Roof soft washing: algae, mix and rinse

The dark vertical streaking on an asphalt roof is not dirt and it is not mould in the usual sense. It is Gloeocapsa magma, a cyanobacterium that feeds on the limestone filler in modern shingles and shelters under a dark protective sheath. It spreads by air, which is why one house in a street starts it and the neighbours follow, and it runs downward in streaks because that is how the spores wash.

Because it is alive, removing it is not the goal. Killing it is. Anything that scrapes the surface takes granules with it and leaves the roof both uglier and shorter-lived.

That single sentence is why this job exists as a category. Every roof-cleaning method that works is a chemical method applied at garden-hose pressure, and every operator who has ever put a wand on shingles has cost that homeowner years of roof life whether or not it showed that afternoon.

ARMA also recommends the work be left to trained professionals, which is worth quoting to a homeowner who is weighing your price against a Saturday afternoon.

ARMA’s method is a solution of liquid household bleach — sodium hypochlorite — and water, gently dispersed on the surface, left 15 to 20 minutes, then rinsed gently with water. The association is explicit that mixing directions vary between shingle manufacturers, so check the maker before you mix.

The working figure this site publishes is roughly 3% sodium hypochlorite at the shingle, which from 12.5% stock is about one part to three parts water plus a surfactant to help it cling on a slope. Weaker and the dwell will not kill it; stronger buys you nothing and makes the landscaping problem worse.

1. Soak everything below. Beds, lawn, shrubs, and the downspout outlets. Then do it again halfway through.
2. Work from ladders where you can. ARMA’s guidance is to avoid walking the roof; a wet shingle roof is one of the more dangerous surfaces on a residential site.
3. Apply across the plane, top to bottom, keeping the whole area wet with mix rather than treating patches.
4. Dwell 15–20 minutes, watching that it does not dry.
5. Rinse gently, top down, at garden-hose pressure.
6. Rinse the landscaping a final time, generously.

Fall protection is not optional on a pitched roof and it is the thing that will be looked at hardest if anything goes wrong. Price it into the job rather than skipping it.

The streaks fade over days rather than instantly — the organism dies on contact but the dark sheath weathers off over the following weeks. Tell the customer that before you leave, or they will call on day two believing it did not work.

What soft washing does not do: replace lost granules, fix curling or cupping shingles, repair flashing, or extend a roof that is already at the end of its life. If the shingles are shedding grit into the gutters in quantity, the honest advice is that the roof needs a roofer, not a wash.