Moss Treatment and Prevention After the Roof Is Clear

Every roof we clear in Nanaimo will grow moss again. That is not a flaw in the work. Spores drift off every fir, fence and neighbouring roof, and from October to March the shingles here can stay damp for weeks at a stretch. This page is about what happens after the clean: how fast the green returns, what changes that pace, and when it is worth having us back. The removal itself is covered on our roof moss removal page.

How quickly it comes back

A freshly cleared roof is a bare surface waiting for spores. On an open lot with full sun on the south-facing slope, it can be several years before you see growth again. Under tall Douglas firs on the lower slopes of Mount Benson, a north-facing pitch can show a green tinge within a couple of wet seasons. Most houses fall somewhere between those two.

The first sign is usually faint colour along the bottom edges of the shingle tabs near the eaves, on the side of the house that gets the least sun. Growth at that stage is a film, not a cushion, and a film is easy to deal with.

Shade and overhanging branches

Nanaimo sits in the Coastal Douglas-fir zone, and the trees are the main reason a roof stays wet. A bigleaf maple limb reaching over the garage, a cedar hedge grown taller than the gutter line, or a row of firs along the back fence can keep a slope in shade from the first fall rain until spring. Shade slows drying after every shower, and the needles that land on the shingles hold water right where moss wants it.

We do not do tree work. What we can do, from up on the roof, is show you which branches shade which slope, so you have something specific to take to an arborist. Pulling back one or two limbs to let afternoon light and moving air across the roof often does more than anything we apply. On a wooded lot in Departure Bay or Lantzville, that conversation is often the most useful thing we leave behind.

Zinc strips along the ridge

A zinc strip is a narrow band of metal fixed along the ridge. Each time it rains, a small amount of zinc rinses down the shingles below it and makes the surface a harder place for new moss to take hold. The effect is strongest in the first metre or two under the ridge and fades toward the eaves. That matters, because the bottom third of a slope, close to the gutters, is where moss usually gets going.

On a short, simple roof, a strip protects much of the surface and is worth having. On a long slope or a roof cut up by dormers and valleys, it only helps part of it, and we say so before fitting one. On cedar shake the strip has to be fastened without splitting the shakes it runs across, so on a shake roof we look at their condition first and tell you whether a strip is worth fitting at all.

Timing the treatment

Treatment is a liquid applied to a cleared roof. It kills spores and any thin growth the clean could not reach, and that dead material weathers off over the following months. It needs a dry roof and a few dry days after it goes on. In Nanaimo that points to late spring through the end of summer, once the rain eases. Sprayed in November, most of it ends up in the gutters before it has had a chance to work.

Treatment does not replace clearing. Over a thick mat, it kills the top layer and leaves a dead cushion that still holds water against the roof. If your roof has more than a thin film on it, it needs a roof demossing first, with treatment going onto the bare surface afterwards.

Runoff from a treated roof goes down the downspouts and into whatever is planted below, so we cover or soak the beds under the eaves before we start and keep the spray on the shingles.

Needles in valleys and gutters

Needles that pile up in valleys, behind chimneys and along the lower edge of the roof are where new moss starts. Getting that loose debris off in late fall, after the maple leaves and fir needles have dropped, slows regrowth on the lower slopes more than most people expect.