Can I apply the repellent over the moss?

No. The product would seal the dirt in instead of reaching the shingle. Clean it, let it dry completely, then apply.

Does a water repellent kill moss?

No, it is not a moss killer. It shortens how long the surface stays wet, which makes conditions less favourable, but it does not destroy what is already there.

How long should it dry after cleaning?

Long enough that the material is dry through, not just on the surface. That depends on weather and exposure. Write to us for the exact window for your situation rather than relying on a general rule.

Roof moss and algae: what to do

Short answer

Clean first, protect second. Never the other way around. A water repellent applied over moss seals the dirt in instead of protecting the shingle. Moss holds water against the surface, and it is that retained moisture that works the shingle at the first freeze.

Why does moss damage a roof?

Moss acts like a sponge sitting on the shingle. It keeps water in contact with the surface long after the rain, where a clean roof dries.

That retained water does the damage, not the moss itself. Every freeze turns the water in the material into ice, and ice takes up more room. Repeated across dozens of cycles a winter, the mechanism fatigues the shingle and loosens granules.

What order should you work in?

Clean, let it dry completely, then apply. Those three steps are in that order and none of them gets skipped.

Applying a repellent to a surface still covered in moss, or still damp, is the main way to waste the product. The treatment has to reach the material to enter its pores. If it meets an organic layer instead, it works on the moss and not on the shingle.

Why does breathability matter so much here?

Because a roof that has just been cleaned has to keep drying. A product that blocks vapour from leaving traps moisture in the material, which leads to efflorescence and rot under a well-established building-science principle.

Liquid water repellency and vapour permeability are two independent properties. They are measured separately, the second by ASTM E96. A penetrating repellent can therefore shed water while letting vapour escape, which a sealed film does not do.

Will the moss come back?

It comes back where conditions favour it: high ambient humidity, shade, low sun. In the St. Lawrence valley and the Saguenay that is a recurring call, far more than in the Abitibi where the climate is cold and dry.

A water repellent does not sterilize the surface and is not a moss killer. What it changes is how long the surface stays wet after rain, and therefore the conditions moss needs.

Frequently asked questions

Can I apply the repellent over the moss?: No. The product would seal the dirt in instead of reaching the shingle. Clean it, let it dry completely, then apply.
Does a water repellent kill moss?: No, it is not a moss killer. It shortens how long the surface stays wet, which makes conditions less favourable, but it does not destroy what is already there.
How long should it dry after cleaning?: Long enough that the material is dry through, not just on the surface. That depends on weather and exposure. Write to us for the exact window for your situation rather than relying on a general rule.

Sources

These are the published sources behind the claims above. They describe the product CLASS (silane and siloxane repellents), not tests on Nanoxeal itself. We keep that distinction.

The science behind Nanoxeal →