Moss is a simple green plant that grows in mats or clumps, while algae is an organism with no root, stem or leaf. Moss and algae are commonly found on roofs and walkways in cool, damp climates.
Moss and Algae
Constant moisture causes moss and algae to grow on roofs, sidewalks, trees and shrubs. Though green and black algae may look like mould on a roof, it's not. Moss has roots and can cause more damage than algae on a roof.
Algae can be removed with a 50/50 spray of bleach and water. Or, use oxygen bleach, a powder you mix with water, which is less toxic and doesn't discolour the way chlorine bleach could. Green algae is easier to remove than black algae. Do not pressure wash shingles, and be sure to protect foundation plantings. Do not use table salt; it's corrosive and ineffective.
Moss on roofs can be swept off, brushed off or scraped off to remove the moss' shallow root system. Work downward to avoid damaging shingles. Remove as much moss as possible before treating it to cut down the amount of chemicals needed.
- Washington State University Extension: Miserable Muddy Moss in the Lawn -What to Do!
- Washington State University Extension: Moss Control in Lawns
- Oregon State University Extension Service Garden Hints: A more tolerant approach to moss, lichen and algae
- This Old House: Getting Algae and Moss Off the Roof
- Ask the Builder: Removing Moss and Algae From Roofs