Paint and Surface Application of Fungicides

A somewhat controversial question was the effect of paint on the decay hazard. A coating may influence decay by acting as a barrier to the entrance of fungi, by hindering the entrance of the water the fungi need, or by hindering its evaporation. These last two effects tend to balance each other, and paint can not ordinarily be regarded as an important preventive of decay. Painting wood that is still green should increase the chance of decay; that should also be true for paint on the inside of ship planking, or on any surface through which water would normally be escaping rather than entering. On the other hand, unpainted wood that is exposed to intermittent wetting by rain may become moist enough for decay during periods of wet weather, when painted wood with the same exposure might not reach the moisture content of 25-30 percent, which the decay fungi require. The effect of cracks and unpainted ends exposed in joints complicates the situation and makes it difficult to predict for a particular structure or weather cycle what effect paint may have. There has been similar doubt as to the usefulness of brush or dip treatments with fungicides, despite the strong claims of some dealers.

In laboratory tests with southern pine sapwood, brief dip treatments in oil solutions of pentachlorophenol were quite effective in preventing the entrance of the important decay fungus, Poria xantha, despite heavy inoculation with it. The treatment was particularly effective in protecting exposed ends. This agrees with expectation since penetration of solutions through end grain is much deeper than through side surfaces. The futility of drawing general conclusions from tests made under only one set of conditions was shown by the results obtained when some of this same treated wood was buried in soil in two widely separated localities. In both places, the pentachlorophenol gave little protection, while copper compounds that had been decidedly inferior in the inoculation test proved much better than the pentachlorophenol in the soil. In general, for nondurable wood in contact with soil no mere surface treatment is adequate, and thorough impregnation is needed.

Experiments were conducted in warm moist surroundings in southern Mississippi in which pieces of 2 X 4-inch pine sapwood were joined in pairs, the end of a diagonal piece being fastened against the side of a vertical one. Part of these assemblies were given 30-minute soak treatment with the oil-soluble preservatives, copper naphthenate, pentachlorophenol, and phenyl mercury oleate. All of the treated and part of the untreated were then painted. The preservatives were allowed to reach all faces of the pieces, but assembling was done before painting, so the end of the diagonal piece that was in the joint was not painted. All were exposed outdoors on racks for 3 1/2 or 4 years; at the end of that time decay had made marked progress in most of the untreated assemblies. There was still very little decay in those with the preservative.

Painted assemblies without preservatives, in which the painting had been done very carefully so as to seal the joint, were in nearly as good condition as those with the preservative. A rather high proportion of the painted assemblies in which the joint was not sealed, and which were thus more like the ordinary paint job, showed decay, but it was mainly limited to the wood near the joint, with total damage slightly less than in the unpainted, a result that needs confirmation by further experiments.

Many additional tests are needed before positive statements can be made as to how much brush or dip treatments with preservatives decrease the decay hazard, and in what situations the increase in safety justifies the cost of the treatment. Pending further evidence it would seem advisable to give such treatments to lumber used in porches and outside steps, for houses in moist climates. For houses built over moist soil, such treatments might well be used on sills and outer ends of joists, unless preservative-impregnated wood is readily available. The lower siding boards might also profitably be treated, at least to the extent of dipping their ends. Treatment to be most effective should be on unpainted wood, but paint applied afterward and properly maintained should prolong the effectiveness of the preservative.

An experiment was set up in southern Mississippi with modifications of construction such as might be applied in rails of porches, stairs, and fences. A rail that passes over the top of a post without a break should lower the absorption of rain water by the post; in the experiment, such assemblies actually remained nearly free from decay after 3 years. Where the end of the rail met the end of another rail on top of the post or where the end of a rail was fastened against the side of the post, decay has started in most of the assemblies that have so far been opened. Builders have gone far in the development of metal flashing and overlapping joints to keep water out of the roofs and walls of frame buildings, but there is still room for improvement in this respect in some details of buildings, and still more in fences, platforms, bridges, tank supports, and similar outdoor structures.

Houses Without Basements

In the defense housing projects started since 1940, the chief concern as to decay hazard was the fact that the houses and barracks were mostly built close to the ground and without basements. The error of putting decay-susceptible wood members in direct contact with the ground or imbedding them in moist concrete was less frequent than in the emergency construction of the First World War. The principal concern was for the lack of ventilation under many of the houses set on moist sites. Particularly in the winter in the colder parts of the country, moisture condenses on the inner faces of the sills of some of the houses, and in a smaller number on the joists near the outside foundation, or throughout the substructure if the building above is unheated. Occupants of the houses usually are unwilling to keep ventilators open during the colder parts of the year.

Evidence was obtained indicating that vapor from soil was an important source of this condensation moisture, and a simple method was devised for shutting it off. Asphalt roll roofing was laid on the soil under buildings that were too moist. There was a surprising decline in the moisture content of the wood; no more moisture condensation has been observed during the 3% years since the cover was put on, although nearby houses that had the same moisture content before the beginning of the test continued to show high moisture and in some cases condensation. Many houses, even on moist sites and with mineral-board skirting completely closed around the base during the winter, do not reach the moisture danger point. The houses that do become moist enough for decay can apparently be safeguarded sufficiently by the use of roofing of the grade weighing 55 pounds per 108 square feet and costing at retail about 3 cents a square foot. Ordinary asphalt-impregnated roofers' felt, the 15-pound grade costing about a fourth as much as the asphalt roofing, also appeared to be a sufficient vapor barrier in the places where it has been tested, but it will deteriorate when lying on moist soil, so its use under buildings should be limited to those that will be needed for only a very few years. Tests have been extended to buildings with ordinary types of foundations in addition to the houses with mineral-board skirting but conclusions are not yet final.

THE AUTHOR Carl Hartley is a pathologist in the Division of Forest Pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering. Except for 3 years of general plant-disease investigations for the Instituut voor Plantenziekten, Netherlands East Indies, Dr. Hartley has been continuously with the Department since 1909. His principal contributions have been on diseases of forest nursery stock and deterioration of forest products.