Retaining walls: what to do and what to avoid

Retaining walls have been in use for thousands of years. Historic Asian rice terraces turned steep hillsides into farmable land through a series of them, which answers the two questions people usually ask first. When do you need a retaining wall, and what does it actually do?

A retaining wall reduces soil erosion and converts steep slope into usable flat terrace. In the process it tends to become a focal point. The hillside that made part of your property unusable suddenly opens up new possibilities.

Some wall types look simple to build. There is still a great deal to plan, because soil is extraordinarily heavy, particularly after rain.

The four main wall types

Gravity retaining walls use their own weight and mass to hold soil back. Because it is mostly about weight, you can use a wide range of materials: brick, pavers, unmortared stone, and dry-stacked stone are all popular. Shorter walls often need no additional reinforcement, though a decent height requires a trench for the wall to sit in, and larger walls need a concrete footer.

Cantilevered retaining walls, also called reinforced walls, run steel bars through concrete. They consist of a relatively thin stem and a base slab, and the weight of the soil above the slab holds it down so the wall cannot tip forward. This is the preferred approach in commercial construction.

Sheet piling walls are used mainly in tight spaces. Sheets of steel, wood, or vinyl are driven directly into the soil. This only works in softer soils, and you need one third of the sheet driven into the ground for every two thirds standing above it. Larger walls will need additional anchoring.

Anchored retaining walls are supported by anchors driven into the ground behind them and attached by cables. This method can add support to any of the above, and is particularly useful for heavier soil loads or when you need a thinner wall.

How much weight are we talking about?

A fairly basic retaining wall five feet tall and fifteen feet long holds roughly twenty tons of soil. Every additional foot of height increases that pressure substantially. Soil type and drainage are therefore the two most important things to establish before you choose anything else.

The rules that matter

Your wall is only ever as strong as its support system. A few non-negotiables:

Choose a workable material. Something you can buy locally and transport. There are many manufactured blocks made specifically for this.

Do not build a wall over four feet tall yourself. Larger walls carry serious weight and need engineered support and drainage. This is the single most common and most expensive mistake we see.

Build a proper foundation. Dig a trench and fill it with crushed rock. A useful rule for depth: dig one eighth of the wall height, plus three inches.

Level your surface. This matters more than it sounds.

Batter the wall backward. Build in a slight backward slope, at minimum one inch for every foot of height, so the wall leans into the hill rather than away from it.

The part nobody sees

Most retaining wall failures are drainage failures, not masonry failures. Water building up behind a wall adds enormous hydrostatic pressure to a structure already holding tons of soil. Gravel backfill, filter fabric, and weep holes or a perforated drain line are not optional extras on anything of height. They are the reason the wall is still standing in ten years. If your slope is steep or your soil is clay-heavy, this is the point at which it is worth bringing in a licensed contractor rather than a weekend.