Retaining walls: drainage, loads and when brick is not enough
Part of our brick walls guides
A retaining wall holds back soil, and wet soil is heavy. The pressure behind a retaining wall is what breaks it, and most of that pressure is water that had nowhere to go.
Drainage comes first
Every retaining wall needs a drainage path behind it: free-draining backfill, a perforated land drain at the base taken to a suitable outfall, and weep holes through the face. Without them, water builds up behind the wall each time it rains and applies a load the wall was never designed for. A wall that is bulging or has cracked in a stepped diagonal is nearly always a drainage failure.
Construction for the height retained
Low retained heights — a step, a raised bed, a small change of level — can be built in solid brickwork on an adequate foundation. As the retained height increases, the answer is usually a reinforced concrete or blockwork core with a brick facing, so the structure does the work and the brick provides the appearance.
Anything substantial, anything retaining ground next to a boundary or a structure, and anything supporting a driveway or parking area should be designed for the actual loads. Where that is the case we will say so rather than guess at it.
Sloping gardens
Terracing a slope with two lower walls is often better than one tall one: less retained height at each level, simpler drainage, and usable flat areas rather than one big drop. It also spreads the cost, because retained height drives price much faster than length does.
- Free-draining backfill and a land drain behind the wall
- Weep holes through the face at regular centres
- A foundation sized for the retained load, not just for the wall's own weight
- A coping that sheds water off the top and away from the backfill
- Terracing rather than a single tall wall where levels allow
Frequently asked
Why is my retaining wall bulging?
Almost always water pressure behind it, sometimes combined with an undersized foundation. Repointing the face does not address it. The wall needs looking at properly, because a bulging retaining wall can fail suddenly.
Do retaining walls need building regulations approval?
Retaining walls are not always covered by building regulations, but they can be where they affect a structure, boundary or highway, and larger walls may need design input. It depends on the height, the ground and what is behind it — worth checking with your council before starting.