Everyone knows that salts cause corrosion (ask anyone who lives where they salt roads in the Winter), but it is often not appreciated what a huge bearing this has on the performance of roof coatings.1. Uniform Atmospheric Corrosion
The corrosion rate of the metal in a roof depends directly on the concentration of salts such as sulfates and chlorides. This is because anions in the salts catalyze the oxidation of iron. So how long your roof coating will last depends, among other things, on how much salt (from acid rain and other pollutants) your roof is exposed to.
2. Salty Coatings?
Many roof coatings that meet Title 24 "Cool Roof" requirements are made of acrylic latex emulsions in water. These emulsions can contain salts in the formulation. Putting on one of these "government approved" coatings is like adding pollutants yourself to your own roof.
Recommendations:
1. Perform a roof inspection.
There are many "do it yourself" forms available. Checking for corrosion is always one of the tick boxes. Uniform atmospheric corrosion looks like whole panels or sets of metal fasteners corroding at the same time.
Galvanized metal corrodes white first (zinc oxide), then red (ferric oxide).
Roof coatings should chalk first (oxidized polymer) before failing.
If you see what looks like orange blooms in an otherwise good looking coating, chances are you have corrosion from salts in the formulation.
2. Test products first.
Even good roof coatings will perform differently in different situations. Ask for a test patch and see how the product performs on your roof before coating the entire roof.
