There are three destructive forces against which most wood protective measures are directed: biological deterioration (wood is attacked by a number of organisms), fire, and physical damage. In this section, well deal with protecting wood products against biological deterioration.
Damage to wood buildings and other structures by termites, wood bores, and fungi is a needless waste. The ability of wood to resist such damage can be greatly increased by proper treatment and continued maintenance. Wood defects are also caused by improper care after preservation treatment. All surfaces of treated wood that are cut or drilled to expose the untreated interior must be treated with a wood preservative.
There are two basic methods for treating wood: pressure and nonpressure. Pressure treatment is superior to nonpressure, but costly and time consuming. Building specifications dictate which method to use.
The capacity of any wood to resist dry rot, termites, and decay can be greatly increased by impregnating the wood with a general-purpose wood preservative or fungicide. Its important to remember that good pressure treatment adds to the service life of wood in contact with damp ground. It does not, however, guarantee the wood will remain serviceable throughout the life of the building it supports.
Woods of different timber species do not treat with equal ease. Different woods have different capacities for absorbing preservatives or other liquids. In any given wood, sapwood is more absorbent than heartwood. Hardwoods are, in general, less absorbent than softwoods. Naturally, the extent to which a preservative protects increases directly with the depth it penetrates below the surface of the wood. As we just mentioned, the best penetration is obtained by a pressure method. Table 8-4 shows the ease of preservative penetration into various woods. In the table, use E for easy, M for moderate, and D for difficult.
Table 8-4.Preservative Penetration
Nonpressure methods of applying preservatives to a surface include dipping, brushing, and spraying. Figure 8-11 shows how you can improvise long tanks for the dipping method. Absorption is rapid at first, then much slower. A rule of thumb holds that in 3 minutes wood absorbs half the total amount of preservative it will absorb in 2 hours. However, the extent of the penetration depends upon the type of wood, its moisture content, and the length of time it remains immersed.
Figure 8-11.Improvised tanks for dip treating lumber.
Surface application by brush or spray is the least satisfactory method of treating wood from the standpoint of maximum penetration. However, it is more or less unavoidable in the case of already installed wood, as well as treated wood that has been cut or drilled to expose the untreated interior.
Pentachlorophenol and creosote coal tar are likely to be the only field-mixed preservatives used by the Builder. The type of treatment or preservative depends on the seventy of exposure and the desired life of the end product.
Preservatives can be harmful to personnel if improperly handled. When applying preservatives, you should take the following precautions: