If you drive through almost any Jacksonville neighborhood, you will see roofs with dark stripes running down from the ridge. A lot of homeowners think it is dirt, mold from a leak, or just an old roof. In most cases it is algae.
It is algae, and it is common here
The streaks are usually a type of blue-green algae called Gloeocapsa magma. It spreads in warm, humid places, which describes Northeast Florida most of the year. It feeds on moisture and on the limestone filler used in many asphalt shingles, and it shows up on tile and metal roofs too.
It tends to start on the north and west sides of the roof, where the surface stays damp longer, then spreads down the slope as rain washes the spores along. That is why it looks like streaks instead of patches.
Is it hurting the roof?
It is mostly a looks problem at first, but it does not stay that way. The dark color soaks up more heat. Left for years, the algae can make room for moss and lichen, which hold water against the roof. Some roofing manufacturers also give instructions on how their roofs should be cleaned, so it is worth checking yours.
Why you should never pressure wash a roof
High pressure takes the streaks off, but it also takes the protective granules off shingles and can crack or loosen tile. It forces water up under the edges. It also does not kill the algae at the root, so the streaks come back sooner.
If someone shows up to clean your roof with a surface cleaner or a pressure wand pointed at the shingles, that is a red flag.
How soft washing works
Soft washing uses low pressure, about what a garden hose puts out, to apply a cleaning solution made for roofs. The solution sits long enough to kill the algae, then it is rinsed. The streaks fade during the cleaning and keep lightening over the following days as rain rinses off what is left.
We cover the plants around the house before we start and rinse them after, and we keep an eye on the gutters and downspouts so runoff goes where it should.
How long does a clean roof stay clean?
It varies with shade, nearby trees and which side of the roof faces north. A soft wash kills the growth instead of just knocking it loose, so it lasts much longer than pressure washing would. Most homeowners can see when it is time again from the street.
What you can do between cleanings
- Trim branches that hang over the roof so it gets more sun and dries faster
- Keep gutters clear so water drains off instead of sitting
- Ask about the cleaning method before you hire anyone
Not sure what is on your roof?
Take a photo from the street and send it to us. We will tell you whether it is algae, and what it would take to clean it. You can read more about how we do it on our roof cleaning page.
Want a price for your property? Send a few photos and we will reply, usually within 24 hours.
