Preventing mold and mildew growth on composite decking

Thursday, October 29, 2009 by Chris Fox

Mold and mildew can be a nuisance on any exterior building surface, regardless of the material. If the conditions are right, they will spawn on wood, plastic, concrete, metal and other surfaces. Mold formation is most prevalent in consistently wet, shaded areas. Spores from the natural environment are carried by the wind and commonly land on decks surfaces. It is important to note that the appearance of mold and mildew is a function of nature, not necessarily a deficiency with any of the material on which it grows.

Deck Cleaning
Periodic washing with soap/detergent and water will help remove surface dirt. This will also help prevent the buildup of pollen, debris and spores that can cause and accelerate mold/mildew growth (it also helps to remove deck mold). Caution: A pressure washer should not be used to “blast” mold/mildew or soils from a deck surface. The abrasive nature of the water stream can potentially damage the material by driving the spores into the material, which may create a moreMold on composite deck before/after cleaningchallenging problem to remediate. A pressure washer with a fan-tipped nozzle should be used only to lightly wet or rinse wood or composite deck surfaces. 

There are many deck wash and exterior cleaning products available at retail. It is important to make sure you use a cleaner specifically intended for your application. After selecting a product, be certain to read, understand and follow all instructions supplied by the manufacturer. Some cleaning products and inhibitors may be more effective than others, depending on the environmental conditions your deck is subjected to. Additionally, it is always a good idea to test the cleaner in a small, inconspicuous area prior to applying it to the entire deck.

Mold Inhibitors
As with deck washes, there are several mold-inhibitor products available from paint stores, hardware stores, online outlets and home centers to help prevent long-term mold/mildew growth. For any product selected, be certain to read, understand and follow all instructions provided by the manufacturer. Depending upon the environmental factors affecting your composite lumber or treated wood deck, some preventive cleaning products may be more effective than others. It may be necessary to try more than one product. For ongoing preventive maintenance, follow the manufacturer’s recommendation.

Comments for Preventing mold and mildew growth on composite decking

Wednesday, June 23, 2010 by D:
I'm re-building our patio picket fencing (cement base/floor) but am getting black spots overnight on a lot of the wood. I'm using my 15 year old cedar pickets (power sanded down to good new wood) + New cedar horizontal rails. All wood has been treated with Thompson's Water Seal and allowed to dry/sun cure for several weeks. The pickets have been yearly re-sealed for at least 13-15 years. Pickets look new after power sanding down to "new wood". One day after re-installing the fencing, most (old and new) cedar was covered with Black Dots. It seems to be growing each day (rain or shine) since then. Do you have any advise to help me out here? Is this Mold, Mildew, or what messing up the project? ANY help you have to offer will be greatly appreciated. Thank you dan The Thompson's Water Seal is from last year but been stored in the attic over winter. Can the Water Seal go bad with and be carrying some exotic fungus/mold/mildew?
Wednesday, June 23, 2010 by Chris Fox:
Dan, every indication would be that the black spots are a fungus growing on the cedar wood. Most cedar is resistant to fungus, but fungus can grow on virtually any surface given the right conditions. There also are many airborne fungi that attack wood under certain conditions. Many of the airborne fungi, once in the wood cannot be removed, but applying chemicals that stop entry can prevent them. Most mold/mildew can be removed by apply a mold inhibitor with a pump up sprayer. After it dries you can apply a water repellent and UV protective coating. If the conditions that fostered the growth don't change the fungus will probably return. Contact the chemical manufacturer to determine if there is a shelf life on the sealant. I suggest testing a mold-cleaning product, on a scrap piece of your cedar or an inconspicuous spot on the fence. If the mold/mildew remover is effective then the entire fence will need to be addressed, either by a thorough cleaning with mold/mildew remover or treatment with a mold inhibitor product

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