Expert: Dan Griffin - 7/15/2011

Question
This was originally a question I asked of Bill C, who is now on holiday. The concrete tiles on my roof are about 30 years old and seem to be in good condition I forgot to say that I had cleaned them thoroughly and removed all the moss, and there was plenty. Bill said that I should leave them alone to do the job that they were designed to do. Sealing them with the dirt and moss on would not have been very effective and a waste of time and effort. This I agree with. However, as they are now pristine, does this advice still hold good?

Answer
Angus, the tile served well for 30 years, but it is always a good idea to keep concrete from absorbing water which can lead to freeze/thaw problems.  The original waterproofing in the tiles is usually mixed into the concrete, not surface applied.  There are a number of silanes and siloxanes that would seal the concrete tiles without changing the color.  They are often used for masonry and concrete walls that are prone to absorbing water.  I suspect that your oringinal tiles were treated with some form of sealer, either in the design mix or applied .  The problem becomes chemistry.  If the original sealer isn't compatible there may be a reaction.

The very best answer would be to contact the roof tile manufacturer for their recommedation.  Even if you do not know the exact manufacturer, any manufacturer should be able to tell you about industry standards.  I hesitate to give you brand specific information as I do not know what might be available on the other side of the pond...  The normal application of silanes/siloxanes would be to use a pump sprayer or airless.