Retaining walls shift in wet weather primarily because water builds up behind them, creating hydrostatic pressure that pushes the wall outward or causes the soil to become unstable. The good news is that most movement can be prevented — or stopped from getting worse — by addressing drainage, backfill, and structural design. Here's what you need to know as a NZ homeowner.
Why Wet Weather Causes Retaining Walls to Move
When rain saturates the soil behind a retaining wall, the water has to go somewhere. If it can't drain away quickly, pressure builds up against the wall face. This is called hydrostatic pressure, and it's one of the leading causes of retaining wall failure in New Zealand's wet climate — particularly in regions like the West Coast, Northland, and Wellington.
Other contributing factors include:
- Poor or blocked drainage behind the wall
- Inadequate footing depth or undersized foundations
- Wrong backfill material (clay-heavy soils hold water and expand)
- No geotextile filter fabric allowing fines to clog drainage
- Surcharge loads (vehicles, buildings, or slopes above the wall adding extra pressure)
Check Your Drainage First
The single most effective thing you can do is ensure water can escape freely from behind the wall. A well-built retaining wall should have:
- Weep holes — openings at the base of the wall (typically every 1–1.5 m) that allow water to escape through the face
- A drainage aggregate layer — at least 300 mm of clean, free-draining gravel (e.g., 20 mm AP20 chip) directly behind the wall
- A perforated drainage pipe (ag pipe) at the base of the wall, sloped to daylight or a stormwater outlet
- Geotextile filter fabric wrapped around the drainage aggregate to stop fine soil particles from migrating in and blocking the system
If your existing wall has weep holes, check they aren't blocked with soil or debris — clear them out with a piece of wire or a hose.
Use the Right Backfill Material
What goes behind the wall matters enormously. Never backfill with clay or silty soil — these materials hold water, expand when wet, and contract when dry, creating ongoing movement. Instead, use:
- Free-draining granular fill such as AP20 or AP40 crushed aggregate
- Compacted in layers (no more than 150–200 mm lifts) to avoid settlement
- Geotextile separation between the drainage gravel and any topsoil above
If your wall was backfilled with unsuitable material, excavating and replacing it is the most reliable long-term fix — though this is a significant job best handled by a professional.
Foundation and Structural Considerations
A wall that's moving may have an undersized or poorly placed footing. In NZ, retaining walls over 1.5 m in height typically require a building consent and must be designed to meet the NZ Building Code — particularly in terms of structural stability and drainage (refer to Clause E1 for surface water).
Key structural points:
- Footings should extend into stable, undisturbed ground
- Batter (leaning the wall slightly back into the slope) helps counteract soil pressure
- Deadman anchors or geogrid reinforcement are used in taller walls to tie the wall back into the hillside
- Timber walls should use H5-treated timber for ground contact — check with your supplier at Mitre 10 or PlaceMakers
Always confirm consent requirements with your local council before starting or modifying a retaining wall. Some lower walls may be exempt under Schedule 1 of the Building Act, but this depends on height, soil conditions, and proximity to boundaries or buildings — your council is the definitive source.
Ongoing Maintenance to Prevent Shifting
Even a well-built wall needs regular attention:
- Inspect after heavy rain — look for bulging, cracking, or leaning
- Clear weep holes and drainage outlets at least once a year
- Remove large plants or trees growing close to the wall — roots can displace structure and block drainage
- Avoid parking vehicles or placing heavy loads near the top of the wall
- Repoint or reseal any cracks in concrete or masonry walls before water infiltrates and freezes or expands
Temporary Measures During Wet Weather
If your wall is actively moving during a wet spell, you can take some short-term steps while arranging professional assessment:
- Divert surface water away from the top of the wall using temporary channels or sandbags
- Avoid adding any load (soil, materials, vehicles) above the wall
- Do not attempt to push or prop a failing wall yourself — this can be dangerous
When to Call a Professional
Retaining wall failure can be sudden and dangerous. You should engage a qualified retaining wall builder or geotechnical engineer if:
- The wall is visibly leaning, cracking, or has moved more than a few millimetres
- The wall is over 1.5 m high
- There is a building, driveway, or boundary close to the wall
- You're unsure about the soil conditions or drainage setup
- A building consent may be required
For walls that require engineering input, a Licensed Building Practitioner (LBP) may need to be involved — check the LBP register to find someone qualified. You can also find experienced local retaining wall builders on tradedirectory.co.nz.
WorkSafe NZ has guidance on excavation and earthworks safety if you're planning any digging around an existing wall.
Licensed Building Practitioner (LBP)
