How Do I Stop a Concrete Block Retaining Wall from Cracking in Frost?

Frost cracking in concrete block retaining walls is almost always caused by water getting trapped behind or inside the wall and then expanding as it freezes. The good news is that with the right drainage, quality materials, and a bit of seasonal maintenance, you can prevent most frost damage — or stop it from getting worse.

Why Frost Causes Concrete Blocks to Crack

When water freezes, it expands by roughly 9%. If that water is sitting in the voids of your concrete blocks, in the mortar joints, or in saturated soil pressing against the wall, the expansion creates enormous pressure. Over several freeze-thaw cycles — common in inland and elevated areas of New Zealand such as Central Otago, the Mackenzie Basin, Hawke's Bay hill country, and parts of Canterbury — this repeated stress causes spalling, joint cracking, and eventually structural failure.

New Zealand's climate varies enormously. Coastal areas rarely see hard frosts, but if your property sits above 300–400 m or in a frost hollow, this is a real concern worth taking seriously.

The Number-One Fix: Get Your Drainage Right

Poor drainage is the root cause of almost every frost-related retaining wall failure. Water must be able to escape freely from behind the wall rather than building up and saturating the backfill.

• Install a continuous drainage layer of clean, free-draining aggregate (e.g., 20 mm crushed rock) at least 300 mm wide directly behind the wall from the base to near the top.
• Use a geotextile filter fabric between the aggregate and the native soil to prevent fine particles from clogging the drainage layer over time.
• Fit weep holes or ag-pipe at the base of the wall so collected water has somewhere to go. Weep holes should be spaced no more than 1.5–2 m apart and kept clear of debris.
• Grade the surface above the wall so stormwater runs away from the wall rather than pooling behind it.

If your existing wall lacks drainage, retrofitting ag-pipe or additional weep holes is often possible and well worth the effort before winter sets in.

Choose Frost-Resistant Materials

Not all concrete blocks are equal when it comes to frost resistance. When building or repairing, look for:

• Dense, low-absorption concrete blocks — higher-density blocks absorb less water and are far more resistant to freeze-thaw damage.
• Frost-grade mortar — use a mortar mix with a low water-to-cement ratio. Adding a plasticiser can improve workability without adding excess water.
• Quality control joints — planned control joints allow the wall to move slightly without cracking randomly. Your block supplier or a retaining wall builder can advise on spacing.

Check with your block supplier about the freeze-thaw rating of their products if you're in a known frost zone.

Proper Construction Practices That Prevent Cracking

If you're building a new wall or repairing an existing one, these practices make a big difference:

• Solid base course — the first course of blocks must sit on a compacted, level foundation (typically a concrete footing or compacted AP20 gravel). A wall that settles unevenly will crack regardless of frost.
• Batter the wall slightly — most concrete block retaining walls are built with a slight lean (batter) back into the slope, typically 10–25 mm per 300 mm of height, which improves stability and reduces lateral pressure.
• Don't overfill voids with soil — keep the cores of hollow blocks filled with concrete or aggregate, not loose soil that can hold moisture.
• Cure mortar properly — don't lay mortar in freezing conditions. Fresh mortar that freezes before it cures will be permanently weakened.

Seasonal Maintenance Tips

Repairing Existing Frost Damage

If your wall already shows spalling (surface flaking) or cracked mortar joints, act before the next frost season:

1. Rake out cracked mortar to a depth of at least 15 mm and repoint with a fresh, frost-resistant mortar mix.
2. Replace badly spalled blocks — a block that has lost significant face material will continue to deteriorate.
3. Apply a penetrating concrete sealer to reduce water absorption. Products from Sika or Ardex are widely available in New Zealand and can help protect sound masonry.
4. Address the drainage before repointing — repointing without fixing drainage is a short-term fix at best.

Building Consent and the NZ Building Code

In New Zealand, retaining walls over 1.5 m in height generally require a building consent, though consent may also be required for lower walls depending on factors such as proximity to boundaries, structures, or slopes. Even walls under 1.5 m may need consent depending on your council's rules and site conditions. Always check with your local council before building or significantly modifying a retaining wall. MBIE Building Performance has useful guidance on when consent is required.

Retaining walls must comply with relevant structural standards for masonry. NZS 3604 applies to timber-framed buildings rather than retaining walls specifically. A qualified engineer may be required for walls over 1 m in some situations.

When to Call a Professional

Call a qualified retaining wall builder or structural engineer if:

• Your wall is leaning, bulging, or has moved — this is a safety issue, not just cosmetic.
• The wall is over 1 m tall and showing significant cracking.
• You're unsure whether your drainage is adequate.
• You're planning a new wall that may require building consent.
• Cracks are widening progressively season to season.

A professional retaining wall builder will assess the drainage, structural integrity, and frost exposure of your specific site. You can find qualified local tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — Retaining Walls and Consent
• BRANZ — Build Magazine and Technical Guidance
• Sika NZ — Concrete Repair and Protection Products
• Ardex NZ — Masonry and Concrete Products
• WorkSafe NZ — Safe Work on Slopes and Excavations
• Standards NZ — NZS 3604 and Structural Standards