Our wonderful Pacific Northwest climate gives us lots of green foliage, but unfortunately that foliage can include an awful lot of moss on your roof. Letting moss grow uncontrolled on your roof will damage the roof and lessen its life, not to mention that it looks bad. Unfortunately, many home owners end up doing more damage when they hire someone to clean the moss off of the roof.
Roof cleaning services are common, and many of them involve a dude with a ladder and pressure washer. He goes up on the roof and pressure washes it for an hour or two and it looks great. Problem is, pressure washers are too strong for roofing materials and you end up washing away loads of roof granules at the same time. Roof granules protect the asphalt under layer of the shingles, and when they are worn away, it is an indication that the roof is nearing a leak and potentially in need of replacement.
I cannot tell you how many home inspections we have been apart of for home buyers when the inspector comes down off his ladder and says “The roof is damaged from excessive pressure washing. It’s only got a few years of life left.” Guess what, those buyers end up negotiating for a new roof. A strong pressure washing of a composition roof could easily take 5 years of life off of a roof, maybe more, in the matter of an hour. If you are planning to sell your home soon, this little cleanup job may have taken $5000 or more out of your pocket if the buyer starts requesting concessions based on roof condition.
If you need to have your roof cleaned, hire someone that actually knows what they are doing and will be gentle with the roof. You should never use high-pressure water to clean a roof. Low-pressure air, water and hand-cleaning with brushes will get the same job done without the damage. There are also chemical treatments (usually zinc) which can kill the moss and prevent its return.
Replacing a roof is expensive. You need to keep moss off it, but make sure you use low-impact methods to keep it clean, otherwise you’ll end up costing yourself a bunch of money.