How to Build an Allan Block Retaining Wall (BC Homeowner's Guide)

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."

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.

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