Why is my stucco developing dark streaks and mould spots near the roofline?

Dark streaks and mould spots near the roofline on stucco are caused by persistent moisture from roof runoff, poor ventilation, and the shaded, slow-drying conditions that exist where the wall meets the soffit. This is one of the most common stucco complaints across the GTA, particularly on north-facing walls and in neighbourhoods with mature tree canopy that keeps walls shaded.

The area where the stucco meets the roofline is uniquely vulnerable for several reasons. Roof runoff that overshoots or leaks from gutters runs down the wall surface, depositing organic material and keeping the stucco wet for extended periods. Failed or missing kick-out flashing where the roof slope meets the wall allows water to flow behind the stucco rather than into the gutter, saturating the substrate and creating a perpetually damp zone that supports mould and algae growth. The soffit overhang creates a sheltered area that receives less direct sunlight and air movement, so the stucco dries much more slowly than the rest of the wall. In Toronto's humid summers — especially in neighbourhoods within 5 kilometres of Lake Ontario where humidity runs consistently higher — these conditions are ideal for black mould (typically Gloeocapsa magma, the same organism that causes black streaking on roof shingles) and green algae.

To address the problem properly, you need to fix the moisture source before treating the staining. Inspect your gutters and downspouts first. Clogged or undersized gutters overflow directly onto the wall below, which is the single most common cause of stucco staining near the roofline. Clean gutters, verify downspouts are directing water away from the foundation, and install gutter guards if tree debris is a recurring issue. Next, check the kick-out flashing at every point where a roof slope terminates against a wall. Missing or improperly installed kick-out flashing is epidemic on GTA homes — many builders skip it entirely, and it allows thousands of litres of roof water per year to flow behind the stucco rather than into the gutter. Installing proper kick-out flashing costs $200 to $500 per location and is one of the most cost-effective repairs you can make. Also verify that the soffit ventilation is working — blocked soffit vents reduce air circulation and contribute to moisture buildup in the wall cavity.

Once the moisture source is corrected, clean the existing staining with a solution of one part oxygen bleach (sodium percarbonate) to four parts water, applied with a garden sprayer and allowed to sit for 15 to 20 minutes before rinsing with a garden hose. Do not use chlorine bleach, which damages stucco and kills surrounding vegetation. Do not pressure wash stucco at close range — the high pressure can blast away the finish coat and drive water into the wall assembly. A soft wash (low-pressure rinse after chemical treatment) is the safe approach. For stubborn mould, commercial masonry mould removers are available at building supply stores for $20 to $40 per container.

To prevent recurrence, apply a penetrating silane or siloxane concrete sealer to the cleaned stucco at $3 to $7 per square foot. This repels water absorption without trapping moisture vapour, significantly reducing the surface dampness that mould and algae need to grow. Trim back any tree branches within 2 metres of the affected wall to increase sunlight exposure and air circulation. If the staining is severe and the stucco surface has become pitted from biological growth, a skim coat of acrylic stucco finish can restore the surface and provide better mould resistance than the original cement finish.

Cleaning and prevention measures are reasonable DIY projects for a capable homeowner, but if the investigation reveals failed flashing or moisture entry behind the stucco, hire a qualified stucco contractor with WSIB coverage to make the structural repairs. Covering up mould staining without fixing the moisture source guarantees the problem returns within a season.