Retaining Walls
Low retaining walls without mortar or curing time
Terrace a garden without a bricklayer. PolyBrick® blocks push-fit together on a properly prepared base, so low retaining walls go up in hours and stay put for decades.
Why PolyBrick works here
A retaining wall is the job most people put off. Masonry means mortar, mixing, curing and getting it right first time. Sleepers rot from the back where you cannot see them. Gabions need a mason's eye to look deliberate rather than industrial.
PolyBrick sidesteps most of it. Blocks interlock by pushing together, so there is no mortar, no mixing and no curing: the wall is finished when you stop building rather than three days later. Nothing rots, rusts, shrinks or warps, so the back face is as sound in year fifteen as the front.
What it still needs is a properly prepared foundation. A retaining wall of any kind concentrates load into a narrow strip of ground, so the trench, the compacted fill and the level of that first course carry everything above them. What that foundation should be depends on the height of the wall, the ground it sits on and what it is retaining, so establish it for your site before you start rather than assuming.
Where this works well
- Garden terracing and level changes
- Raised bed and border retention
- Driveway and path edge retention
- Sloping garden steps and platforms
- Low boundary and screen walls
- Planter and seating walls
- Bin and log store surrounds
- Allotment and community garden terracing
At a glance
| Assembly | Push-fit interlock. No mortar, no cement, no specialist tools |
| Reinforcement | [Confirm: bar diameter, grade, spacing, embedment, max height, soil conditions] |
| Foundation | Excavated to firm subsoil, compacted and screeded dead level. Specify for your site |
| Drainage | Free-draining backfill essential. Water pressure is what fails retaining walls |
| Material | 100% recycled polypropylene, UV stabilised, made in the UK |
| Maintenance | None. No rot, no rust, no repointing, no treatment |
| Compression | Tested to 150 tonnes/m² under fixed static load |
| Service life | 25 years under warranty |
Know the limits before you start
Worth being straight about this, because retaining walls are the one garden structure where getting it wrong has consequences beyond appearance.
A retaining wall holds back soil, and soil is heavy, especially when saturated. The taller the wall, the more force it carries, and the relationship is not linear: a wall twice the height carries considerably more than twice the load.
For low garden terracing, raised beds and edge retention, a properly built PolyBrick wall on a compacted base is well within its comfort zone. For anything taller, anything retaining a driveway or a structure, anything on a steep or unstable slope, or anything where failure would affect a neighbour, a building or a highway, take engineering advice and have the wall designed. That applies to any material, not just this one.
If you are unsure which side of that line your project sits on, tell us the height, the slope and what is above and below the wall, and we will give you an honest answer rather than a sale.
Drainage is what keeps a wall standing
Most retaining wall failures are water failures. Soil behind a wall holds rain, that water has weight and nowhere to go, and the pressure builds until something moves.
Build free drainage in from the start. Backfill directly behind the wall with clean angular stone rather than the soil you dug out, so water travels down through the stone instead of loading the back face. Where a wall is more than a couple of courses, run a perforated pipe at the base of that stone to take water away to somewhere sensible.
The advantage PolyBrick has here is that it does not care about the water. There is nothing to rot, rust or spall, and the joints are not mortar that can be washed out. Get the drainage right and the wall is genuinely maintenance free.
How it goes up
- Set out the line of the wall and check what is behind it. On a slope, work out where water currently runs, because the wall will interrupt it.
- Excavate a level trench along the line, wider than the wall itself, and dig out any soft or made ground until you reach firm subsoil.
- Lay a woven geotextile in the trench, then fill with compacted granular material and screed it dead level. This base carries the whole wall, and a wall built on an uneven base will show it in every course above.
- Lay the first course and check it with a spirit level along its length and across its width. Everything above follows this course, so take the time.
- Build up in courses, pushing blocks together and staggering the joints between courses so no vertical joint runs through the wall.
- Backfill behind the wall with clean angular stone as you go, not soil, and add drainage at the base of that stone on anything more than a low wall.
- Finish the top course with caps, and bring topsoil up behind to the level you want.
Full detail is in our project guidance, and we are happy to advise on a specific build before you order.
Getting the base right
This is the part that decides whether the wall is still straight in ten years, and it is the part most likely to be rushed.
A retaining wall concentrates a lot of load into a narrow strip of ground. If that ground is soft in places, made ground, or has never been compacted, the wall settles unevenly, courses open up and the line goes.
Dig out to firm subsoil, put a proper compacted granular base in, and screed it level rather than raking it. Check level along the run and across the width. It is an hour of work that determines everything above it, and it is the only part you cannot go back and fix.
Terracing a slope
Two low walls usually beat one tall one.
Where a garden falls away, stepping the level change into two or three shorter terraces reduces the load on each wall considerably, gives you usable planting space between them, and looks better than a single high face. It is also far more forgiving to build.
Set each terrace back from the one below rather than stacking them vertically, and drain each one separately. Water that runs off the top terrace should not arrive behind the bottom one.
Finishing and planting
A PolyBrick wall reads as a deliberate structure rather than a landscaping compromise, and it takes finishing well.
Cap the top course for a clean line. Plant trailing species along the top to soften the face over a season, or run planting in front of the wall so the structure becomes the backdrop rather than the feature.
Where a wall doubles as seating, keep the height comfortable and cap it properly. Where it forms a raised bed, line the soil face if you want to keep the fill separate, though the material itself is inert and will not leach into the soil.
Get it right first time
The one thing worth knowing: drain behind the wall, not just in front of it.
Almost every retaining wall that fails, in any material, fails because of water pressure behind it rather than the weight of soil alone. Saturated ground is dramatically heavier than dry ground, and it pushes.
Backfill with clean angular stone rather than the soil you dug out, run a drain at the base of it, and give that drain somewhere to discharge. It costs a bulk bag of stone and an afternoon. It is the difference between a wall that is still straight in twenty years and one that starts leaning after the first bad winter.
Ask our engineer
Questions about wall height, reinforcement, drainage or whether your project needs designing? Ask below, or get in touch directly.
Common Questions
How high can a PolyBrick retaining wall be built?
For low garden terracing, raised beds and edge retention, a properly built wall on a compacted base is well within its comfort zone. Beyond that the answer depends on the slope, the soil, the drainage and what sits above and below the wall.
For anything taller, anything retaining a driveway or structure, or anything where failure would affect a neighbour, a building or a highway, take engineering advice and have the wall designed. Tell us your height and site and we will give you an honest answer.
What foundation does a PolyBrick retaining wall need?
A trench excavated to firm subsoil, filled with compacted material and screeded dead level. What that foundation should consist of depends on the height of the wall, the ground beneath it and what it is retaining, so establish it for your site rather than assuming.
Whatever you build, that base carries the whole wall: screed it rather than raking it and check level along the run and across the width. It is the only part of the build you cannot go back and fix.
What should I backfill behind a retaining wall with?
Clean angular stone, not the soil you dug out. Most retaining wall failures in any material are water failures: saturated soil is dramatically heavier than dry soil and it pushes.
Free-draining backfill lets water travel down through the stone instead of loading the back face, and on anything more than a low wall a perforated pipe at the base of that stone should take it away.
Do PolyBrick walls need maintenance?
No. There is nothing to rot, rust, spall or repoint, and no mortar joints for water to wash out. The material is UV stabilised recycled polypropylene, so it does not need treating, painting or sealing.
Get the base and the drainage right at the start and the wall is genuinely maintenance free.
How does it compare to sleepers or concrete blocks?
Sleepers rot from the back where you cannot see it, and need replacing. Concrete blocks mean mortar, mixing and curing, and a wall that is finished three days after you stop working.
PolyBrick pushes together dry, so the wall is finished when you stop building, and nothing degrades from the buried side.
Is it better to build one tall wall or two shorter ones?
Two shorter ones, usually. Stepping a level change into two or three terraces reduces the load on each wall considerably, gives usable planting space between them, looks better than a single high face and is far more forgiving to build.
Set each terrace back from the one below and drain each one separately.
Will the plastic leach into my soil?
No. The material is inert recycled polypropylene and will not leach into soil, which is why the same blocks are used for raised beds and planters growing food. It locks the waste plastic up rather than releasing it.
Do I need planning permission for a garden retaining wall?
Low garden walls are usually permitted development, but height limits apply and they are lower where a wall adjoins a highway.
Rules differ across the UK and are tighter for listed properties and conservation areas. Check with your local planning authority if your wall is anything other than low and clearly within the garden.
