Building a retaining wall that stays standing

Retaining walls rarely fail because the blocks were weak. They fail because water built up behind them, or because the footing was too small for the load. Get those two right and almost any good block or brick will do the job for decades.

Drainage first

Soil holds water, and wet soil pushes far harder than dry soil. After heavy rain, the pressure behind an undrained wall can double. That is why every retaining wall we build gives water a way out:

• A slotted ag pipe in a filter sock at the base, falling to a stormwater point

• Free-draining gravel behind the wall, at least 300 mm wide

• Geotextile fabric between the gravel and the soil, so the drain doesn’t silt up

• Weep holes through the wall in brick and block construction

If a quote doesn’t mention drainage, ask how the water will get out. It is the cheapest part of the job, and the most expensive to add later.

The footing

The footing is the part nobody sees and the part that does most of the work. The taller the wall and the heavier the load behind it, the wider and deeper the footing needs to be. A low garden wall can sit on a modest strip footing. A wall holding back a driveway needs far more.

Above about a metre, or wherever a wall supports a building, a driveway or a boundary, an engineer should design the footing and the reinforcement. We build from their drawings, and we can recommend engineers we have worked with.

Choosing the material

Concrete block with steel and core-fill suits taller walls and tight sites. Brick suits garden walls and anything that has to match the house. Segmental retaining blocks stack without mortar and work well for terraced gardens. Each has a sensible height range, and the drainage rules apply to all of them.

Height, boundaries and approvals

Most councils limit how high a retaining wall can be near a boundary before a permit is needed, and the limits vary between councils. Walls that hold up a neighbour’s land can also need their consent. Check before you dig, or ask us to check as part of the quote.

Signs a wall is in trouble

Walk the wall after a wet week and look for:

• Leaning or bulging, even slightly

• Cracks that open wider over a season

• Water pooling at the base or seeping through the face

• Soil washing out from under the footing

Caught early, most of these can be fixed by adding drainage or rebuilding a section. Left alone, they tend to get worse after every wet winter. If you are not sure, send us a photo and we will tell you what we see.