Thinking like Water
How to outsmart the wood eaters. Most houses and small buildings are built from giant plants and other organic products. This is good since doing so means we're using renewable and potentially sustainable natural materials. The problem is that nature also knows how to break our houses down and recycle the nutrients. In every climate region there are critters large and small who have evolved for the sole purpose of chewing away at your tasty wood house. The long term lifespan of your house will depend to a large extent on how you deal with these critters. First principles: Each of these many organisms needs two things in order to settle in and get to work digesting your house - moisture and oxygen. Deny them either one and they die. For the most part, it is pretty hard to deny these guys oxygen. Short of shrink wrapping every piece of wood, the main place oxygen depletion stops wood rot is in wooden posts embedded deeper than 8" to 12" in packed soil (as in a pole or pier foundation). There you can have moisture without rot. The soil closer to the surface will have oxygen however, and there protection is vital. OK, now let's focus on the house above the foundation. Since we are building and living in an oxygen rich environment, how do we keep the surface of walls, windows and doors dry and therefore rot free? The first line of defense is rather obvious - stop the water that falls or splashes against it.
Gutters carry the roof water to a drain or drywell down slope from the house. Head flashing takes water running down or behind the siding out and over the housewrap or building paper and to the outside of the windows and doors. Window sills and a drip stop at the bottom of the siding completes the process of wall drainage.
The drawing above shows more about what is happening behind the siding where the building paper laps over the top of flashings.
This drawing demonstrates the concept of a vented rain screen behind the siding that allow moisture to escape and dry the siding from the back. For more detailed information on rain control see this link to the Building Science.com article on "Rain Control in Buildings". The diagrams above are from this online article and many excellent articles are available at the main site. http://tinyurl.com/2zoqrt For additional information on Flashings and water control see the September 2007 issue of the excellent Canadian BCBuilding.info online newsletter: http://tinyurl.com/2b45x8
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