Why Is My Roof Staining & Growing Moss? (What PNW Homeowners Need to Know)
- Ryan Ihrig
- Oct 2
- 2 min read
If you’ve noticed dark streaks, green patches, or fuzzy growth on your roof, you’re not alone. In the Pacific Northwest, roof staining and moss growth are incredibly common.
But what causes it? And more importantly, is it dangerous?
Let’s break it down.
What’s Causing the Stains on My Roof?
Those black or brown streaks are likely algae (Gloeocapsa magma), a type of bacteria that thrives in moist, shaded areas. It feeds on the limestone filler in shingles.
The green, fuzzy stuff? That’s moss (we all know that from living here!), and it’s not just ugly. Moss absorbs and holds moisture like a sponge, causing premature roof aging and even structural damage if left untreated.
Why Is This So Common in the PNW?
Rain and humidity create the perfect environment for moss and algae.
Shaded roofs (especially on north-facing sides) dry slower.
Trees and debris drop organic matter that feeds growth.
Is Moss and Algae Really a Problem?
Yes indeed. Here’s why:
Moss can lift and curl shingles (more common than you think), leading to leaks.
Moisture retention causes rot and wood damage.
Algae shortens roof lifespan and ruins curb appeal.
How to Fix It (Without Damaging Your Roof)
The best solution is a soft wash, which is a low-pressure treatment that uses eco-safe cleaners to gently kill and remove moss, algae, and mildew without harming your roof.
Avoid high-pressure washing, it can blast away granules and shorten your roof’s life.
Prevention Tips
Trim overhanging branches.
Keep gutters clean.
Schedule annual roof inspections and cleaning.
Need Help? We’ve Got You!
At Pure Pressure Washing, we specialize in safe, effective soft washing for homes in Snohomish, Lake Stevens, and cities within King, Skagit, and Snohomish counties. If your roof looks green or grimy (like the pic above), let us help you bring it back to life, safely.
Reach out for a free estimate today!
Call or Text: 425-238-0104
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