Of the three conditions necessary for wood decay (water, oxygen, and a temperature between 35 and 100 degrees F), only one can be managed by the building design – water.
Overhangs, a home's guard against wood decay, are critical in protecting modern wood-based materials used in wood-frame housing. The critical factor of the overhang of course, is the width. Though it may seem like common sense, there are a number of variables that make overhang-width a bit more complicated depending on where you live.
Here is a summary to use on one- and two-story residential buildings utilizing overhangs and possibly offsetting floors in order to provide protection to the exterior building envelope (siding, trim, window and door frames) from rain water.
Equilibrium Moisture Content
When wood is manufactured into various building products and installed into buildings, the moisture escapes or is absorbed until an equilibrium moisture content (EMC) is reached between the wood and the surrounding atmosphere. The EMC of wood is dependent on the temperature and humidity of the surrounding environment. Typical EMCs for exterior framing in the U.S. range from 7% to 14%, depending upon location.