How do Ottawa contractors prepare a foundation wall before applying parging?

Surface preparation is genuinely the most important part of a parging job — more important than the mix, the application technique, or the finish. A beautiful parging coat applied to a poorly prepared foundation will delaminate within a few Ottawa winters. Here's what proper prep looks like.

The Full Preparation Process

1. Remove all existing failed material. If there's old parging that's crumbling, hollow, or delaminated, it all comes off. Contractors use a cold chisel and hammer, an angle grinder with a masonry wheel, or in some cases a small jackhammer for stubborn material. Every bit of loose or poorly bonded old parging needs to go — leaving any behind creates a weak layer that the new parging will eventually separate from.

2. Clean the surface thoroughly. The exposed foundation needs to be free of:

• Dust and loose particles (power washing or stiff wire brushing)- Efflorescence — those white salt deposits that indicate moisture migration- Paint or sealers — these prevent the parging from bonding to the concrete- Oil, grease, or tar — sometimes found on foundations near driveways or where damp-proofing was applied above grade- Organic growth — moss, mildew, or algae that trap moisture

Most Ottawa contractors use a pressure washer at 2,000–3,000 PSI for this step. It's fast, effective, and reveals any additional areas of deterioration that weren't visible under the dirt.

3. Repair significant defects. Large voids, holes from old tie wires, crumbling mortar joints (on block foundations), or deep cracks get filled with a hydraulic cement or patching compound before the parging coats go on. This creates a more uniform substrate and prevents weak spots.

4. Dampen the surface. Concrete and block are porous and will suck moisture out of fresh parging too quickly if applied to a dry surface, weakening the bond and causing premature cracking. The wall should be damp — not dripping wet, but uniformly moistened. Experienced contractors will wet the wall the evening before and again the morning of application.

5. Apply a bonding agent (when needed). On very smooth poured concrete foundations, a liquid bonding agent or slurry coat (thin cement paste brushed on) helps the parging grip the surface. Block foundations with their textured mortar joints usually don't need this step — the joints provide mechanical keying.

What to watch for as a homeowner: If a contractor shows up and starts applying parging over dirty, dry, or painted foundation without any of these steps, that's a red flag. Proper prep adds $200–$500 to the job compared to skipping it, but it's the difference between parging that lasts 20 years and parging that fails in 3–5 Ottawa winters. The prep work should take at least half a day for a full-perimeter job — if someone is done in an hour, corners are being cut.

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