How to build a retaining wall
A retaining wall holds back a slope and turns it into usable, terraced space — and with modern interlocking blocks it's a no-mortar, stack-and-go project for a low wall. The secret isn't the stacking; it's the compacted base and the drainage behind it. Here's how to build one, plus how many blocks and caps to buy.
First: height, permits, and drainage
Before you buy a single block, settle three things:
- Height. A DIY interlocking-block wall is realistic up to about 3–4 ft. Taller walls carry serious soil loads and usually need geogrid reinforcement, a permit, and an engineer — check local code.
- Drainage. Water trapped behind a wall is what topples it. Plan for a layer of free-draining gravel and a perforated drain pipe along the base from the start.
- Utilities. Call 811 to have buried lines marked before you dig.
How a block wall works
Interlocking blocks have a lip or pin that sets each course back slightly, so the wall leans into the hill (the "setback" or batter) and the soil helps hold it. Underneath, a compacted gravel pad spreads the load; behind, drainage gravel and a pipe carry water away; and the bottom course is buried so the base can't kick out.
How many blocks and caps?
Enter your wall length and height and our calculator gives the blocks and caps you need, plus the estimated cost.
Open the Retaining Wall CalculatorWhat you'll need
- Retaining-wall blocks and matching caps
- Crushed stone for the base and for drainage backfill (clean ¾″ gravel)
- 4″ perforated drain pipe and landscape fabric
- A plate compactor (rented) — the single most useful rental for this job
- A hand tamper, a long level, and a string line
- Construction adhesive for the caps, and a masonry saw or splitter for cuts
- Marking paint, a shovel, and a wheelbarrow
Step by step
- Lay out the wall and call 811. Mark the line, and wait for utilities to be located before digging.
- Dig the base trench. Excavate a level trench wide enough for the block plus about 12″ of gravel behind it, and deep enough to bury the entire first course (roughly the block height plus your base).
- Build a compacted base. Add 4–6″ of crushed stone, then compact it in lifts and level it carefully. This pad carries the whole wall — don't rush it.
- Set the first course dead level. Lay the bottom blocks and level each one front-to-back and side-to-side. Any error here multiplies all the way up, so take your time on this course.
- Stack the courses. Sweep each course clean, then add the next, staggering the joints (running bond) so seams don't line up. The block lip creates the setback automatically.
- Add drainage. Lay perforated pipe along the base behind the wall, backfill behind the blocks with clean gravel, and separate the gravel from the soil with landscape fabric.
- Backfill and compact. Fill behind the wall in 6–8″ lifts, compacting each before adding the next.
- Cap the wall. Glue the caps on with construction adhesive to finish the top and lock the back course.
Mistakes to avoid
| Mistake | Why it matters |
|---|---|
| A soft or uncompacted base | The wall settles and leans — compact a gravel pad first |
| First course not level | Small errors snowball into a crooked, unstable wall |
| No drainage gravel or pipe | Water builds up behind the wall and pushes it out |
| Not burying the bottom course | The base has nothing to resist it and kicks out |
| Building too tall to DIY | Above ~3–4 ft you need engineering and usually a permit |
| Joints not staggered | Stacked seams create weak vertical lines |