Slate Roof Cleaning is SH safe?

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.


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.


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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