How to plan a block retaining wall

Step 1: Check the height and whether you need a permit

The retained height — from the bottom of the wall to the top of the soil it holds back — is what matters. Many jurisdictions require a permit, and often an engineered design, for walls around 4 feet tall or more, and for walls that hold up a driveway, a building, a slope or another wall close behind. Check with your building department first.

Also check the block itself: small garden-wall blocks are generally meant for low walls of about 2 feet, while larger retaining blocks can build taller gravity walls. The manufacturer's literature lists maximum heights for its products.

Step 2: Lay out the wall

• Mark the face of the wall with string lines or a garden hose for curves.
• Call 811 to have buried utilities marked before you dig — it's free.
• Plan where water will go. Walls fail mostly because water builds up behind them.
• Measure the length along the face and use the retaining wall calculator for blocks, caps, base and drainage stone.

Step 3: Dig the base trench

Block manufacturers generally call for a trench wide enough for the block plus room behind it — commonly around one and a half to two times the block's depth — and deep enough for a compacted gravel base of at least 6 inches plus the buried course.

Step 4: Build a level, compacted base

Spread crushed stone in the trench and compact it in lifts with a plate compactor or hand tamper. Level it carefully front to back and side to side. Any error in the first course shows up magnified at the top of the wall, so take your time here.

Step 5: Set the first course

• Start at the lowest point and step the base up in block-height increments if the ground slopes.
• Level every block in both directions, and check it against the string line.
• This course is buried — it keeps the base of the wall from sliding out.

Step 6: Drainage and backfill as you go

• Drain pipe: lay a perforated pipe behind the first course, sloped to drain to daylight or a storm drain. Manufacturers require one for taller walls and poorly drained sites; it's cheap insurance for any wall.
• Drainage stone: fill at least 12 inches behind the blocks with clean crushed stone (no fines), course by course. Many blocks have hollow cores that are filled with the same stone.
• Filter fabric between the drainage stone and the soil behind it stops fines from clogging the stone.
• Compact the backfill behind the drainage zone in lifts as the wall rises.

Step 7: Stack the courses and cap the wall

Stagger the vertical joints like brickwork, sweep each course clean before placing the next, and keep checking the line and level. Finish with cap units glued with a concrete landscape adhesive, and cap the drainage zone with a few inches of low-permeability soil so surface water doesn't pour straight into it.

Materials checklist

All of these except the last row come straight out of the retaining wall calculator.

Sources

• Segmental retaining wall manufacturers' installation guides (for example Allan Block) — base, buried course and drainage recommendations.