Regular cleaning is an important part of keeping a slate roof in working order. Debris and moss buildup hold moisture in the slates that can split them during winter freezes. While you might be tempted to reach for a strong chemical or a pressure washer, it's better to leave them behind and give your slate roof a little tender loving care.
Rock Solid Roofing
Slate is a sedimentary rock with miniscule layers that easily cleave. This quality is what distinguishes slate as a suitable roofing material. Once turned into roofing material, slate absorbs water and can naturally de-laminate during the freeze-and-thaw cycles of winter. Harder slates, generally more expensive, are less absorbent and fare better through many decades. Softer slates absorb more water and easily de-laminate.
Because a slate roof should last a lifetime, flashing, fasteners and accessory materials are often the highest quality so they don't fail before the slates need replacing. A quality slate installation in North America usually features heavy-gauge copper components. Copper will corrode when exposed to incompatible chemicals, such as ammonia or chlorine bleach. While cleaning agents won't usually gouge holes in your copper gutters, they can turn them prematurely black or mar a lovely, aged verdigris patina.
Up On The Rooftop
You can't walk on a slate roof, no matter how shallow its pitch. Slate shingles do not lie flat against each other and can easily snap under pressure. Often, the damage is hidden under overlapping slate courses and becomes apparent only when the roof starts to leak. Also, slate is slippery when wet. The best access for most homeowners comes in positioning a long extension ladder from the ground to the roof at the same pitch as the roof. The upper part of the ladder will lie flat against the roof and distribute your weight among many shingles.
Getting to Work
Limit cleaning to removing organic debris and delaminated slate flakes from valleys, snow guards, gutters and the area behind the chimney. Use heavy rubber gloves to scoop up material in areas you can safely reach. A vinyl leaf rake can clean harder-to-reach spots. If you have moss, mold or rust spots that are reachable, gently scrub them with a medium-stiff brush and a solution of 1/4 cup general household cleaner to one gallon of water. Wash away soap residue and remaining bits with a gentle spray from the garden hose.
Easy Does It
Pressure washers can break apart slates nearing the end of their usefulness. They also drive water up and under slates and cause leaks. A strong blast from the garden hose will do the same damage. Don't lean your body outside of the ladder when reaching. Instead, descend the ladder and move it over a few feet so you can climb up again and get to work on the next section. Gray or gray-green streaks usually come from zinc or copper flashing and nails. They don't harm the stone. In fact, the chemicals that cause them are lethal to moss, mold and algae.