What this guide answers
Four decisions go into ordering posts for a sleeper retaining wall: which section, which weight class, what length, and how far apart. Get the length wrong and you are either short of embedment or paying for steel you bury for no reason. Get the spacing wrong and the sleepers do not fit.
This guide works through all four using the posts we actually stock, so every recommendation lands on a real length you can order. Where a wall is above the approval threshold, the engineer's design governs everything below.
C posts or H posts: decide it on the job, not in the abstract
The section shape is not a quality ranking. It is about which sides of the post the sleepers arrive from.
- H posts have a channel on both sides of the web, so sleepers slide in from the left and the right. That makes them the post for every intermediate position along a run.
- C posts have a channel on one side only. That makes them the post for the two ends of a run, and for anywhere the wall stops against something: an existing structure, a boundary you cannot build past, a set of stairs, or a change in direction handled by a corner post.
A straight wall in one run therefore takes a C post at each end and H posts in between. Order both from the steel posts collection: galvanised steel H posts and galvanised steel C posts. If you want the section shapes explained properly, we have an article on the difference between H beams and I beams.
Standard or heavy duty 150 series
We stock two weight classes.
- The standard range. Galvanised C and H posts in twelve lengths from 0.4m to 3.8m. This is what most residential sleeper walls use.
- The heavy duty 150 series. The 150UC23 H post and the 150PFC C channel, each in nineteen lengths from 2.0m to 6.0m.
We are not going to publish a wall height at which the 150 series becomes mandatory, because that is not a decision a supplier's web page can make. What we can tell you is how the ranges line up with reality. A wall that needs 150 series posts is an engineered wall, so the section, the embedment and the pier come out of the design rather than out of a table. Sections above the 150 series are quoted to your engineer's specification. If you have a drawing, send it with your quote request and we will price to it.
One useful observation from the two ranges: the standard range tops out at a 3.8m post and the 150 series starts at 2.0m and runs to 6.0m, so they overlap deliberately. If an engineer has specified a 3.0m post in a heavier section, we hold that length in both classes.
Post length: the rule and the table
Our materials calculator sizes posts on a simple rule: the pier depth equals the wall height, so the post length is twice the wall height, set in a 450 mm diameter pier.
That is deliberately more conservative than the common benchmark. Our article on how deep retaining wall posts should be sets out the widely used minimum of at least one third of the total post length embedded, with a 1:1 ratio for soft soil or high load cases. Pier depth equal to wall height puts half the post in the ground, so it satisfies the one third minimum with room to spare and sits at the conservative end for reactive or soft ground. If your engineer has specified something different, use the engineer's number.
Because our standard posts come in 0.4m steps, each step maps to a 0.2m step in wall height:
| Wall height | Pier depth | Post length to order | Available in |
|---|---|---|---|
| 0.2 m | 0.2 m | 0.4 m | Standard C and H |
| 0.4 m | 0.4 m | 0.8 m | Standard C and H, corner posts |
| 0.6 m | 0.6 m | 1.2 m | Standard C and H, corner posts |
| 0.8 m | 0.8 m | 1.6 m | Standard C and H, corner posts |
| 1.0 m | 1.0 m | 2.0 m | Standard C and H, corner posts, 150 series |
| 1.2 m | 1.2 m | 2.4 m | Standard C and H, corner posts, 150 series |
| 1.4 m | 1.4 m | 2.8 m | Standard C and H, 45 degree corner, 150 series |
| 1.5 m | 1.5 m | 3.0 m | Standard C and H, 150 series |
| 1.6 m | 1.6 m | 3.2 m | Standard C and H, 150 series |
| 1.7 m | 1.7 m | 3.4 m | Standard C and H, 150 series |
| 1.8 m | 1.8 m | 3.6 m | Standard C and H, 150 series |
| 1.9 m | 1.9 m | 3.8 m | Standard C and H, 150 series |
| 2.0 m to 3.0 m | equal to wall height | 4.0 m to 6.0 m | 150 series only |
This table is indicative and is not a substitute for a design by a Registered Professional Engineer of Queensland. Above the approval threshold in your area, the post size, embedment depth and footing come from the engineer. See our Queensland council approval guide for where that threshold sits, and confirm your own situation with your council or a licensed private building certifier.
Two practical notes on length
- Round up, not down. If your wall height falls between two steps, take the next length up. The extra sits above ground where it can be dealt with, rather than coming out of the embedment where it cannot.
- Height means retained height. If you are cutting into a bank and filling on top, the wall height for this purpose is the total retained height, not just the visible face.
Post spacing is set by the sleeper, not by a rule of thumb
This is the point most buyers get wrong. Post centres are not a free choice. The sleeper has to sit between the posts, so the centres are set by the sleeper length plus a small tolerance, and adding a fence to the wall changes the tolerance again.
| Sleeper length | Post centres, no fence | Post centres, with fence |
|---|---|---|
| 1480 mm | 1500 mm | 1510 mm |
| 1980 mm | 2000 mm | 2010 mm |
| 2380 mm | 2400 mm | 2410 mm |
| Any other length | Sleeper length plus 20 mm | Sleeper length plus 30 mm |
Those are the figures from our own alignment and steel post positioning guide, which also covers setting out with string lines. Our concrete sleepers come in all three lengths, so the spacing follows from which sleeper you choose rather than the other way around.
How many posts
Posts equal bays plus one. A 12 metre wall built with 2380 mm sleepers and no fence has 2400 mm centres, which is 5 bays, so 6 posts: two C posts at the ends and four H posts in between. Get the count wrong by one and you are a post short at the far end, which is a second delivery.
The materials calculator does this arithmetic for you, along with course counts, drainage volumes and fixings. It is an estimate rather than a quote.
Corners, and the constraint nobody mentions
Where a wall changes direction you need a corner post rather than a mitre. We stock two:
- 90 degree corner posts, in 0.8m, 1.2m, 1.6m, 2.0m and 2.4m
- 45 degree corner posts, in 0.8m, 1.2m, 1.6m, 2.0m, 2.4m and 2.8m
Here is the constraint worth checking before you finalise a design. The corner post range is shorter than the straight post range. On the pier depth equals wall height rule, the tallest 90 degree corner post covers a wall of about 1.2 m and the tallest 45 degree corner post covers about 1.4 m, while the straight range runs to 1.9 m and the 150 series well beyond.
If your wall turns a corner and is taller than that, the corner is the limiting element and it needs to be resolved in the design, not on site. Ring us before you order and we will work through the options with you.
Composite posts
For lower walls where weight and handling matter more than capacity, we also carry C composite posts and H composite posts in the composite posts collection. They pair with composite sleepers and suit garden terraces and low landscape walls rather than structural work.
Galvanising, and why the buried part is the part that matters
Our steel posts are galvanised. That coating matters most where you cannot see it. The lower half of a post on the pier depth equals wall height rule is permanently in the ground, in concrete, behind a drainage zone that is doing its job by staying wet after rain. That is the section that has to last, and it is not one you can inspect or recoat later.
The same logic applies to the fixings. Use galvanised hardware rather than untreated, and we stock galvanised M12 bolts and nuts for that reason. Finish the top of the post with a cap so water does not sit in an open section: we carry an H post cap, a C channel post cap, a 90 degree post cap and a 45 degree post cap in the post caps collection.
None of that helps if water is allowed to build up behind the wall in the first place. Drainage is a structural component, not a finishing touch. See the retaining wall drainage guide.
When the wall stops being a table exercise
Everything above is a starting point for a straightforward residential wall on ordinary ground. Hand it to an engineer when any of these are true:
- the wall is above the approval trigger in your area, which for most of South East Queensland is a total retained height of one metre, but not everywhere
- there is a surcharge in the zone of influence: a driveway, slab, shed, pool or another wall stepping above
- the wall is within 1.5 metres of a building or another retaining wall
- a fence is going on top, which adds load including wind load
- the ground is soft, filled, reactive, steeply sloping or wet
- the wall forms part of a pool barrier
In Queensland, professional engineering services must be carried out by a Registered Professional Engineer of Queensland or under their direct supervision, and you can check any engineer on the public register held by the Board of Professional Engineers of Queensland. Confirm your own situation with your council or a licensed private building certifier before ordering material. Our council approval guide goes through it in detail, and the wall height guide sorts materials by height band.
Putting the posts in
Set out first, then dig. Our step by step installing steel posts guide covers the sequence, and the installation cross section diagram shows how the post, sleepers, drainage zone and backfill sit together. One set out detail worth knowing before you dig: the wall is built with a lean back of at least 50 mm per metre of height, so it is not plumb by design.
Order or ask
Browse the steel posts and posts and corners collections, or send us the wall and we will do the take off. Request a quote with the length, the height, whether a fence is going on top, your suburb and a couple of site photos.
28 Shettleston Street, Rocklea QLD 4106. Monday to Friday, 6am to 4:30pm. Phone (07) 3716 0191 or email enquiry@hybuilt.com.au.