Build Green: Wood Can Last for Centuries by Carol Clausen and Sam Glass tells about how rain seepage can cause wood decay. Decay frequently occurs in joints, where boards or beams are joined together, while the rest of the structure remains sound. One reason is that water gets trapped in the joints. Another reason is that the ends of the boards or beams absorb water much more rapidly than the sides do. That is because of the arrangement of the wood cells. Just as these cells conduct water up the stem of the living tree, they similarly conduct water lengthwise in boards or timbers cut from the tree stems. Each of the many thousands of cells that are exposed at the cut ends of boards behave like a soda straw. Water moves more quickly through the length of each cell than it does through the thick cell walls. So, water is absorbed rapidly from the ends of a cut board, but water is absorbed more slowly through the sides of the board.
In thin wood materials such as siding or fascia boards at the edges of roofs, water is absorbed primarily through the ends. In larger beams or timbers, water is absorbed in two ways: through the ends and through checks or cracks that occur naturally in the upper surface as wood weathers. Water collects in the checks during each rain, swelling the wood, which in turn closes the checks and hinders drying. Similarly, exposed beams comprised of several layers of smaller timbers nailed together may be wetted by water that gets trapped between each layer. Trapped moisture dries slowly and contributes to a decay-hazard situation. Large, load-bearing timbers that are exposed to weather should be preservative-treated. Additionally, their top surfaces should be flashed to prevent water from entering through checks.
Preventing absorption of water at the ends of solid wood siding and entry of water around window and door openings is most important in high-decay hazard zones, especially where storms come predominantly from one direction. Butting siding against vertical trim provides maximum opportunity for rain seepage into the siding. Capping or sealing the ends of wood siding provides excellent protection against end-grain absorption. When siding is being replaced, a thorough building inspection for hidden water damage and decay is possible. Re-siding provides an opportunity to replace faulty window flashings and to install kick-out flashings and building wrap. These are all techniques that the homeowner or your contractor can easily apply.