What are those black stains on the roof?
Those black stains are algae.
Algae appears as dark stains on shingles and usually cover more area on the lower section of the roof and taper as they reach the ridge.
Dark streaks are algae that live off the minerals in some types of roof shingles. Algae thrives in moisture, so it’s seen most often on shaded or north-facing roof slopes that don’t get a lot of direct sun.
How does a roof get algae?
Algae spreads from one infected roof to another via airborne spores.
These spores land randomly on a roof and take root.
A protective sheath creates the dark color the algae forms to protect itself from UV rays.
How does it grow?
Algae requires moisture and proper temperatures to grow. Dew is the primary moisture source.
Algae is found predominantly in the Southeast, but it can and will grow nationwide.
The algae generally grows on the north side of roofs since this exposure generally receives less sunlight. Algae are well adapted to extreme conditions.
What solutions do I have?
Install Algae Resistant Shingles such as GAF shingles which have a specially formulated granule that inhibits algae growth, keeping your roof looking new for years to come.
If a new roof is not an option, GAF recommends cleaning the roof with a special mixture. That mixture is 4 gallons (18.2 L) of water, 1 gallon (4.5 L) of bleach and 1 cup (236 ml) of TSP (Tri-Sodium Phosphate or acceptable phosphate-free substitute) found at many home centers. Apply this mixture with a garden sprayer, let it sit for up to 20 minutes and rinse with low-pressure water. Be sure to protect any shrubs below. Remember, the roof surface will become slippery when wet so be careful!
What not to do:
Do not power wash the shingles. Some roof cleaning companies offer this service. However, it is not recommended as it may dislodge granules which can lead to premature shingle failure. Composition roofing is a relatively soft material.
Any abrasive action removes the protective granular surface, breaks the bond between the shingles and damages the fabric of the material.
When pressure washing removes moss and debris from the roof, it also removes some of the roofing material.
What GAF shingles can I use?
GAF Shingles with StainGuard® Protection Available Nationwide:
All GAF Lifetime Designer shingles except Grand Sequoia®
Timberline® American Harvest®
Timberline Cool Series®
Timberline® ArmorShield II™