part of why the maximum stacking height varies between suppliers. If a quoted price looks unusually low, it's worth asking what grade of concrete and what reinforcing bar size is actually used.

Why These Dimensions Became the Standard

The 2,340mm length and 200mm height aren't arbitrary. They evolved to match two things tradies and suppliers already had settled on: standard Colorbond and timber fence panel spacing (so a plinth needs no cutting on a typical job), and a height that clears the usual gap left when a fence line crosses uneven ground without needing a crane or heavy machinery to lift into place. A 50–56kg plinth is heavy, but two people can still position it by hand — go much bigger and you'd need mechanical lifting for every install.

Maximum Soil Retention Height

This is the question most people are really asking when they search for plinth sizes: how much soil will this actually hold back?

Single Plinth Capacity

A single 200mm plinth is built for exactly that: closing a soil gap of up to around 200mm. This is the classic scenario after a fence goes in and the ground on one side sits higher than the other — often because the neighbouring block was filled or built up at some point, or because your side has settled slightly lower over time.

Why "Maximum Height" Isn't One Fixed Number

It's worth being upfront about something here: not every plinth on the market is rated the same. Standard 50–60MPa reinforced plinths from most Australian suppliers are commonly rated to a two-high stack, around 400mm. A small number of heavier-duty ranges advertise engineering to go beyond that. Don't assume every plinth you see for sale carries the same rating — check the specific product's stated maximum before you plan around it, and if in doubt, treat 400mm as the sensible ceiling for a standard reinforced plinth.

Soil step	Recommended solution
Under 200mm	One standard plinth
200–400mm	Two plinths stacked (check product-specific rating)
Over 400mm, or sloped ground	Retaining wall (concrete sleepers), engineered for the load

Drainage Behind a Plinth Run

Height rating assumes normal drainage. Retained soil that stays saturated behaves very differently to soil that drains freely — waterlogged soil is heavier and pushes harder against whatever's holding it back, which matters even at the modest heights a plinth deals with.

• Avoid directing downpipes or surface runoff straight at a plinth run.
• A thin gravel strip behind the plinth helps water move away rather than pooling against the concrete.
• If the retained area holds garden beds that get watered regularly, factor that ongoing moisture into your height decision — it's another reason to stay conservative on reactive clay sites.

Stacking Plinths for Extra Height

How Stacking Works

Two plinths stacked one on top of the other gets you to roughly 400mm of retaining height, which is the standard maximum for reinforced concrete plinths sold in Australia. This isn't a workaround — it's a documented, supported way to use the product, and most suppliers design their plinths with this exact two-high configuration in mind.

Load Considerations

Stacking changes the load picture. The bottom plinth carries the weight and lateral pressure of everything above and behind it, so a few things matter more once you go two-high:

• Fence posts need to be sound enough to carry the extra fixed load — don't stack against a post that's already leaning or rotted.
• Both plinths need to sit flush against each other and against the fence, with no gap for soil to work through.
• The base under the bottom plinth needs to be compacted properly — an uneven base is the most common cause of a stacked run settling or leaning over time.

Installation Differences When Stacking

Fixing methods for a stacked run typically use brackets or cleats at both plinth levels, not just the top, so the whole stack is tied back to the fence posts rather than relying on weight alone to hold position.

Stacking two plinths is the practical ceiling for this product category. Adding a third course isn't standard practice with reinforced concrete plinths — at that point, the soil load and structural requirements call for an engineered retaining wall instead.

When Height Signals You Need a Wall Instead

Plinths are designed for small, defined soil steps — not ongoing retention of a sloped block or a garden bed that keeps creeping higher. A few signs you've outgrown a plinth solution:

Warning Signs

• The soil step is close to or over 400mm.
• The ground behind the fence is on a slope, not a single flat step — plinths handle a defined height difference, not a gradient.
• You're seeing ongoing soil movement or erosion, not just a one-off level difference.
• The area needs to support garden beds, paving, or foot traffic loaded above the retained soil.
• You're on reactive clay or a site with known ground movement — worth an engineering opinion regardless of height.

Why Plinths Aren't Engineered Retaining Structures

A retaining wall built from concrete sleepers is designed as a standalone structure — engineered posts set in concrete footings, sized to the retained height and soil type. A plinth relies on the fence it's fixed to for part of its stability. That's fine for a small soil step, but it's not a substitute for proper retaining wall engineering once the height or the load gets serious. As a general Australian guide, any retaining structure over 1 metre typically needs council approval and engineering sign-off — plinths sit well under that threshold, but it's a useful reminder of where the line is.

Factor	Under-fence plinth	Concrete sleeper retaining wall
Typical height range	Up to 400mm (stacked)	From around 400mm up to several metres, engineered per site
Structural support	Relies partly on the existing fence	Standalone posts and footings
Best suited to	A defined, small soil step	Sloped blocks, terracing, larger level changes

If any of this sounds like your situation, our concrete sleeper retaining wall range is the more durable, code-appropriate choice. Our guide on plinth vs retaining wall walks through the full decision framework if you're not sure which side of that line you're on.

How to Measure Your Fence Line

Getting an accurate measurement before you order saves a return trip — or worse, a mismatched course where the last plinth doesn't line up with the fence post.

Step-by-Step

1. Measure the total fence run in metres, end to end.
2. Divide by 2.3–2.4m to estimate how many plinths you'll need per course — round up, since you'll rarely get an exact fit.
3. At several points along the run — not just the ends — measure the vertical soil gap under the fence. It's rarely perfectly even.
4. Use your largest gap measurement to decide between a single plinth course or a stacked pair.
5. Note any fence posts, gates, or corners along the run — these can affect how plinths fit together and may need a plinth trimmed to length.

Common Measuring Mistakes

• Measuring the gap at one spot and assuming it's consistent along the whole run.
• Forgetting to allow for gate openings, which usually aren't plinthed the same way.
• Not checking post spacing against the plinth's actual length before ordering — a 5–10mm mismatch per bay adds up over a long fence line.

Ordering Tips

• Order a spare plinth or two if you're stacking, in case one needs trimming around a post.
• Confirm colour and finish before ordering the full run — mismatched batches can be a slightly different shade.
• If your run mixes a single course and a stacked section (common where ground height varies), order the two quantities separately so nothing gets mixed up on delivery.

FAQs

Can I cut a plinth to a non-standard length?

Yes, with a concrete saw and the right blade, though it's a job best left to whoever is doing your install rather than a DIY cut on-site. Most runs won't need it if post spacing is standard.

Do all plinths on the market handle the same maximum height?

No — treat 400mm (two standard plinths) as the sensible default, but always check the specific product's rated maximum before planning a stacked install, since not every range is engineered the same way.

What if my soil step varies along the fence line?

It's common. Measure at multiple points and plan for the tallest section — you can run a single course where the gap is small and step up to a stacked pair where it's larger, using the same plinths throughout.

Does plinth height affect fence warranty or council requirements?

A plinth used within its rated height generally sits below the threshold that triggers council retaining wall approval (commonly around 1m in most Australian states), but rules vary by council and by whether the plinth is also functioning as a retaining structure for fill. If you're unsure, a quick check with your local council before you start is worth the five minutes it takes.

Does soil type change what height I can safely retain?

Yes, to a degree. Reactive clay soils expand and contract with moisture, which puts extra cyclical pressure on anything retaining them — including a plinth. On reactive clay sites, it's worth staying toward the conservative end of the height guide above and paying closer attention to backfill and drainage, rather than pushing right up to the maximum rated height.

Can I mix plinth heights along the same fence line?

Yes — this is actually the norm on sites where the ground level varies. Run a single 200mm course where the gap is small, and step up to a stacked 400mm pair where the ground drops away more. Just keep the top of the plinth run reasonably level relative to the fence line, even if the number of courses changes underneath.

Is a plinth run a one-off cost, or does it need maintenance?

Once installed correctly on a compacted base, a concrete plinth is essentially a fit-once product. It doesn't rot, warp, or need repainting the way a timber solution would. The main thing worth checking every year or two is that backfill hasn't eroded away from the top edge and that fixings haven't loosened — a five-minute visual check, not ongoing maintenance.