Green Staining Below Eavestroughs in Westboro — Causes & Fixes

Those green streaks running down the walls beneath your eavestroughs are caused by oxidation of the aluminum combined with organic material washing over the gutter lip. In heavily treed Westboro neighbourhoods, decomposing leaves, algae, and tannins mix with moisture inside the gutter and create a green-tinged runoff that stains siding, brick, and stucco. Copper eavestroughs produce an even more dramatic green patina runoff, but since most Westboro homes have aluminum gutters, the culprit is almost always a combination of algae buildup and oxidized aluminum residue.

Stopping the Stain at Its Source

The most effective long-term solution is keeping your eavestroughs clean. In Westboro, where mature maples and elms line nearly every street, debris accumulates rapidly from September through November and again when spring buds and pollen drop in April and May. Twice-annual professional cleaning at $150 to $350 per visit removes the organic sludge that feeds the staining process. Installing gutter guards rated for fine debris — micro-mesh systems at $18 to $30 per linear foot installed — dramatically reduces the amount of organic material sitting in the trough and breaking down into that green-tinted slurry.

For the existing stains on your siding, a solution of oxygen bleach (not chlorine bleach) mixed with warm water and applied with a soft-bristle brush works well on vinyl and aluminum siding. On brick, a pressure washer at a low setting removes most oxidation staining. Avoid harsh chemical cleaners on painted surfaces, as they can strip the finish.

Another contributing factor specific to Ottawa's climate is the 50-plus freeze-thaw cycles each winter. Each cycle traps moisture and organic residue against the gutter face, accelerating the oxidation that causes green streaking. Ensuring your eavestroughs have a proper slope of at least one-quarter inch per 10 feet of run toward the downspout prevents standing water from sitting in the trough and intensifying the oxidation process.

If your eavestroughs are more than 15 years old and the factory enamel finish has worn thin, the bare aluminum oxidizes much faster. At that point, the staining will keep returning no matter how often you clean. Replacing aging eavestroughs with new seamless aluminum at $8 to $18 per linear foot installed gives you a fresh factory-baked finish that resists oxidation for another decade or more. If the staining problem is severe or your eavestroughs need attention beyond simple cleaning, browsing eavestrough professionals through the Ottawa Construction Network directory at justynrookcontracting.com is a good next step to get quotes and find contractors experienced with Westboro's older housing stock.