Roof Cleaning for Streaks, Lichen and Needle Build-Up

Ask a Nanaimo homeowner what is wrong with their roof and the answer is usually “moss”. Look closer and the moss is often only part of the picture. There are black stripes running down from the ridge, grey and orange crusts the size of a coin dotted over the shingles, and a drift of needles packed into the valley where two slopes meet. Each of those is its own problem, and none of them comes off the way a moss cushion does. Thick moss is covered on our roof moss removal page. This page is about the rest of the roof, and about washing it without doing harm.

Black streaks are not dirt

The dark vertical stripes on so many roofs around town are algae. They begin as a faint shadow near the ridge on the slope that dries slowest, then lengthen downhill each wet season as the runoff carries the growth along. From the street they make a ten-year-old roof look twenty.

Nanaimo gives algae a long run at it. From October to March the rain rarely lets up for long, and the roofs that sit in shade under the trees on the slopes of Mount Benson or along the bluffs above Departure Bay can stay damp for days after each storm. You may have noticed a clean band below a galvanized chimney flashing or a metal vent. Traces of zinc washing down from the metal keep that strip clear, and it is the same reason zinc strips are part of our treatment and prevention work.

Streaks are mostly a looks problem. They are also a sign that the slope holds water, which is the same condition moss needs, so a streaked roof is often a roof that will grow moss next.

Lichen holds on harder than moss

Lichen is a partnership between a fungus and an alga, and it grows slowly enough that most people only notice it once it is well established. It shows up as flat, crusty rings in pale green, grey or rusty orange, and it is common on older asphalt in Harewood and on cedar shake close to the water.

Moss sits on the surface and lifts away with a plastic scraper. Lichen does not. It grips the granules so tightly that pulling it off takes the top of the shingle with it, leaving a bare dark spot that ages faster than the rest of the roof. The method that works is patient: treat it, leave it, and let the weather finish the job.

Needles, cones and the dams they build

This is the Coastal Douglas-fir zone, and the trees overhead decide how much ends up on your roof. Douglas fir drops needles and cones, western red cedar sheds flat brown sprays, and bigleaf maple adds a heavy layer of leaves each fall. That material does not spread out evenly. It slides down until something stops it, and then it piles up.

The usual stopping points are the valleys, the uphill side of a chimney, the top edge of a skylight, and the gap behind a dormer. Once a pile forms it acts as a small dam. Water backs up behind it, the shingles under it never dry out, and moss and algae start in the shelter it gives them. We clear those piles by hand and with soft brooms before any washing starts, because a wash will not move a wet needle mat.

Why we wash softly

A pressure washer makes a roof look clean in an afternoon, and that is why it is tempting. On asphalt it strips the mineral granules that protect the shingle from sunlight. On cedar shake it raises and splits the fibres so the wood soaks up more water than before. Either way you are trading years of roof life for one good-looking summer.

A soft wash works the other way round. A cleaning solution goes on at low pressure, sits long enough to kill the algae and loosen the film, and is then rinsed off gently. The roof surface stays intact. Before anything is applied we wet down the beds and shrubs under the eaves, and we rinse them again afterwards. The steps for asphalt shingle roofs and cedar shake roofs differ in the details, and those pages explain how.