Moss and Algae

Q. I live in Olympia, Washington, where moss tends to grow on roofs. The moss growth on my cement tile roof had reached an unacceptable level. I had the roof pressure washed. I also had it treated with zinc. I have heard that after a pressure wash, there is some sort of sealant or treatment I need to apply to prevent water leaks through the tiles. Is that true? If so, what treatment is required?

A. It is not necessary to seal the tiles after washing although sometimes a sealer coat will help slow down the regrowth of moss. While the sealer may slightly reduce water absorption, it is the tile density that prevents water passage and the method of installation that prevent leaks.

Q. We live in an area where moss on roofs is prevalent. How does your roofing material hold up under wet, damp conditions and what precautions need to be taken to prevent moss build-up?

A. Roof tiles hold up very well in the type of climate that you described but, unfortunately, we can offer no magic solution for avoiding the growth of moss on the roof. The good news is that there is no indication that moss or algae do any harm to the concrete tiles themselves. Small amounts of moss on a roof are not a problem but, left unchecked, moss will continue to grow and may eventually block and divert water flow, thus diminishing the water-shedding capacity of the roof. Heavy moss accumulation can also reduce the flow of air between the tiles and prevent them form drying out between storms. Periodic cleaning of the roof can prevent this and the use of biocides or zinc strips may be incorporated to slow the growth of these organisms.