Keeping Mildew off Siding

House Doctor | Gary Dymski

October 30, 2003

Q. We've had aluminum siding on our house for about 30 years. A few years or so ago, on the north side, green mildew appeared on the siding. We washed it off with a solution specifically made to clean aluminum siding, and it did an excellent job. Now, we must do it again.

After we clean it this time, is there a product we can apply that will keep the mildew from coming back? - Mrs. Walter Kranz, Bethpage

A. First, you don't really need a specially made product to remove the mildew. I've yet to discover a mildew preventer, but from my experience, the best mildew remover is chlorine bleach. Try using one or two parts of bleach to four parts water. You can apply it with a small pump sprayer, then rinse it off with a hose. Just be careful about your eyes and skin; wearing googles and latex gloves is wise.

To protect lawn and shrubs, spray them with water before applying the bleach solution. The water will keep the solution from sticking to the lawn and shrubs, and you can give them a quick rinse after applying the solution. If you have sensitive plants, you might want to cover them with plastic.

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I would leave the bleach solution on the siding for about a half hour or so. Then, before rinsing, I would apply a trisodium phosphate substitute (TSP solution) or a nonphosphate cleaner to the siding. Use a hose-end sprayer to apply the TSP or nonphosphate cleaner, then rinse with a hose. If the mildew does not disappear, repeat the process.

Mildew could be returning every few years because the siding is not getting enough sunlight. Trimming tree branches, shrubs, bushes and plants could provide enough sunlight to properly dry the siding after moisture hits.

Another reason might be that the siding is getting old. Generally, the lifespan of most siding is 20 to 30 years, and by then, you've gotten your money's worth. Siding also should be ventilated properly to allow moisture to escape. The appearance of mildew every few years could be a warning sign that your siding should be replaced.