Cedar shake and shingle roofs are a defining feature of many homes across the Seattle Eastside — particularly in established neighborhoods in Bellevue, Kirkland, Sammamish, and Mercer Island. They're beautiful, naturally durable, and well-suited to the Pacific Northwest's aesthetic. They're also among the most demanding roof types to maintain in a wet climate.
This guide covers everything homeowners need to know about cleaning and maintaining cedar roofs in Washington State.
Why Cedar Roofs Are Especially Vulnerable in the Pacific Northwest
Cedar is a naturally rot-resistant wood, but that resistance has limits — and the Seattle area's climate tests those limits constantly. The combination of frequent rainfall, persistent cloud cover, and abundant tree canopy creates conditions where cedar roofs stay damp for extended periods, accelerating the biological growth and moisture damage that shortens cedar's lifespan.
The primary threats to cedar roofs in Washington are:
- Moss: The most damaging threat. Moss roots penetrate between shakes, retaining moisture and causing wood to soften and decay from beneath
- Algae and lichen: Surface growth that stains cedar and, in the case of lichen, bonds to the wood surface and causes erosion
- Debris accumulation: Leaves, pine needles, and organic matter that trap moisture against cedar surfaces
- Moisture retention: Shaded cedar that never fully dries between rain events is particularly susceptible to accelerated decay
The Right Cleaning Method for Cedar: Soft Washing Only
This is the most important thing to understand about cedar roof cleaning: high-pressure washing will damage cedar.
Pressure washing cedar shake or shingle roofs raises the wood grain, forces water deep into the wood, can split or crack individual shakes, and strips the natural oils that give cedar its weather resistance. A single improper pressure washing can cause more damage than years of moss growth.
The correct method for cedar roof cleaning is soft washing — low-pressure application of professional-grade biodegradable cleaning solutions that kill moss, algae, and lichen at the root without damaging the wood surface.
How Often Should Cedar Roofs Be Cleaned in Washington?
In the Seattle metro area, cedar roofs typically need professional cleaning every 1–2 years. Homes with significant tree cover — particularly those with overhanging Douglas firs or big-leaf maples — may need annual cleaning. The key indicator is moss visibility: if moss is visible on the cedar surface, cleaning is overdue.
Preventative moss inhibitor treatments applied after cleaning can extend the interval between cleanings by creating an environment that's hostile to moss regrowth.
Signs Your Cedar Roof Needs Immediate Attention
- Visible moss or green growth on cedar surfaces
- Dark staining or black streaks on shakes
- Shakes that appear cupped, cracked, or splitting
- Debris accumulation in valleys or along the ridge
- Gutters filling rapidly with organic debris
What Professional Cedar Roof Cleaning Involves
- Free roof assessment to evaluate cedar condition and the extent of biological growth
- Soft wash treatment with biodegradable solutions formulated for wood surfaces
- Low-pressure rinse that removes loosened growth without damaging cedar
- Debris clearing from valleys, ridges, and gutters
- Optional preventative moss inhibitor treatment
If your cedar roof hasn't been cleaned in the past two years, or if you're seeing any of the warning signs above, request a free assessment from Oasis Restores. We serve homeowners with cedar roofs throughout Bellevue, Sammamish, Kirkland, Mercer Island, Issaquah, Redmond, and the greater Seattle Eastside.