Why roof moss is a problem, not just a look
Moss grows in the gaps between shingles and slowly wedges them up. Once a shingle edge lifts, wind-driven rain can get underneath it. A thick mat also holds water against the roof for days after a storm, which wears shingles out faster and can lead to rot in the roof deck. The clumps that break off end up in your gutters.
It grows fastest on:
- North-facing roof slopes that rarely get direct sun
- Sections shaded by firs, cedars or the house next door
- Valleys and spots where needles and leaves collect
- Low-slope roofs that drain slowly
How roof moss is removed
- Inspection. Checking the roof material, its condition and how heavy the growth is.
- Physical removal. Thick moss is brushed or scraped off by hand, working down the slope so shingle edges aren't pried up.
- Treatment. A moss and algae treatment is applied to kill what's left. Some products keep working for weeks as rain carries them down the roof.
- Cleanup. Debris is cleared from the roof and the gutters, since that's where the moss ends up.
- Prevention (optional). Zinc or copper strips near the ridge, or a scheduled preventive treatment, to slow regrowth.
Why high pressure is the wrong tool for roofs
Blasting an asphalt roof with a pressure washer strips the protective granules, can tear shingles and drives water underneath them. Shingle manufacturers generally warn against it. Cedar shake and tile roofs need their own gentle approach too. A careful provider uses hand removal and low-pressure soft washing, and will tell you if a roof is too fragile to walk on.
Notes by roof type
- Asphalt composition shingles: the most common roof in the area. Hand removal plus a soft-wash treatment.
- Cedar shakes: damage easily. Usually cleaned at low pressure and treated, sometimes followed by a preservative.
- Concrete or clay tile: can be slick and brittle underfoot. Ask how the provider plans to access it.
- Metal: moss is less common, but algae and lichen still grow in the shade.
How often to remove roof moss in Seattle
Most roofs here benefit from moss removal every few years, with a lighter preventive treatment in between, and more often under heavy tree cover. Fall and early spring are popular because moss is moist and comes off more easily, but treatments need a few dry hours to stick, so providers schedule around the forecast.
What affects the cost of roof cleaning
- Roof size and pitch; steep roofs need harnesses and roof anchors
- Roofing material: asphalt, cedar shake, tile or metal
- How thick the moss is and how long it's been growing
- Number of stories and how easy it is to get onto the roof
- Whether gutter cleaning and debris haul-away are included
- Prevention add-ons like zinc strips or follow-up treatments