Allan Block is one of the best DIY-friendly retaining wall systems there is — no mortar, no forms, and each block locks into the course below it. But here's what most install videos skip: the block stacking is the easy 30%. The trench, the base, the compaction, and the drainage are the 70% that decide whether your wall is still straight in ten years.
This is the same process our crews follow on every wall we build in the Lower Mainland — with the actual dimensions and materials, not "dig a small trench and add some gravel."
The short answer
- • The block stacking is the easy 30%. The trench, base, compaction and drainage are the 70% that decide whether it is still straight in ten years.
- • A gravity wall — block, base and drainage alone — is good to roughly 3 ft 6 in. Past that it needs geogrid in the backfill and usually an engineer.
- • Anything sitting above the wall — a driveway, a parking pad, a building — drops that threshold fast.
- • 6 in of compacted, dead-level base, the first course buried, wall rock through the cores and a 12 in zone behind.
Six things decide whether this is a weekend job or ours. Most walls are a mix — one on the right is not a no, it is the thing to settle before you order block.
Wall height
YESUnder about 3 ft 6 in (1.07 m) of exposed wall, built as a gravity wall
NOT QUITETaller than that. It needs geogrid layered into the backfill, and in most Lower Mainland cities an engineer's stamp
What sits above it
YESLawn or garden — nothing parked, driven or built on the ground it holds back
NOT QUITEA driveway, parking area or building above the wall. That is a surcharge, and the safe height drops fast
The ground behind
YESFlat ground behind the wall
NOT QUITEThe wall is holding back a slope, which keeps pushing after the wall is built
One wall, or tiers
YESA single wall
NOT QUITETerraced tiers. Too close together and both physics and most building departments treat them as one tall wall
Your city's trigger
YESUnder the height your city lets you build to without a permit — check yours, it takes a minute
NOT QUITEOver it. That means a permit, and in most cities an engineer as well
Somewhere for the water to go
YESGround lower than the wall, on your own land, for the drain to run out to
NOT QUITENowhere lower to send it, or the only low spot is the neighbour's
Mostly left? This is a wall you could build, with a plate compactor and a few weekends. Anything on the right? It is not a DIY wall — and those are the ones we build routinely, engineering coordination included.
Before You Dig: Three Things That Aren't Optional
- Request BC 1 Call before excavating — it's free to create an account and have the buried lines under your property located and marked. Buried gas and utility lines don't care that it's just a garden wall.
- Check your city's height trigger. Every Lower Mainland municipality has a height where a retaining wall legally requires a permit — and often an engineer. In the strictest cities it's as low as 0.6 m (2 ft); others allow up to 1.2–1.5 m. If anything sits above the wall — a driveway, parking pad, building, or slope — the threshold drops. Check your city's exact rules with our free permit checker.
- Plan where water exits before you place a block. Your drain pipe needs a "daylight" point — somewhere downhill where water can physically leave. If you can't find one, that's a design problem to solve now, not later.
The Full Tool & Material Checklist
Honest list — this is what it actually takes to build a wall that lasts:
Tools
- Plate compactor (rental)
- Shovels, spade, and a digging bar for roots and rocks
- Wheelbarrow (you will move literal tonnes of material)
- String line, stakes, and marking paint
- 4-foot level plus a torpedo level — every block gets checked
- Rubber mallet or dead-blow hammer
- Angle grinder or masonry saw with a diamond blade (for cuts and corners), or a block chisel and heavy hammer
- Caulking gun and concrete adhesive (for the cap course)
- Utility knife (filter fabric), tape measure, gloves, eye and ear protection
Materials (beyond the blocks)
- ¾" minus angular road base for the foundation
- Clear crushed "wall rock" — enough to fill every block core plus a 12" (300 mm) zone behind the entire wall, full height
- Perforated drain pipe, long enough to reach daylight
- Geotextile filter fabric
- Cap blocks and adhesive
If you're adding up rental days, aggregate deliveries, and a weekend or three of labour — it's worth knowing what the done-for-you number looks like before you commit.
Two things are worth knowing before you buy block. What your city requires at your height you can check yourself in about a minute. The other is whether your ground and whatever sits above the wall change the build — send us a photo of the slope and we will tell you whether it is a wall you could build yourself, a job for us, or one that needs an engineer. No visit needed for that answer, and if you are set on building it yourself but the rules are not clear to you, it is worth a call first.
Step 1 — Mark the Wall and Dig the Trench
Mark your wall line with paint or a string line. Then dig — deeper and wider than feels reasonable:
- Walls under 4 ft (1.2 m): trench 18" (460 mm) wide, deep enough for 4" (100 mm) of base material plus a full buried block course.
- Walls 4 ft and over: 24" (600 mm) wide, 6" (150 mm) of base, plus 1" of burial for every 1 ft of wall height. (A wall this tall is also almost certainly in permit territory — check first.)
Yes, the bottom course gets buried on purpose. That buried course is the wall's toe — skip it and the wall walks forward over time.
Step 2 — Compact the Trench (and Fix Bad Soil Now)
Run the plate compactor along the trench bottom — minimum two passes. If the bottom of your dig is heavy clay, wet, or previously disturbed fill, remove it and replace it with granular material, compacted in layers. A wall built on mud settles unevenly, and no amount of block-straightening later can fix it.
Step 3 — Set the Drain Pipe
If your wall holds back a slope, sits in clay soil (most of the Lower Mainland), or the ground ever feels wet — you need perforated drain pipe at the base of the wall, on a continuous slope to your daylight point. Water trapped behind a retaining wall is what actually destroys retaining walls: saturated soil can multiply the pressure on the wall. The pipe is cheap. The lean it prevents is not.
Step 4 — Place and Level the Base
- Minimum 6" (150 mm) of ¾" minus angular road base — angular aggregate locks together under compaction; round drain rock and pea gravel never do. Don't use them as base.
- Compact — minimum two passes.
- Level it obsessively. Any irregularity in the base multiplies upward with every course: a ¼" hump here is a visible wave by course five.
Step 5 — Lay the Base Course, Block by Block
The base course is where the time goes. Set each block, check it level front-to-back and side-to-side, and check it against its neighbours. Fill the area in front of the blocks with soil so they can't shift during compaction. Slow here means straight forever.
Step 6 — Wall Rock In and Behind, Backfill in Lifts
- Fill the hollow block cores and a 12" (300 mm) zone behind the wall with clear crushed wall rock, up to block height — every course. This turns the whole back of your wall into a vertical drain.
- Behind the wall rock, backfill with soil in lifts of 8" (200 mm) maximum, compacting each lift — minimum two passes, starting directly behind the block.
- Never backfill directly against the block with topsoil or organic material. That's a sponge strapped to your structure.
Step 7 — Stack the Courses (and Compact ON the Wall)
Sweep each course clean, stack the next with joints offset, and repeat the wall rock + backfill + compact cycle every course. From the second course up, run the plate compactor directly on top of the blocks and behind them, in paths parallel to the wall — that's what locks each course into the one below. It feels wrong the first time. It's how the system is designed to work.
Step 8 — Know Where DIY Ends
An Allan Block gravity wall — block, base, and drainage alone — is good to roughly 3.5–4 ft, in ideal conditions. Past that, the wall needs geogrid reinforcement layered into the backfill, and in most Lower Mainland cities it also legally needs an engineer's stamp. The threshold drops fast when:
- A driveway, parking area, or building sits above the wall (that's a "surcharge" — extra load the wall must carry)
- The wall retains a slope rather than flat ground
- You're building terraced walls — tiers too close together are treated as one tall wall by both physics and most building departments
None of those are weekend projects. All of them are walls we build routinely, engineering coordination included.
Step 9 — Finish: Fabric, Caps, and Grade
- Lay geotextile filter fabric between the wall rock and the soil behind it — it lets water through but keeps fines from clogging your drainage within a few seasons.
- Glue the cap course down with concrete adhesive.
- Finish grade so surface water runs away from the wall, and only put topsoil in the top few inches for planting.
The Five Mistakes Behind Almost Every Leaning Wall
- No buried base course — the wall has no toe
- Base dumped and raked instead of compacted in lifts
- No wall rock, no pipe — water trapped behind the wall
- Topsoil backfilled straight against the block
- Building past gravity-wall height (or under a driveway) without geogrid and engineering
We build Allan Block and segmental wall systems across the Lower Mainland — base, compaction, drainage, permits and engineering handled. Book a site visit and we will walk the grade with you; the phased price comes out of Planning & Design. Still deciding whether to build it yourself? Start with your city's rules — and give us a call if the answer is not clear.
