Most homeowners are aware of the potential threat that termites and other insects pose to the wooden parts of their houses, but they often forget that wood has one other constant natural enemy -- wood rot. Although some people call it dry root and others call it wet rot, they are basically the same thing -- a form of decay or rot caused by tiny plant-like organisms known as fungi.

Similar to the fungus growths which cause mildew (on paints, fabrics and other surfaces) and food spoilage, this organism is everywhere and its spores are carried through the air. Although these can infect almost any material, like all other plant organisms they can only thrive when moisture is present.

To prevent this it is important that wood which is close to (or worse yet, in contact with) the ground be protected against rot by chemically treating it with a wood preservative. Many people mistakenly believe that paint affords this kind of protection. Paint does help wood shed water and keep it from soaking in but it is merely a surface coating and does not really act as a wood preservative.

For example, porch floors or steps made of wood could be painted every year, but this paint would only be applied to the top and on the parts that can be easily reached with a brush.

If the underside is close to damp earth and thus is continually moist, rot could still develop and cause the wood to decay rapidly. In the same way, window sills can rot because the joints tend to open near the ends, or the wood gets damp in the corners and never really dries out.

One can check for dry rot by probing with a knife or similar sharp tool. If rot is present, the wood will feel spongy and soft and in advanced cases will cut as easily as a piece of cheese. The most likely places to look for conditions of this kind are along the top of the basement foundation where wood beams are close to the ground, the edges of all exterior wood doors, especially garage doors, the frames around windows and doors, exterior wood shutters, outside porches and steps made of wood, fence posts and other garden structures, and in the joints and bases of wood posts, garden sheds, play equipment and lawn furniture.

For years, the only effective wood preservative available was creosote. But it has an unpleasant odor and a dark ugly color and it is almost impossible to paint over. In addition, creosote tends to leach out of the wood in time and thus lose its effectiveness. However, homeowners now can use wood preservatives which contain a special chemical called pentachlorophenol. Sold under various brand names in hardware and paint stores, products containing penta (as it is normally called) come in various solutions which contain water-repellent ingredients, in addition to the preservative.

Most of these are clear, odorless, nonstaining liquids which protect against rot and repel termites and other insects, and reduce the likelihood of warping, checking and swelling. When dry, most of them leave the wood partially primed so that it actually takes paint better than raw wood.

The best time to treat wood for dry rot is before construction -- that is, before all parts are assembled, but after they are cut to size (so the material can soak into the end grain after cuts are made). It can be applied with brush or spray, but maximum protection is provided if the wood can be soaked or dipped into the solution.

When dipping is impractical and the material must be applied by brush or spray, then it is important that it be flooded on as liberally as possible. Where possible, use pans or basins to catch the run-off. Wear rubber gloves to protect the hands since some people find that their skin is sensitive to the chemical, and when applying by spray, a mask should always be worn. Work out-of-doors or in a well-ventilated area.

In addition to applying these wood preservatives to new lumber before and during construction -- at least to lumber in sensitive locations, such as pieces that will be close to the ground or in damp locations -- they can also be applied to existing houses or other structures in places that are susceptible to rot. This would include the various parts of the house mentioned above, as well as under door edges and sills, wood gutters, all joints in fences where the end grain of wood is exposed to weather or likely to absorb moisture, wood screens, porch posts and enclosures.

The material must be applied to bare or raw wood only -- it will not "take" or soak into wood that has been painted, varnished or otherwise sealed.

In joints and cracks (for example, in outdoor lawn furniture or where siding boards meet end-to-end), this is seldom a problem since the end grain is quite porous and will absorb liquid worked in with brush or spray. However, bear in mind the preservative will only soak into dry wood.

To reach into narrow crevices and tight corners, an ordinary pump-type garden sprayer can be used -- or even an oil can with a long spout. It must be remembered that while this will help to prevent rot in wood that is still sound, when applied to wood that has already begun to develop rot it will not restore the wood that has decayed -- it will just help keep the decay from spreading.

In some cases instead of cutting out the rotted wood and replacing it with new material it may be practical to solidify or "rejuvenate" the old wood by injecting a special type of epoxy liquid which is sold for this purpose in most marine supply stores and hardware stores catering to the marine trade. This is a two-part liquid (mixed before use) which soaks into rotted wood and hardens it; it is widely used on wood boats where rot is a frequent problem.

Because preservatives containing penta are clear, they are often used as the basis for a stain that will be applied to decks or wood shingles where the owner wants a colored preservative. Pigment can be added to create a stained effect by following the directions on the label.