Q: I have a new home, and used a composite decking on all of our exterior decks, and porches. The brand is [a certain one]. The first spring we began to see black splotches all over the deck surface. The lumber supplier told us they were handling claims, and gave me a high-strength deck cleaner to use. I used the cleaner, and the decks looked great. But it didn't last. It seems I would have to clean the decks every other month to keep down the spotting. Is there anything happening with this manufacturer ? Thanks for your help.

A: Mold growth on composite lumber is not limited to a specific manufacturer or deck material in general. Composite lumber is a mixture of sawdust and plastic, often high-density polyethylene, from recycled sources. Like any wood product that is exposed to moisture, microbial growth is possible while the plastic components are likely to be hydrophobic and resist absorbing water.

Exterior building surfaces are going to collect pollen and other organic debris that, if the moisture conditions allow, will be conducive to microbial growth. Exterior surfaces that are well ventilated and exposed to sunlight (UV light) will not grow mold. This is true of siding and roof shingles as well as decks. Those surfaces that are shaded and not allowed to dry can support mold, algae or even moss in some cases.

Some manufacturers of composite lumber state in their literature that composite lumber used in decks can support microbial growth. Their recommended steps to prevent growth include; 1) frequently clean the deck of leaves and other debris, 2) treat the deck with a mold preventative treatment and 3) avoid chronic moisture sources such as gutters overflowing onto the deck, proper pitching and ventilation of the deck. Additional guidance can be found on the Internet by searching for “composite lumber.” One Internet site, www.correctdeck.com, may be particularly useful.

One final point is that mold growth on the exterior surface of a building does not pose the same health issues as mold growth within a building. The great outdoors has fungal growth everywhere -- in the soil, on the dead trees and decaying leaves. Airborne fungal levels outside are expected to be much higher than those inside buildings except when snow and ice cover the ground. Concern about damages mold growth can cause to a deck esthetically or to the deck’s long-term structural integrity is reasonable, but it is unlikely to pose a health impact any different than the outdoor air itself. Outdoor mold growth is different from microbial growth within a building, which can seriously impact air quality.

Thanks to Mark Goldman, who is a senior air quality scientist for Engineering and Fire Investigations (EFI), a firm that provides mold investigation and indoor air quality services nationwide. Mark can be reached at mark_goldman@efiglobal.com

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