Preserving a cedar deck

      Best methods for treating and protecting wood decks from decay. 1998.

by Professor Gene Wengert

I recently purchased a house that has a cedar deck attached. What product(s) would be best for treating and preserving the wood? And how often do you suggest treating it?

The heartwood of cedar (the wood that is red in color) has natural decay resistance that is quite good. Certainly, if you can avoid continual or frequent contact with water, there will be little risk of decay over the next several decades. The sapwood (white colored wood) has no such protection, however.

Decay fungi (as well as mold, mildew, and stain fungi) have four essential requirements for growth--temperature between 70F and 100F (approximately), oxygen, moisture (over 22% MC or 95% RH), and food. The food is the wood itself for decay fungi. (Food is the sap for blue stain fungi and is dust, dirt, and microorganisms in the air for mold and mildew fungi). To control fungi of all types in or on wood, eliminate one of these four essential items. For your deck, can you perhaps treat the deck with a sealer (like the one you see on TV all the time) to eliminate much of the water--there is no such thing as dry rotting, by the way! (One concern about a water sealer--if there are puddles on the deck, when it freezes, they can form miniature skating rinks.) If all else fails, then the last resort is to poison the food (which is what cedar does naturally). There are a few poisons available at a paint/hardware store, but keep in mind that these "home" treatments are just skin deep--if the wood cracks, it may expose untreated wood.

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.

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