How long does it take for moss killer to work?

The Victoria pattern is a spring treatment that browns the moss by June, a dry summer in which the dead mats sit crisp and ugly on the north slope of a Gordon Head or Broadmead roof, and then the October and November storms that finally strip them into the gutters. Homeowners who treat in May and expect a clean roof by July are usually disappointed; the ones who treat in September see the winter do the removal by February. That is why local moss companies sell the removal visit as part of the job rather than leaving the weather to it.

Thick, old growth behaves differently from a haze. A two-year cushion on a Highlands or Metchosin roof under cedar is dense enough that a single application does not penetrate it, and the top browns while the base stays alive and re-greens by the next winter. Those roofs need physical removal first and treatment second, in that order, which is what Pacific Northwest extension guidance recommends. Copper gutters on older Oak Bay and Uplands homes add a constraint: zinc-sulfate products can corrode them, so ask what is being sprayed.

Four things set the speed: how thick the moss is, whether the roof was dry when treated and stayed dry for a day or two after, whether the moss was actively growing (spring and fall) or dormant (high summer), and how much rain and wind follow to break down the dead growth. A thin haze treated in May on a sunny Sidney roof can be gone by August. A cushion treated in a wet March on a shaded Saanich roof can still be green in June because the product never reached the base.

• Thin growth dies faster than cushions
• Dry roof at application, dry day after
• Active growth takes up treatment better
• Rain and wind do the removal; brushing shortens it

The temptation at week four, with a roof covered in brown crust, is to blast it. On asphalt that strips the granules and shortens the roof's life; extension guidance and Victoria contractors are unanimous on it. The right tools are a soft broom stroked downhill or a low-pressure rinse, used by someone with fall protection on any roof over one storey or with a real pitch. If the crust is stubborn, wait for it; more rain, not more pressure, finishes the job.

If a properly applied treatment fails twice on the same slope, the slope is the problem: too much shade, a tree dripping onto it, or a valley that never dries. Trimming the tree, installing zinc or copper strips after a clean (they protect about 15 feet below the strip) and clearing the valley change the conditions the moss needs. And if the shingles under the moss are bald or lifting, no treatment schedule will fix a roof that is at the end of its life; an inspection at $150 to $400 in our model is the honest next step.