When a retaining wall leans, bulges or starts to come apart, people usually blame the blocks. Most of the time the real cause is behind the wall: water. Wet soil is heavy, and in our climate it also freezes. A wall that can’t drain ends up holding back far more than it was built for.
What water does to a wall
After rain or snowmelt, water soaks into the soil behind a wall. If it can’t get out, it builds up pressure against the back of the wall. In winter, that saturated soil freezes and expands, pushing again. Over a few seasons the wall tips forward, the top course goes out of line, or the face bulges in the middle.
What should be behind a retaining wall
Wall manufacturers publish installation guides for segmental block walls. The details vary by product, but the pieces are consistent:
- A compacted levelling pad. The first course sits on a compacted granular base — commonly around 6 inches thick — and every course above depends on it being flat.
- A buried first course. Guides typically call for burying the bottom course about 10% of the wall’s height, or at least 6 inches, whichever is greater.
- A drainage zone. Around 12 inches of clear, free-draining stone directly behind the blocks, so water can move down instead of pushing out.
- A perforated drain pipe. Laid at the base of the wall to collect that water and carry it to an outlet where it can drain away safely.
- Filter fabric. Keeps the surrounding soil from washing into the drainage stone and clogging it over time.
- Setback. Each course steps back slightly into the slope so the wall leans into the soil it’s holding.
Source: Nicolock installation instructions.
You can see this section on our retaining walls page.
Where the water goes matters too
A drain pipe needs somewhere to go. Pointing it at your neighbour’s yard or letting it dump next to the foundation just moves the problem. Planning the outlet — and the grading above the wall so less water gets there in the first place — is part of the job. That’s where our drainage and grading work often ties in.
When a wall needs an engineer
Height changes the rules. Manufacturer guides commonly say unreinforced block walls are practical up to about 3 feet, and that taller walls, terraced walls, weak soils and walls carrying extra load need special design.
In Ontario, the building code also treats some retaining walls as designated structures. The City of Barrie’s building services information sheet says a permit is required when a retaining wall exceeds 1,000 mm (about 3.3 ft) in exposed height and is next to public property, access to a building, or private property the public can access — which includes front and side yards where couriers or emergency services may approach. Those walls need a design from a professional engineer licensed in Ontario, and guards are required where the height exceeds 1.0 m.
Sources: Nicolock; City of Barrie.
Rules vary by municipality. If your wall is getting close to a metre, check with your town before you build — we’ll flag it at the first site visit.
Signs your wall has a drainage problem
- Leaning forward or bulging in the middle
- Water seeping out of the face or pooling at the base
- Gaps opening between blocks or courses
- Soil washing out from under the bottom course
Small, low walls can sometimes be taken down and rebuilt with proper drainage using the same blocks. Larger walls may need an engineer’s input first.
If you have a wall that’s moving — or a slope you want to hold — we’re happy to take a look. Get a free estimate.
