Retaining wall base, drainage and boulder walls
Retaining walls fail from behind. Not from the block being wrong, but from water building up behind it and frost pushing it out over a few winters. The base and the drainage are the wall — everything else is the part you can see.
The short version
- Base trench: 6″ of compacted road crush, wider than the wall, and bury the first course.
- Drainage: a column of 20mm washed rock behind the wall, full height. Not road crush.
- Fabric between the drainage rock and the backfill soil, or the rock silts up and stops draining.
- Boulder walls need no block and no engineering hardware — set the biggest stones at the bottom and batter the face back into the slope.
- Taller walls hold back real tonnage. Permit requirements and height limits vary by municipality — check yours before you build anything substantial.
Boulder wall or block wall
Boulder wall. Limestone or fieldstone boulders set on a compacted road crush base. Informal, very durable, forgiving of ground movement, and well suited to a sloped yard — everything you need is on the site.
Block wall. Engineered block on the same base, with a washed rock drainage column behind it. The base and drainage materials are all here; wall block and caps are stocked at the yard but aren’t listed online — call 780 910 8882 or come and see what’s in. Worth doing in person for a wall, since block colour and texture read very differently in daylight than on a screen.
Materials
Tools: plate compactor (rental) · level · string line · machine for boulders over ~200 lb. From a hardscape retailer if building in block: wall block · caps · construction adhesive · geogrid for taller walls
Depth & coverage
Round the drainage rock up — the column always takes more than the arithmetic suggests once the wall face is uneven.
Steps
1 · Check the height before anything else
Low garden walls are a weekend job. Taller walls hold back real tonnage and become an engineering question rather than a landscaping one. Permit requirements and height thresholds differ between municipalities in the Edmonton region, so check with yours before you start — and if the wall is holding up anything you’d mind losing, get it designed properly.
2 · Call before you dig
Alberta One-Call, every time.
3 · Excavate the base trench
Twice the width of the wall, deep enough for 6″ of compacted road crush plus the buried first course.
4 · Compact the trench bottom
Before any material goes in. Everything above inherits whatever you leave here.
5 · Build and compact the base
Road crush in 2″ lifts, compacted each time, then levelled precisely. Every error here is multiplied by the height of the wall — an eighth of an inch out on the base course is an inch out at the top.
6 · Set the first course, and bury it
The bottom course sits below finished grade — fully buried on a low wall, roughly a tenth of wall height on a taller one. It’s what stops the wall walking forward. Level each unit or stone individually, front to back and side to side.
7 · Boulder walls: biggest at the bottom, battered back
Set the largest stones on the base course with their flattest face out. Each course steps back into the slope — a wall leaning slightly into the hill stays; a vertical one doesn’t. Fit stones so they bear on each other rather than balancing, and fill voids with smaller stone.
8 · Block walls: build the drainage as you go
Every course, place washed rock behind the block — a 12″ column, full height. Don’t leave it to the end, and don’t substitute road crush.
9 · Fabric between rock and soil
Separates the drainage column from the backfill. Without it, silt fills the voids and the drainage column becomes a wet clay column.
10 · Backfill and compact in lifts
Behind the drainage rock, backfill soil in 6″ lifts, compacting each. Uncompacted backfill settles and takes the grade above the wall with it.
11 · Give the water somewhere to go
The drainage column has to daylight — at the end of the wall or through outlets at the base. A drainage column with no outlet is a reservoir.
12 · Finish the grade
Screened topsoil above and behind, graded to shed water away from the top of the wall rather than over it.
Retaining walls in the Edmonton area
Frost is the load nobody designs for. A wall is usually built to hold soil, but here the force that takes walls out is water freezing in the backfill and expanding against the back face. Twenty-plus freeze-thaw cycles a year makes that relentless — which is why the washed rock column matters more in this climate than almost anywhere else.
- Clay makes it worse. Clay backfill holds water directly against the wall and expands as it wets. The drainage column is the barrier between the two.
- Build when the frost is out, and get backfill compacted before freeze-up. Frozen lumps in the backfill leave voids when they thaw.
- Boulder walls handle movement better than block. They’re not rigid — individual stones can shift a little without the wall failing. In a freeze-thaw climate that’s a real advantage, and it’s part of why they suit Alberta yards.
Common mistakes
- No base, or an uncompacted one. The wall settles unevenly and starts leaning in year two.
- Not burying the first course. The wall slides forward at the bottom.
- Road crush behind the wall instead of washed rock. The fines block drainage — the exact failure the column exists to prevent.
- No fabric between rock and soil. Silt migrates in and clogs the column.
- No outlet. Water collects behind the wall and freezes.
- Backfilling in one go without compacting. The ground behind sinks over the following year.
- Building vertical. Every retaining wall needs batter — a lean back into the slope.
- Going too tall without advice. The point where a DIY wall becomes a structural risk arrives sooner than most people expect.
What it costs
Priced by material and volume — calculate base, drainage rock and boulders separately.Boulders are priced by weight and vary with size and type, so a boulder wall is best quoted against your actual run and height. Call the yard with the wall length and height and we’ll work out both the stone and the base and drainage behind it.
FAQ
A compacted base of 20mm road crush, at least 6″ deep and twice the width of the wall, with the first course buried into it.
20mm washed rock, in a column at least 12″ wide, running the full height. Washed is essential — material with fines in it will block and hold water.
Almost always water behind it, an inadequate or uncompacted base, or a first course that wasn't buried. Frost then does the rest over a few winters.
Height limits and permit requirements vary by municipality across the Edmonton region, so check with yours before building. As a rule of thumb, the taller the wall the more it stops being a landscaping job and starts being a structural one.
Yes — wall block and caps are stocked at the yard, though they aren't listed on the site. Call 780 910 8882 or come and see what's in. We also carry boulders for boulder walls, and all the base and drainage material either type of wall needs.
Different, rather than weaker. They're not rigid, so they tolerate ground movement well, which suits a freeze-thaw climate. They need more space and a batter back into the slope.
Yes — between the drainage rock and the backfill soil, so silt can't migrate into the rock.
Order the materials
Tell us the wall length and height and we’ll work out the stone, the base and the drainage rock together. Delivered across Edmonton metro inside 24 hours.
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