Weathering is not only sun and rain. A large part of what ages a roof, a driveway or a wall is biological: algae, moss, lichen and the roots and moisture that come with them.
What biological weathering is
Living growth changes a surface mechanically and chemically. Roots work into small cracks, growth holds water against the material, and the organisms themselves produce mildly acidic compounds as they live and decay.
On a roof this shows up as streaking and moss. On concrete it shows up as green film, dark patches and, eventually, spalling where roots have opened the surface.
Algae: the streaking
The dark stains running down a roof from the ridge are usually algae feeding on limestone filler in the shingle. They are most common on north-facing slopes and in humid climates.
Algae does not usually destroy a shingle on its own, but it signals that the surface is staying damp, and damp is what everything else needs.
Moss and lichen: the water holders
Moss holds water like a sponge and keeps the surface beneath it wet for far longer than rain alone would. Lichen anchors itself into the material and is harder to remove without damage.
Both lift shingle edges and block the drainage paths a roof relies on.
Breaking the cycle
Cleaning removes what is growing. Water repellency removes the reason it grew. Shade and debris control remove the rest of the invitation.
That is why we clean before treating, and why a treatment on a surface still covered in growth is a waste of money.
Source
Written for Dr. Armor. The original GoNano article this guide draws on: Biological weathering: the hidden threat to asphalt roofs
Questions
Common questions, answered in full
- How long does roof rejuvenation last?Coverage runs from 3 to 20 years, and the roof’s condition score decides which.
- Can you treat a roof that is leaking?No. A leak is a repair job and has to be fixed before any treatment.
- What does the warranty cover?Peeling, curling or warping caused by the treatment, and water getting through.