Stucco is one of the toughest exterior cladding materials available — but it’s not maintenance-free. And in Toronto, where winter brings repeated freeze-thaw cycles, any weakness in your stucco becomes a liability. Water enters a hairline crack in October. It freezes in January. By March, that crack has expanded into a chunk of missing stucco — or worse, rot and mould in the wall assembly behind it.
The good news: stucco gives you warning signs before it fails. If you know what to look for, a late-summer or early-fall inspection takes about 20 minutes and can save you thousands in repair costs. Here’s exactly what to look for.
1. Hairline Cracks — Don’t Dismiss Them
Hairline cracks are the most common early warning sign and the most frequently ignored. Homeowners often assume a crack that thin can’t be letting in water — but stucco is porous, and water finds its way through even the smallest opening.
What to watch for: cracks that run diagonally from the corners of windows and doors are especially common, as these corners concentrate structural stress. Horizontal cracks along the base of the wall or near the roofline are also worth flagging.
What to do: Any crack wider than 1/16″ — roughly the thickness of a credit card — should be sealed before winter. Cracks narrower than that can be monitored, but note their location and length so you can track whether they’re growing.
2. Stucco That Sounds Hollow When Tapped
Walk along your exterior walls and knock on the stucco every 12–18 inches with your knuckle. Healthy stucco bonded to its substrate produces a solid, dense sound. Stucco that has delaminated from the lath or substrate produces a distinctly hollow, drum-like sound.
Hollow sections indicate that the bond has broken — usually due to moisture infiltration or, in older homes, aged lath that’s no longer holding. Delaminated stucco can hang in place for months before it falls away, but once freeze-thaw cycles start, deterioration accelerates rapidly.
What to do: Hollow-sounding areas need to be cut out and re-patched. This is not a DIY sealant job — the delaminated material needs to come off and be replaced with a properly bonded patch coat.
3. Staining, Efflorescence, and Rust Marks
White chalky streaks on your stucco surface are efflorescence — salt deposits left behind when water moves through the stucco and evaporates. Efflorescence itself doesn’t damage the stucco, but it’s a reliable indicator of water movement through the wall system.
Brown or rust-coloured staining is more serious: it usually means the metal lath or fasteners behind the stucco are beginning to corrode. If you see rust staining bleeding through to the surface, moisture has been present long enough to start working on your structural components.
What to do: Efflorescence: investigate the source of water infiltration (commonly a failed sealant at window or door frames). Rust staining: have a contractor inspect the area — it may require removing a section of stucco to assess the condition of the lath.
4. Cracks or Gaps at Penetrations and Trim
Look carefully at every point where your stucco meets something else: window frames, door frames, electrical boxes, hose bibs, dryer vents, and roof lines. These transitions are the highest-risk points in any stucco system because caulking and sealants age and fail faster than the stucco itself.
Even a small gap at a window frame can allow enough water in during a heavy rain event to saturate the sheathing behind the wall. In a Toronto winter, that moisture has nowhere to go — and it does damage the whole time it’s sitting there.
What to do: Any gap larger than 1/8″ at a penetration should be recaulked before winter. Use a high-quality polyurethane or silicone sealant rated for exterior masonry — not latex paintable caulk, which shrinks and cracks within a season or two.
5. Soft or Crumbling Areas
If any section of your stucco feels soft when you press on it, or if material crumbles off when you run your finger along a crack edge, the stucco has absorbed significant moisture and its structural integrity is compromised. This is particularly common at the base of walls where water splashes up from grade, and on walls with chronic drainage issues.
Soft stucco won’t survive a winter. Freeze-thaw cycles will accelerate the deterioration, and you may end up with large sections failing when the weather turns.
What to do: Soft or crumbling sections need to be removed and re-applied before any cold weather arrives. This is time-sensitive — stucco needs above-freezing temperatures to cure properly.
6. Interior Signs You Shouldn’t Ignore
Sometimes failing stucco announces itself from the inside before you see the damage outside. Water staining on interior walls or ceilings near exterior walls, peeling paint on interior surfaces adjacent to the outside, and musty odours in rooms against the building envelope can all be signs of stucco that’s been letting water in for some time.
By the time stucco failure shows up as interior water damage, the problem is typically significant. A contractor inspection of the corresponding exterior stucco is urgently warranted.
When to Call a Professional vs. Handle It Yourself
Here’s a practical breakdown:
- Single hairline cracks in sound stucco: fill with matching elastomeric caulk (DIY-friendly)
- Recaulking at window and door frames: DIY with the right product and good preparation
- Hollow sections, soft stucco, rust staining, or large cracked areas: call a stucco contractor
- Interior water damage linked to exterior stucco: call immediately and don’t wait until spring
Toronto’s ideal window for stucco repair work is late August through October — temperatures are stable, the stucco has time to cure before the first frost, and contractors have availability after the busy summer season. Don’t wait until spring if you can address it now.
Book a Pre-Winter Inspection
Not sure what you’re looking at? We offer free on-site consultations and can give you a clear picture of your stucco’s condition, what needs to be addressed before winter, and what can wait. We’re GTA-based and typically available within a few days.
Contact Alasya Construction: 905-917-4568 for a free pre-winter stucco assessment.
