Waterproofing the inside face of a brick basement wall is a practical DIY project for many NZ homeowners, particularly in older homes where damp basements are common. While interior waterproofing won't fix severe structural water ingress, it can significantly reduce moisture penetration and protect your basement from dampness, mould, and deterioration. Here's how to approach it properly.
Understand the Limits of Interior Waterproofing
Before you start, it's important to be realistic. Interior waterproofing — sometimes called negative-side waterproofing — works by creating a barrier on the inside face of the wall to resist moisture pushing through from outside. It works well for minor to moderate seepage and general dampness, but it is not a substitute for proper exterior drainage if you have significant water pressure building up against your foundation.
If you're seeing large cracks, active water flowing in, or signs of structural movement, stop and get a professional assessment first. Check the MBIE Building Performance website for guidance on what may require a building consent — significant remediation work can trigger consent requirements, and it's always worth confirming with your local council.
Step 1 — Identify and Fix the Source
Before applying any waterproofing product, find out where the moisture is coming from:
- Condensation — moisture in warm air condensing on cold walls (common in NZ winters)
- Seepage — groundwater or rainwater pushing through the brickwork
- Cracks or failed mortar joints — allowing direct water entry
A simple test: tape a piece of plastic sheeting to the wall and seal all edges. Leave it for 24–48 hours. If moisture appears on the wall side of the plastic, water is coming through the wall. If it appears on the room side, you have a condensation problem — which is better addressed with ventilation and insulation.
For condensation issues, Energywise has useful guidance on improving ventilation and reducing moisture in NZ homes.
Step 2 — Prepare the Surface Thoroughly
Good preparation is the most critical part of the job. Waterproofing products will fail if applied to a dirty, loose, or damp surface.
- Remove any paint, efflorescence (white salt deposits), or loose render using a wire brush or angle grinder with a masonry disc
- Rake out cracked or crumbling mortar joints to a depth of at least 15–20 mm and repoint with a suitable mortar mix
- Fill cracks with a hydraulic cement or cementitious repair mortar — products from Sika or Ardex are widely available in NZ and well-suited to this
- Allow all repairs to cure fully before proceeding
- Clean the wall with water and let it dry to a damp but not wet surface (most cementitious waterproofing products actually require a damp substrate)
Step 3 — Choose the Right Waterproofing Product
For brick basement walls, the two most common interior waterproofing approaches are:
| Product Type | How It Works | Best For |
|---|---|---|
| Cementitious slurry coat (e.g. Sika, Ardex) | Bonds to masonry and fills pores | General seepage and damp walls |
| Crystalline waterproofing | Grows crystals into the concrete/mortar to block capillaries | Active seepage, long-term protection |
| Bitumen/rubber membrane | Flexible coating applied by brush or roller | Damp-proofing, lower water pressure |
For most NZ DIYers, a cementitious slurry waterproofing product is the most practical choice. Brands like Sika and Ardex offer products specifically designed for this application. Always read the manufacturer's datasheet carefully and follow their mixing and application instructions exactly.
You can source these products from Bunnings, Mitre 10, or PlaceMakers.
Step 4 — Apply the Waterproofing
- Mix the product strictly to the manufacturer's ratio — consistency matters
- Apply with a stiff masonry brush, working the slurry firmly into the surface
- Pay extra attention to mortar joints, corners, and wall-to-floor junctions — these are the most common failure points
- Most products require two coats, applied in opposing directions, with the first coat allowed to reach a firm (not fully dry) set before the second is applied
- Cove the wall-to-floor junction with a fillet of mortar or waterproofing slurry before coating — this prevents water tracking under the coating
- Keep the coating damp-cured for 24–48 hours after application (mist with water) to prevent it drying out too quickly and cracking
Step 5 — Manage Ongoing Moisture
Once the waterproofing is in place, reduce the moisture load on the wall:
- Ensure gutters and downpipes are clear and directing water away from the foundation
- Grade soil away from the house perimeter where possible
- Use a dehumidifier in the basement, especially in winter
- Ensure adequate ventilation — this is also a requirement under the Healthy Homes Standards for rental properties
When to Call a Professional
Interior waterproofing has its limits. Call a qualified tradesperson if you notice:
- Structural cracks or signs of wall movement
- Significant water inflow that doesn't slow between rain events
- Rising damp from the floor as well as the walls
- Any work that may require a building consent — always check with your local council first
For complex basement waterproofing, an exterior tanking system or a French drain may be needed — work best left to experienced professionals. You can find qualified local bricklayers and waterproofing specialists on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
