Cleaning a Slate Roof

new england slate roof with ladders

At New England Slate we are often asked, “what’s the best way to clean a
slate roof?” Well, that’s a tricky question. First it depends what you’re
trying to clean off of your slate roof. And second it depends how much time
and effort you want to put in.The photo above shows a slate roof during a cleaning. The slates are 9 years
old and had a build up of moss & algae compounded by tree sap. Our
customer used a basic solution of simple green dish soap & water. He chose
an “eco” product purposefully because of all the run-off from the roof onto
the landscaping. Our customer told me, “I’ve tried all the specialty products
over the years and have little to no evidence they work any better.” We’ve
also heard that chlorine, bleach or copper solutions don’t seem to have any

The goal when cleaning a slate roof is to soften and loosen the undesirable
buildup. The trick to doing this successfully is applying the solution with a
garden type sprayer and letting it sit and reapply and reapply and reapply.
The more applications and sitting time the better. You want to keep it wet
once you’ve begun.

With moss and algae type growth, a soft brush, a hose sprayer and a lot of
“elbow grease” will get it off. With more severe environmental staining
from tree sap and/or pollution, you may need a pressure washer (3800psi).
We’ve heard that a 24 ft extension pole and a 12.5 inch wall washer can
work well when attached to a pressure washer. This wall/roof washing
attachment has a brush edge and two rotating nozzles. The unit can be
handled to “float” over the slate. BUT BE CAUTIOUS – Do not apply too
strong a spray and do not use the nozzles too aggressively.
Also, it is imperative that you get above the roof (ideally on a man lift) and
direct the spray downwards. You do not want to be spraying the water up
and under the courses of slate. Also, go inside the home frequently to
check for leaks. If you can’t get a lift, it’s ladders and chicken ladders.
Remember—do not walk on slate.

How much time and effort should you allow? Well, the photo above
is a 3 car garage with a 12:12 pitch. It took two workers, two long days to
complete. According to our customer, he quotes this type of work, time &
material only. It’s too difficult to estimate b/c the work is very variable.
We hope you’ve found this article interesting and helpful.

Please write to with questions or stories of
your own.

Thank you for reading!

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