Will this damage my shingles?

Not by the method the shingle makers' association specifies. The damage people worry about comes from pressure, which that association forbids on asphalt shingle; in its method the solution does the work and the rinse is low-pressure. If a roof is old enough that it is already shedding granules on its own, we will say so at the estimate.

Yes — sodium hypochlorite, diluted, with a surfactant. It is the chemistry that kills this organism.

Will it kill the plants under the roofline?

They get wet down before we start and rinsed again after. Tell us about anything you are particularly attached to before the day. If something is growing tight against the wall under a slope we are cleaning, it is worth moving it before the day rather than after.

How long before the streaks come back?

It depends on the roof rather than on the wash, and the honest answer is that we cannot give you a number that would be true of every house. Shade is the main driver: a slope that stays damp under trees goes again far sooner than one in full sun. Cutting back a limb that lies over the roof takes away the shade that keeps that part damp.

Roof cleaning (soft washing)

The dark streaks running down a Florida roof are not dirt and they are not stains. They are alive, they are why the roof looks older than it is, and getting rid of them without damaging the roof is a question of chemistry and patience rather than pressure.

The black streaks are a living thing

What you are looking at is Gloeocapsa magma, a cyanobacterium — closer to algae than to mold. It travels on the wind, lands on a roof, and takes hold wherever it stays damp long enough. It wants warmth and humidity, and it takes hold wherever moisture lingers on the surface. Where a roof stays damp longest — under an overhanging limb, on the plane that dries slowest — you also get moss, which is the one that actually damages a roof rather than just discolouring it. It also explains the streaking pattern: it spreads downhill with the water. Because it is an organism and not a deposit, the job is to kill it rather than to blast it off, and anything that only removes what is on the surface leaves the roof to go dark again from what is still living in it.

Why we will not pressure wash a shingle roof

This is the part worth reading twice. The coloured granules on an asphalt shingle are not decoration — they are the shingle's sunscreen, and the asphalt underneath them ages fast once they are gone. High pressure takes them off. You can watch them come down the valley and into the gutter, and the roof looks wonderful until the bare patches start to show as a different colour and the shingle in those places gives up early. Nothing puts them back. Pressure also drives water sideways, up under the laps the roof relies on to shed it, which is a leak you will not find until it rains hard. A cheaper wash that costs a roof several years of life is not a saving.

What soft washing actually involves

A cleaning solution is applied at low pressure, left to work, and rinsed off gently. The active ingredient is sodium hypochlorite — the same chemistry as household bleach — diluted and mixed with a surfactant. The dwell time is the part that matters and the part that cannot be rushed: the solution has to kill the organism, and a rinse that comes too early just moves it around. Plants and shrubs under the roofline get wet down before the work starts and rinsed again afterwards. Everything coming off the roof ends up in the gutters and downspouts.

Tile, metal and low-slope roofs are their own problem

Concrete and clay tile fails differently. Tile is strong where it is supported and brittle where it is not, so it is walked on the lowest part of the tile where it laps the one below and sits over the batten, and not in the middle of the span. Unglazed tile is porous, so growth sits deeper in the surface than it does on a shingle and needs longer to kill rather than more pressure. Metal panel is a different question again: the coating is what is being protected, and the wrong product on a painted panel will dull it. Low-slope commercial roofs hold water instead of shedding it, which changes both the risk and the method. Ask us to look at any of these in person before quoting.

Common questions

Will this damage my shingles?: Not by the method the shingle makers' association specifies. The damage people worry about comes from pressure, which that association forbids on asphalt shingle; in its method the solution does the work and the rinse is low-pressure. If a roof is old enough that it is already shedding granules on its own, we will say so at the estimate.
Do you use bleach?: Yes — sodium hypochlorite, diluted, with a surfactant. It is the chemistry that kills this organism.
Will it kill the plants under the roofline?: They get wet down before we start and rinsed again after. Tell us about anything you are particularly attached to before the day. If something is growing tight against the wall under a slope we are cleaning, it is worth moving it before the day rather than after.
How long before the streaks come back?: It depends on the roof rather than on the wash, and the honest answer is that we cannot give you a number that would be true of every house. Shade is the main driver: a slope that stays damp under trees goes again far sooner than one in full sun. Cutting back a limb that lies over the roof takes away the shade that keeps that part damp.

This service, city by city

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