How to Remove Moss from a Flat Roof

What You'll Need
Grout brush
Water brush
Garden hose
Garden sprayer
Plastic sheeting
Bleach and water
Copper sulphate powder mixed with water
Moss killer

Removing moss and algae from a flat roof can seem like a never-ending task. Most homeowners will have noticed that their flat roof is regularly invaded by the green pest, and while it does no damage to flat roofing materials, it can retain moisture, which will accelerate wear and tear on the flat roof. It also doesn’t look very nice.

Moss on Roofs

A flat roof of any kind of material can become a host to moss. While it does not directly damage the building, unlike termites, for example, it can shorten the life of wood, metal, asphalt, concrete or clay roofs, simply by encouraging water to stick around in its shallow root system. Moisture promotes wood rot and concrete decay, will erode asphalt in shingles, and can cause staining and buckling in a metal flat roof.

Moss is attracted to northern sides of buildings, where moisture stays for longer; they are also prone to grow near large trees, that produce shade, and morning dew, or rainfall which does not move away quickly will also attract moss.

Getting Rid of Moss

There are a couple of methods of removing moss; the first is to use a bleach and water combination. First knock away any large lumps of moss with a brush or broom. Put water over the roof and moss, and then put bleach onto the moss—this will turn the moss brown in minutes. After a few minutes, hose the area down with water, and check. Repeat until all the moss is gone. Use a water brush to sweep away excess water and dead moss.

Removing moss from a singled flat roof can be more complicated—shingles provide cracks and crevices where the moss can be protected. One method of removal is to mix copper sulphate powder with around a gallon of water, and add this to a garden sprayer. Protect any flowers or trees below the flat roof by covering them with plastic sheeting. Spray the moss, and let the mix sink in. Don’t allow the liquid to stay for too long on the roof, as it can corrode parts of the roof, including nails. Rinse off the mixture, and then sweep away with a water brush. The moss should turn brown in a few weeks—dislodging dead moss with a broom should not be difficult.

On a flat roof, another solution is to clean the moss away using a hard bristled brush, such as a grout brush. Metal or toughened plastic bristles are best. Spray the area with a moss-killer, which can be bought at local hardware stores, and then spray the moss with water and soap. Begin to scrub away at the roof with the brush—a hard scrub will pull up the moss at the roots, ensuring that it cannot regrow. Brush off the roof, and spray lightly with water to insure that all the moss has been removed. This is also a good time to check that the roof has not been damaged by moss and moisture build-up.