Decay in Exterior Millwork

Decay of exterior millwork often gives the impression that wood is an inferior material for this use. Wood will not decay if the moisture content is less than 20 percent, so by controlling the moisture, decay is controlled. If the moisture cannot be controlled, naturally resistant or preservative-treated wood should be used. While the heartwood of naturally resistant woods provides adequate protection against decay, sapwood of all species is very susceptible to decay. Sapwood is, however, easily treated with preservatives, so decay resistant exterior mill work can be manufactured from any species, even if it contains sapwood. The heartwood of cypress (Taxodium distichum), redcedar (Thuja plicata), and redwood (Sequoia sempervirens) is highly durable. The heartwood of longleaf (Pinus palustris), eastern white (Pinus strobus), and sugar pine (Pinus lambertiana) are somewhat more resistant than the heartwood of ponderosa (Pinus ponderosa), western white (Pinus monticola), red pine (Pinus resinosa), jack pine (Pinus banksiana), loblolly pine (Pinus taeda), and lodgepole pine (Pinus contorta). Western hemlock (Tsuga heterophylla), noble fir (Abies nobilis), white fir (Abies concolor), and Engelmann spruce (Picea engelmannii) have the lowest resistance to decay. Wood which is rewetted, if previously affected by decay, will decay more rapidly than uninfected wood. The colorless stage of several of the common wood molds penetrates deeply into the wood, increasing its ability to absorb water by 4 or 5 times. Such wood may have sufficient moisture present for decay to take place even though normal wood in the same location would not. It also absorbs preservatives much more rapidly, resulting in discoloration and failure of paint. Decay is more rapid in warm, moist regions. To prevent decay, wood that is already infected-should be avoided. Kiln-drying the wood green from the saw is the greatest protection against infected or too wet lumber. Use of construction designs that reduce rain seepage and that promote rapid drying are useful. Paint alone cannot be relied on to keep wood dry. One of the main reasons for this is that moisture usually enters at joints where the paint has cracked due to the swelling and shrinkage of wood. Pressure-treated woods are the best solution to exterior-treated millwork which will be exposed to high moisture conditions. Pressure-treating is the most uniform and lasting protection against decay, but a good method of treating millwork involves a brief dip, usually for about 3 minutes, in oil-preservative solutions. The incorporation of water repellents to the preservative solution appears to give added protection.

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