Roof Cleaning: Pressure Washing, Soft Wash, or Leave It Alone?

If your roof has dark streaks running down it, you're not alone. That's Gloeocapsa magma, a type of blue-green algae that feeds on the limestone filler in asphalt shingles. It's incredibly common in the Midwest, especially on north-facing roof slopes that don't get as much direct sun. It looks bad, but here's the honest truth: it's mostly cosmetic. It's not eating through your roof. It's not going to cause a leak. But it does look terrible, and a lot of homeowners want it gone.

Pressure washing will damage your roof

This is the big one we want people to understand. A pressure washer puts out 2,000-3,000+ PSI of water. Your asphalt shingles are not designed to handle that kind of force. Pressure washing strips away the granules, those tiny pieces of crusite or ceramic that protect the asphalt from UV light. Once those granules are gone, you've basically fast-forwarded the aging of your roof by years. And here's the kicker. Most shingle manufacturers specifically state that pressure washing voids the warranty.

Soft washing is better, but not perfect

Soft washing uses low-pressure water combined with a chemical solution, usually sodium hypochlorite (basically pool bleach), to kill the algae. It's gentler on the shingles and it actually kills the organism instead of just blasting it off. If you're going to clean your roof, this is the method to use. That said, there are still risks. The chemicals can damage plants and landscaping below if not managed carefully, and whoever's up on the roof needs to know what they're doing to avoid overapplication.

Sometimes the best answer is to leave it alone. If your roof is within a few years of needing replacement anyway, spending $500+ on a cleaning doesn't make a lot of sense. And if the algae isn't severe, it's purely cosmetic, your roof is still doing its job. When it's time for a new roof, we can install shingles with algae-resistant granules (copper-infused) that slow the growth way down.

Not sure whether your roof needs cleaning, repair, or replacement? We'll take a look and give you the straight answer. Call us at (913) 954-4501.