Are Algae and Moss Bad for Roofs?

Moss and algae are recurring problems on roofs, and can cause collateral damage if left unattended. However, once we understand how algae and moss work we can do something about them. We therefore begin this week’s post with a ‘botany lesson’.

About Algae and Moss Life Forms

Algae are useful organisms, because they produce much of the oxygen we breathe. However, they are neither plants nor animals, but a unique species that are a bit of both. Therefore they do not have roots or stems that could work their way between asphalt sheets.

Mosses, on the other hand are simple plants with tiny leaves attached to stems. They convert nitrogen oxides and other urban pollutants to oxygen. However, they don’t have proper roots. They use threadlike protrusions to anchor to surfaces. These can cause open edges of asphalt shingles to lift and curl.

Algae and moss both disrupt water flow as they spread over a roof. This can cause moisture runoff to dam, and spread across before entering the roof space.

However, they can cause more than damage. They can affect the value of a home if left unchecked. That’s because they present as unsightly stains, and can create the impression that a perfectly good roof on its way out.

How the Two Organisms Propagate

Algae organisms colonise damp spots on roofs, particularly in coastal and humid climates. They generate a dark-coloured cover over them to ward off harmful ultraviolet rays after a few months and this is what we see.

Mosses, on the other hand travel in wind as tiny spores like seeds. Animals can also coincidentally spread them between roofs which is why moss tends to infect entire neighbourhoods.

How to Deal with Algae and Moss on Roofs


Trim back over-hanging branches to admit more sunlight and reduce packing of dead leaves. Clear accumulations away regularly with a leaf blower, taking care not to block eaves troughs and down pipes.


Spray affected areas with a 50:50 dilution of liquid chlorine laundry bleach, and water. Rinse away after twenty minutes under low pressure while still moist. The algae will gradually vanish. However dead moss must be blown away after it loosens.

Take precautions when working with the liquid chlorine laundry bleach dilution. Avoid rinsing away under high pressure because this could cause protective shingle granule loss. Get free advice from Valiant Exteriors. Think twice before climbing onto a roof.


Shingles Blowing Off the Roof

Image: Moss Spreading on a Shingle Roof: Roland Tanglao BY Public Domain