Moss Prevention on Oceanside Roofs, Season by Season

On this coast, moss runs to a calendar. It does almost all its growing in the wet months, sits brown and still through the dry summer, and swells up green again once the fall rain settles in. Keeping it down on a cleared roof is mostly a matter of doing the right small job at the right point in that year, some of it by us and some of it by you from the lawn. This page goes through the year in order. If your roof is still carrying a thick mat, the first step is roof moss removal. Treatment and prevention come after the clear, never in place of it.

October and November: leaves down, rain back

The first steady rains wake up anything left on the roof, and the trees choose the same weeks to unload. Bigleaf maples drop their broad leaves and winged seeds. Douglas fir and western red cedar keep shedding needles and dead fronds. All of it slides down until something stops it: a valley, the uphill side of a chimney, the back of a skylight, the bottom row of shingles above the gutter. Those damp little piles are where new moss gets its start.

That is why our most useful fall job is a sweep, not a spray. Once the leaves are down, we clear the loose litter from valleys and roof edges and lift out any moss that has washed down into the gutter trough. Treatment in these months is mostly wasted. A roof that is wet for days at a time rinses it off before it can work, and we will not sell you something that will end up in the downspout.

December to February: watch it from the ground

Midwinter is the season for looking, not climbing. A shaded slope in January may not dry out for weeks, and wet shingles carrying a film of algae are slippery even in soft-soled shoes. Whatever your age or your ladder, a winter roof is not worth the risk.

What you can do is stand back on the lawn, or across the street, and look at the north-facing slope and the bottom edge of the roof. A faint green tinge along the bottoms of the shingle tabs is the first sign of growth. At that stage it is a film, and a film is cheap to deal with. Take a photo from the same spot each winter and you will have a record of how fast your roof greens up, which is the most useful thing you can send us.

March and April: the branch conversation

As the rain eases, we can get onto roofs again for clearing and checks, and that is the best time to talk about trees. Standing on the ridge, we can see which limb keeps which slope in shade, and where a cedar hedge has grown up past the gutter line. Tree cutting is not something we offer. We point out the specific branches, and you can take that list to an arborist.

Removing a single limb, or two, often does more for a roof than anything we put on it, because a slope that dries quickly after each shower is a hard place for moss. On a shaded acreage in Hilliers, where a workshop or barn may sit under trees all day, this is usually the biggest single change on the property.

Late spring to early fall: the treatment window

We book treatment for a dry stretch between late spring and early fall, because it only works on a cleared roof that is dry when we apply it and stays dry for several days afterwards. It is a liquid that finishes off spores and the faint film a brush leaves behind. Nothing visible happens on the day. Over the following weeks and months, the dead film browns, loosens and weathers away.

Before any treatment, we tarp or water down the garden beds and shrubs below the eaves, since whatever lands on a roof eventually comes off it through the downspouts. We apply in a controlled line along the courses and keep it to the roof, and we tell you which product we used.

The dry months are also when we fit zinc strips. A strip along the ridge releases a trace of zinc every time it rains, which makes the shingles just below it harder for new moss to settle on. The effect is strongest near the top and fades on the way down. On a plain gable roof with short slopes, that covers a fair share of the surface. Where the slopes run long, or dormers break them up, it only protects the upper part, and we tell you that before we fit one.

July and August also bring the most common misunderstanding we hear. Moss that has dried out looks brown and dead, and many people assume the problem has solved itself. It has not. It is waiting for the rain.