Covered Bridge

amounts of moisture. Usually we dis-courage decay either by treating the wood with chemicals that make it unfit for food, by selecting a wood species that is unappetizing to the microorganisms, or by keeping wood completely dry. Paradoxically, wood that is completely and permanently submerged in water will not decay. This is because water does not allow the organisms in the wood access to air. Thus, we sometimes use untreated wood for foundation pilings in soils that are completely saturated with water. This works well as long as the water level in the soil does not drop. In cases where the water level has fallen, piles have rotted off at the level where the saturated soil joins the drier soil above, the one location where air, water, and wood are all available to the decav organisms.

4. All wood-destroying insects require water. Many species can live only in

Mia 6"

Dec^y-resistant wood sill

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