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If the weathered effect is not to your liking, you may be able to do something about it.
If the surface is polluted from particles in the air over time, you may be able to power-wash the roof. You must be careful not to use enough force to break the slate. Some old slates are too soft for power-washing.
If the weathering is from lichen or some other biological growth, you can spray the roof with bleach, then rinse with a hose. If left unrinsed too long, the bleach may alter the color of the slate. Also, as you rinse the bleach from the roof, it will collect in the gutters and run down to the ground where it may harm flowerbeds or your lawn.
All in all, you should choose a contractor who has experience in cleaning slate roofs. Ask for references and photos of work he's done.
Good luck! www.durableslate.com