Planning retaining walls and drainage

Water is the cause of most retaining wall failures and wet-yard problems. A wall holds back soil that is heavy, and heavier still when it is wet. A good drainage layer behind the wall, and a place for the water to go, matter as much as the blocks. For problems that are not about a wall, a French drain moves water from where it collects to where it can leave.

This planner covers the slope and the layout first, then a block retaining wall, and then drainage and masonry.

Step 1: Measure the slope and decide if you need a wall

Find the rise and the run, and work out the grade. Gentle slopes can often be regraded or terraced with planting. A wall higher than about 4 feet usually needs an engineer and a permit, and the rules vary by place. Call 811 before you dig.

Step 2: Plan the wall and its base

A segmental retaining wall sits on a buried, compacted base of crushed stone, with the first course below the ground. The wall leans slightly back into the slope. Taller walls need geogrid layers tied into the soil. The base and the drainage behind it decide how long the wall lasts.

Step 3: Excavate and haul the soil

The trench for the base and the space behind the wall need to be dug out. Soil swells when you dig, so allow for the extra volume, and for the weight if you are hauling it.

Step 4: Backfill with stone and fit the drain

Fill behind the wall with free-draining crushed stone, not the soil you dug out. A perforated drain pipe at the bottom of the backfill, sloped to daylight, carries the water away. The same idea, a trench of stone with a pipe and a filter fabric, is a French drain for a wet spot or a damp foundation.

Step 5: Block, brick and mortar walls

For a mortared wall, work out the number of blocks or bricks from the area, and the bags of mortar from the count. Mortared walls need a proper footing and, for taller walls, reinforcement. Allow for openings and for cuts.

Before you start

• The height of the wall and the local rules for walls above a certain height.
• Where the water will go: a drain outlet, a lower area, or a dry well.
• Call 811 before you dig.
• The soil type, since clay holds water and puts more pressure on a wall.
• Access for the delivery of heavy blocks and stone.

Mistakes that cost people money

• No drainage behind the wall, so water pressure pushes it over.
• Backfilling with the dug-out soil, especially clay.
• A base that is too thin, uneven or not compacted.
• A drain pipe with no outlet, or one that slopes the wrong way.
• Building a tall wall without an engineer or a permit.

Frequently asked questions