Moss on a roof is a symptom. It grows where water stays long enough, in shade, on a surface that holds moisture. Removing it without changing those conditions means it comes back.
Why it takes hold
Moss needs moisture, shade and a rough surface to anchor to. North-facing slopes, areas under trees, and valleys where water lingers are the first to grow it. Aging shingles give it more texture to hold on to.
Algae is a different problem and appears as streaking rather than growth. It feeds on the limestone filler in the shingle and spreads where humidity is high.
What cleaning does, and does not, do
A proper soft wash removes the growth and the staining and stops the surface holding water against the roof. It also removes loose granules, which is why pressure washing an asphalt roof does more harm than good.
What cleaning cannot do is restore the shingle’s ability to shed water. Clean a dry, porous surface and it starts collecting moisture again in the same places.
Making the surface a bad host
A surface that repels water gives moss less to live on. That is the durable part of the answer: not just removing the growth, but changing how the shingle behaves when it rains.
Trimming back overhanging branches and clearing debris from valleys does the rest of the work, by getting light and air to the slope.
Cleaning, then protecting, in that order
Growth has to come off before anything is applied, or you seal it in place. On our assessments, cleaning and minor repairs are included only where your written estimate says so.
If the shingles are already brittle or the growth has lifted them, the assessment will say so rather than sell a treatment that cannot hold.
Source
Written for Dr. Armor. The original GoNano article this guide draws on: Measures to keep your roof moss-free
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.