Order of work
- Call 811 (in the US) a few days before digging so buried utilities are marked. Ask your building department whether the wall needs a permit: walls over 4 ft from the bottom of the base, and walls with a driveway, building or slope above them, usually do — and need an engineer’s design.
- Mark the wall with stakes and a string line at the front face, and plan where the drain pipe will come out to daylight at the low end.
- Dig the trench to the depth and width shown above, and cut back the slope behind it far enough for the drainage stone. Compact the bottom of the trench.
- Spread and compact the base gravel, 6 in deep, with at least two passes of a plate compactor. Level it front to back and along the wall; this is the step that decides how straight the wall ends up.
- Lay the first course on the base, level each block both ways and check it with a string line. Take your time: every course above follows this one.
- Lay the drain pipe behind the first course, sloping to its outlet, and line the cut slope with filter fabric.
- Stack the next courses, staggering the joints by half a block. After each course, fill the space behind the block with drainage stone and compact it, and backfill behind the fabric in layers of no more than 8 in.
- Finish the top: fold the fabric over the stone, cover it with 6 in of soil sloped away from the wall, and glue the caps on with two beads of block adhesive.
Tools you’ll need
- Plate compactor for the base and the stone behind the wall (rental), and a hand tamper for tight spots
- Shovels, a mattock and a wheelbarrow
- 4 ft level and a short torpedo level
- Stakes, mason’s line and a line level
- Dead blow hammer or rubber mallet to set blocks
- Masonry saw or an angle grinder with a diamond blade for cut blocks and caps (rental), plus safety glasses, a dust mask and hearing protection
- Caulking gun for the cap adhesive, and work gloves
About this planner
A low block retaining wall turns a slope into a level bed or terrace, and it’s one of the most satisfying weekend projects — if it’s built on a proper base and drained. Most walls that lean, bulge or topple a few years on were built straight on the soil, with no buried course and nothing to let the water out behind them. The blocks themselves are the easy part.
This planner turns your wall’s length and height into the full shopping list: blocks, caps and adhesive, the compacted gravel base, the drainage stone behind the wall, the perforated pipe and the filter fabric, priced with your local numbers. It also gives the trench to dig, how much of the wall goes underground, how much the blocks weigh, and — just as important — the tallest wall your block and soil allow before you need an engineer. It deliberately stops at 3 ft above the ground.
How to use it
- Measure the wall’s length and the height it needs above the lower ground, without the cap. For a wall with a corner, plan each straight run separately and add the lists.
- Pick the block. Small 4 in blocks are light and cheap for edging and walls up to about 1½ ft; large 8 in blocks weigh 75 lb but go higher.
- Tell it about the soil behind the wall and whether the ground above is level or sloping. Clay and slopes push harder, so they lower the safe height.
- Choose caps, drain pipe and fabric, open Your local prices, and print the project sheet to take to the store.
How the quantities are worked out
Courses = (height above ground + 6 in buried) ÷ block height, rounded up
Blocks = courses × (wall length ÷ block length, rounded up) × (1 + extra%)
Caps = wall length ÷ cap length × (1 + extra%)
Base gravel (tons) = length × (block depth + 12 in) × 6 in × 1.15 ÷ 27 × 1.5
Drainage stone (tons) = length × 12 in × (block height in all − 6 in) ÷ 27 × 1.4
Filter fabric = length × (stone height + 12 in) × 1.10
Adhesive tubes = 2 beads × length ÷ 30 ft per tube
Trench depth = 6 in base + buried block
The build follows Allan Block’s residential installation guide: at least 6 in of block below the ground, a base of at least 6 in of compacted gravel, and a 12 in zone of clean drainage stone behind the block, with the pipe vented to daylight at least every 50 ft. The base runs 6 in past the front and back of the block. The height limits come from Allan Block’s maximum gravity wall heights for its 8 × 18 × 12 in AB Classic: 3 ft of total wall on clay and 4 ft on sand with level ground above, and 1¾ ft on clay or 3 ft on sand when the ground slopes up behind the wall. The small 4 in block is also held to its maker’s 2 ft maximum. On top of that, the planner keeps the wall within 4 ft from the bottom of the base to the top of the block, the limit the International Residential Code (R105.2) sets for walls built without a permit, and stops at 3 ft above the ground. Densities are typical supplier figures; clean stone barely compacts, so only the base is ordered heavy.
Worked example
These are the planner’s starting values and the results they produce:
| Wall length | 20 ft |
|---|---|
| Wall height above the lower ground | 2 ft |
| Wall block | Large block, 8 × 18 × 12 in (about 75 lb) |
| Soil behind the wall | Clay or silt – sticky when wet |
| Ground above the wall | Level |
| Wall caps | Yes, glued on |
| Cap length along the wall | 12 in |
| Perforated drain pipe behind the wall | Yes – clay, wet ground or downspouts nearby |
| Filter fabric behind the drainage stone | Yes – keeps soil out of the stone |
| Extra blocks and caps for cuts and breakage | 5 % |
| Large wall block, each | $6.50 |
| Small wall block, each | $3.00 |
| Wall cap, each | $4.00 |
| Base gravel, per ton | $45.00 |
| Drainage stone, per ton | $45.00 |
| Perforated drain pipe, per ft | $1.00 |
| Filter fabric, per sq ft | $0.30 |
| Block adhesive, per tube | $8.00 |
| Bought | Item | Buy | Cost |
|---|---|---|---|
| Large wall block, 8 × 18 × 12 in4 courses of 14, plus 5% · about 4,425 lb in all | 59 blocks | $383.50 | |
| Wall caps12 in long, plus 5% | 21 caps | $84.00 | |
| Landscape block adhesive, 10 oz tubesTwo ¼ in beads under the caps | 2 tubes | $16.00 | |
| Base gravel (¾ in crushed, “paver base”)0.85 cu yd: 6 in deep × 24 in wide, compacted | 1.28 tons | $57.50 | |
| Drainage stone (#57 clean, ¾ in)1.6 cu yd: 12 in behind the block, 26 in high; more if the blocks have hollow cores to fill | 2.25 tons | $101.11 | |
| Perforated drain pipe, 4 in, with sockAlong the base behind the wall; plus the run to daylight at the low end | 20 ft | $20.00 | |
| Non-woven filter fabricUp the back of the stone and over its top, plus 10% for overlaps | 70 sq ft | $20.90 | |
| Estimated materials total | $683.01 | ||
Tips before you buy
- Order a few extra blocks of the same production run: colors vary between batches, and a replacement block from a later run can stand out.
- Start the wall at its lowest point. If the ground falls along the wall, step the base down in block-height steps instead of making the first course deeper.
- Never backfill with the clay you dug out right behind the blocks — it holds water and pushes. Use the drainage stone there, and the soil behind the fabric.
- Point downspouts and surface water away from the top of the wall; a swale or a slight slope away from the cap keeps the drainage stone working.
- If anything heavy will sit near the top — a driveway, a shed, a pool, a house footing — or the wall is taller than this planner allows, have it designed by a licensed engineer.
Frequently asked questions
How many blocks do I need for a 20 ft retaining wall?
For a 20 ft wall standing 2 ft above the ground in 8 × 18 × 12 in blocks, you need 4 courses (one of them mostly underground) of 14 blocks: 56 blocks, or 59 with 5% extra. That’s about 4,425 lb of block. With 12 in caps you also need 21 caps and 2 tubes of adhesive.
How much gravel goes behind and under a retaining wall?
For that 20 ft, 2 ft wall: about 1.28 tons of base gravel for a 6 in × 24 in compacted pad, and about 2.25 tons of clean drainage stone for the 12 in zone behind the blocks. At $45 per ton, that’s roughly $159 of stone.
How deep should the trench for a retaining wall be?
Deep enough for 6 in of compacted base gravel plus the buried part of the first course — at least 6 in of block. With 8 in blocks and a 2 ft wall, 8 in of block ends up underground, so the trench is 14 in deep and 24 in wide, about 2.2 cu yd of loose soil to move for a 20 ft wall.
How tall can a retaining wall be without an engineer?
It depends on the block and the soil. Allan Block’s gravity tables for its large 8 in block allow 3 ft of total wall on clay and 4 ft on sand with level ground above, but only 1¾ ft on clay when the ground slopes up behind it. Part of that height is buried, so this planner allows at most 26 in above the ground on clay and 34 in on sand. In the US, walls over 4 ft from the bottom of the footing, or with a load above them, usually need a permit and an engineered design.