I am bidding a slate roof cleaning for a customer that I have done a few other jobs for. She will likely accept the estimate. Before I send the estimate I just wanted to know the best way to tackle a slate roof. I have read some other threads that suggest using SH with no surfactant, but I have also heard not to use SH on slate roofs. Anyone on here have experience using SH on slate roofs? The roof isn’t that dirty, mainly just algae streaks on it and a few small spots of moss. I have done a bunch of shingle and metal roofs but never a slate roof. Any help is appreciated.
SH with little surfactant. Because not much surfactant, you use more SH. How steep is it? Just make sure you budget for enough SH and watch your plant protection because you’ll get a lot of runoff.
They are EXTREMELY slick when treated. Make metal roofs look like sticky tape.
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Thanks for the reply. The roof is pretty steep so there will likely be a fair amount of runoff without surfactant. There are gutters on the house so that helps. Do you typically rinse a slate roof or leave it on there?
Your choice, depending on how bad it is> They’re pretty nasty looking with dead stuff on them, so I usually rinse. Plus depending on where slate came from, don’t necessarily want to leave SH on for a long time, talking couple of days.
Sorry to interject, but from what I’ve seen over the years a lot of the nice houses with slate roofs may have copper gutters as well.
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OK i will rinse to be safe. Thanks for the help!
Definitely no copper gutters on this house but thanks for the heads up!
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I’ve done a handful of slate roofs, and honestly, the main rule is don’t treat it like shingles. Slate’s tough as rock (because it is), but it’s brittle, step wrong or hit it with too much pressure and you’re replacing tiles.
You can use SH on slate, but go lighter than you would on asphalt. I usually run 3–4% mix, no surfactant or just a splash if you really need it. Slate rinses easy, and too much surfactant just leaves it slippery and soapy.
Apply it low-pressure, think garden-hose strength, not pressure washer strength and let the chemical do the work. Most of the algae will disappear right there. Moss? Don’t go after it aggressively. Treat it, let it die, and the weather will take it off in a couple of weeks.
If you can, work from a ladder or scaffold instead of walking the roof. And if there’s copper or zinc flashing, rinse those areas well afterward so the SH doesn’t mess with the metal.
If the roof’s only got light streaking like you say, a mild mix, even coverage, and a good rinse will have it looking fresh without risking damage.
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