Wood trim requires diligent maintenance

Inspector's Eye

Cracked paint can lead to moisture, rotting

September 01, 2002

Houses built in the past 30 years tend to be low maintenance compared with older ones. One reason is that the exterior siding is likely to be a type that won't rot and might not need painting. Siding is often vinyl or aluminum, less commonly brick or synthetic stucco, and, increasingly, fiber-cement siding. If wood siding is used, it is usually rot-resistant cedar that can be stained, with less work than painting.

But wood is still a popular material for exterior trim such as by windows, doors and corner boards -- and it is anything but low maintenance. Wood trim normally requires repainting every three years or so to protect it and keep it looking good. Most homeowners have trouble doing this as often as they should. The result is that by the time the painters are called, the trim has been without a good coat of paint for a long time.

The problem with this is not so much that the paint begins to look haggard, but that unpainted wood trim tends to rot. Long before the paint starts to look bad, it will have cracks over joints in the trim, allowing water to penetrate the protective paint layer and saturate the wood. In new houses, this process is particularly prevalent. New wood trim and wood frame walls undergo a drying process during and after construction, continuing until the wood reaches a moisture content that is at equilibrium with the surrounding air. Wood shrinks as it dries, and shrinkage causes joints in the trim to open. This cracks the paint and provides an easy path for moisture penetration.

The worst place for this to happen is at the end grain of wood. Butt joints and miter joints where the end of one piece of lumber meets the end of another have cuts across the grain of the wood, exposing the porous end grain. Water easily wicks into the wood there, raising the moisture content to a level at which fungus can grow and rot the wood.

Contributing to the problem is the softwood tree species typically used for trim. Softwoods include pine, spruce and fir, all of which have a high susceptibility to rot. Also, trees used for lumber today are second growth, harvested young, rather than the mature, large-girth trees available in the past. These young trees consist largely of sapwood -- wood located near the outside of the tree -- rather than heartwood. Sapwood lacks the natural rot resistance of heartwood.

Once wood trim begins to rot, the process has to be arrested by removing the rotted wood, replacing it with new wood or wood filler, then repainting and caulking. An alternative to entirely removing the rotted wood is to stabilize it using a consolidant, usually penetrating epoxy, and then finish it using wood filler.

The best wood fillers are ones that don't absorb water if the paint fails. Vinyl and gypsum-based putties are popular because they're easy to use, but they don't stand up well to water. Since rot only occurs where water has gotten in, it doesn't make sense to use a filler that isn't water-resistant. Good choices are epoxy putty (the most durable), polyester putty (think Bondo automobile body putty) and solvent-base cellulose putties.

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