Though it has been overtaken in popularity by other roof coverings, like wood and asphalt shingles, natural slate is still considered a high-quality roofing material. Natural slate is considerably more expensive to install than other types of roofing, but, once installed, a slate roof can last for 150 years. Like other roof coverings, natural slate can develop issues though, such as the growth of moss. If moss develops on a slate roof, it can be cleaned in a similar fashion to other roofs.
Remove any items from the roof and the gutters that help contribute to moss. Any organic matter, such as leaves, limbs or other plant debris, can hold moisture and cause moss to form on the roof.
Spray the moss with the highest stream on a standard garden hose. Aim downward toward the moss, so that the water rolls down the roof in a natural fashion. If the moss does not budge with the stream of a standard hose, you can also rent a power washer from a home improvement store and spray the roof in the same fashion.
Brush the moss from the natural slate. Ask the Builder recommends attaching a scrub brush to a pole. Stand on a ladder just below the area of the roof where the moss has formed and pull the brush downward against the slate to pull the moss free.
Do not step on a slate roof, if possible. Use a ladder to get to a level above the surface of the roof or use scaffolding to get to the centre of a roof. Good slate won't crack if you walk on it, but the more pressure you put on a slate roof, the sooner it will start to fail. If the slate roof has any problem areas, such as broken or missing tiles, replace the tiles before cleaning the roof to prevent issues that may occur from water getting under the tiles. Avoid using any type of chemical on natural slate. This includes oxygen bleach, which is sometimes recommended for cleaning moss.