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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.

Cross section drawing through a wall, from the footing up to the roof eave, with a window in the wall. Curved arrows show wind driven rain pushed against the wall and thrown clear by the roof overhang, by drips at the heads and sills of the openings, and above a splash height off the sloped ground, where a dimension is marked 200 min. Captioned Figure 3: Principles of Rain Deflection.
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 same wall in cross section, with the path water takes labelled: roof sloped to drains at the eave, head flashing over the window, sub-sill flashing under it, a drainage space over a drainage plane behind the siding, sloped grade at 5 per cent running away from the footing, and a dimension marked 200 min from the grade up to the bottom of the wall. Captioned Figure 8: Drainage and Flashing Concepts.
The drawing above shows more about what is happening behind the siding where the building paper laps over the top of flashings.
The same wall in cross section again, this time showing air movement. Labels read clear air spaces at the eave and behind the siding, vent holes at top and bottom of window, and vent holes at top and bottom of wall, with arrows running up the gap behind the siding. Captioned Figure 11: Ventilation Concepts.
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.

https://www.buildingscience.com/documents/digests/bsd-013-rain-control-in-buildings

For additional information on Flashings and water control see the September 2007 issue of the excellent Canadian BCBuilding.info online newsletter.

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