Why is there white powder forming on my concrete block foundation wall below the parging?

That white powder is efflorescence — mineral salt deposits left behind when water migrates through the concrete block and evaporates on the surface. It is one of the most common foundation issues in the GTA, and while it looks alarming, the powder itself is cosmetic. The real concern is what it tells you about moisture moving through your foundation wall.

Efflorescence occurs when water dissolves soluble salts (primarily calcium carbonate) within the concrete block, carries them to the surface, and then evaporates, leaving the white crystalline residue behind. In the GTA, this is especially prevalent in spring when snowmelt saturates clay soils and drives moisture against foundation walls, and again in fall when heavy rains coincide with cooling temperatures. If the efflorescence is appearing below your parging, it means water is entering the block wall from the exterior, bypassing or undermining the parge coat above. This typically indicates that grading around the foundation is directing water toward the wall, downspouts are not extending far enough away, or the exterior waterproofing (if any exists) has deteriorated.

Addressing the moisture source is the critical first step. Check that the soil grade slopes away from the foundation at a minimum of 6 inches over the first 6 feet — this is an Ontario Building Code requirement under OBC Section 9.14. Ensure downspouts discharge at least 6 feet from the foundation. If your concrete block home was built in the 1950s through 1970s, there is a good chance the original dampproofing (a thin tar coating) has cracked and deteriorated, allowing water direct access to the block. In that case, exterior waterproofing with a liquid membrane and dimpled drainage board is the proper long-term solution, typically costing $3,000–$12,000 in the GTA depending on excavation depth and linear footage.

To clean existing efflorescence, use a stiff nylon brush and a commercial efflorescence remover or a diluted muriatic acid solution (1 part acid to 10 parts water). Always wear safety glasses and gloves when working with acid. Wet the wall first, apply the solution, scrub, and rinse thoroughly. Allow the wall to dry completely before applying any sealer. Once clean and dry, a penetrating silane or siloxane concrete sealer ($3–$7 per square foot applied) can help reduce future moisture migration through the block without trapping vapour inside the wall.

If efflorescence keeps returning after cleaning and sealing, the moisture problem is significant enough that surface treatments alone will not resolve it. At that point, a professional assessment of your exterior waterproofing and drainage is warranted. A qualified masonry or waterproofing contractor can determine whether excavation and membrane application are necessary, or whether interior drainage management (weeping tile and sump pump, $5,000–$15,000) is more practical given your site conditions. Contractors working on your foundation should carry WSIB coverage — always request a clearance certificate before work begins, as Ontario homeowners can be held liable for injuries to uninsured workers on their property.