It's easy to forget about the condition of your roof since you don't really see much of it. But over time moss, algae, bird droppings and other contaminants can build up on a slate roof. Moss can tear through slate shingles with its roots and can promote excess dampness that rots the wood beneath your slate shingles. Clean slate roofs in hot weather so they dry fast--or at least on a clear day with plenty of sunlight.
Set up a ladder so you can access the roof.
Run a hose fitted with a spray nozzle up to the roof. You may need to attach an additional hose to your garden hose to enable it to stretch that far. Turn on the water. Climb the ladder with your bucket and scrub brush in hand.
Prepare a cleaning solution in a large bucket once you're securely on the roof. Thisoldhouse.com recommends a 1 to 1 mixture of bleach and water. Askthebuilder.com advises using oxygen bleach powder, which is non-toxic. Mix the powder with water according to the manufacturer's instructions.
Grab the hose and walk toward one edge of the roof. Spray a 3 foot wide strip of slate. Don't spray more than one small area at a time, this will avoid a slippery hazard. Dip your brush in the cleaning solution.
Push the brush across the slate, working downward. Don't brush against the direction of the slate since this can loosen or damage your slate tiles. Brush across the section of roof you rinsed to remove all moss, algae or stains.
Step back several feet and repeat the wetting and cleaning process. Brush until you've cleaned half your roof.
Walk to the far edge of the roof and begin brushing from that side. Work back toward the centre so you always have dry roof to stand on. By the time you've reached the centre, you should have plenty of dry slate to stand on to finish the job.
Carefully climb down the ladder with your cleaning supplies and turn off the water.
Be careful when climbing a ladder and when working on a roof. Cleaning a roof is dangerous work. An assistant might be necessary to help you climb, wet the slate and brush.
Very dirty roofs require a second cleaning.