Roof moss is a plant that is happy to live in the nooks and crannies of a shingled roof. It can grow to be many inches thick, acting like a sponge to absorb moisture and transfer it to the roof system.
When the roof sheathing becomes saturated, water enters the wall system via the top plates. Here the siding and window trim have a sky-high moisture content.
Wall damage occurs inside as well, ruining insulation and drywall and causing mold growth that can be dangerous to occupants of the home.
The most effective way to remove moss is with a broom. Always avoid power washing a roof, which can cause more harm than good.