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Cir 1258 How To Select And Use Pressure-Treated Wood in Florida1 Robert A. Schmidt and Jacob B. Huffman2 1. This document is Circular 1258, one of a series of the School of Forest Resources and Conservation, Florida Cooperative Extension Service Institute of Food and Agricultural Sciences, University of Florida. First published January 2002. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. Robert A. Schmidt, PhD, Professor of Forest Pathology and Jacob B. Huffman, DF, Emeritus Professor of Wood Technology, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611-0410. All chemicals should be used in accordance with directions on the manufacturer's label. The use of specific trade names in this publication does not constitute endorsement of these products in preference to others containing the same active ingredients. Mention of a proprietary product does not constitute a guarantee or warranty of the product by the author or the publisher. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean. Knowledge of how to select and use pressure-treated wood is especially critical in Florida, where very favorable conditions for wood deterioration place our state in the highest risk category in the continental United States. Millions of dollars are lost each year by the construction industry, insurance companies and homeowners because of the deterioration of wood, often improperly used. These losses can be easily avoided if knowledge about wood deterioration and pressure-treated wood is correctly applied. Many factors contribute to deterioration of wood-in-use, including: tree species, construction practices and biological and physical agents. However, detailed knowledge about these and other contributing factors is not required to prevent deterioration of wood structures. This publication provides the information necessary for the selection and use of pressure-treated wood to minimize or eliminate wood deterioration. The ABC's of pressure-treated wood are: Wood exposed to frequent or continuous moisture in warm environments must be pressure-treated. Not all pressure-treated woods are alike. Precautions are necessary in handling and disposing of pressure-treated wood. What Is Pressure-treated Wood? Pressure-treated (PT) wood is lumber, timber, plywood, piling, poles or posts which have been commercially treated with a chemical wood preservative under pressure to assure penetration of the chemical into the wood. Treatment takes place in a large pressurized cylinder where the amount of preservative placed in the wood is controlled. The process varies depending on the chemical and wood species and its intended use, but generally includes: 1) pretreatment (removing tree bark, milling and drying), 2) treatment (the chemical preservative is forced into the wood with pressure), and 3) post-treatment (cleaning to remove excess chemical and fixing to aid in the retention of the chemical). Some, but not most, PT wood is redried to stabilize wood structure and wood chemistry.
How To Select And Use Pressure-Treated Wood in Florida 2 Chemical preservatives used by the industry to pressure treat wood vary in effectiveness and also in animal, human and environmental toxicity. The choice of preservative depends on the intended use of the wood product. Some preservatives are classified as restricted-use pesticides which only certified persons may purchase and/or use. The treated wood is not a restricted-use product, but precautions for handling and disposal are necessary (see Precautions on page 3). Water repellent products, with or without a fungicide, are sold over-the-counter to the public since these products are not restricted by the Environmental Protection Agency (EPA). These chemicals may be brushed, dipped or sprayed on wood to protect the surface but are short-lived (less than 2 yr above ground). Directions for use of water repellents appear on product labels and are not discussed further since the focus here is on PT wood. Water repellent treatments may be incorporated into PT wood to provide immediate longer-lasting protection. However, this does not eliminate the need to apply repellent coatings periodically to preserve the appearance of the PT wood. Why Use Pressure-treated Wood in Florida? In Florida (and elsewhere in the southeastern USA), a warm moist climate and wet soils provide very favorable conditions for biological organisms to decompose wood. In the USA, the wood decay hazard varies from low in the dry southwest to severe in Florida where warm, wet conditions prevail (Figure 1). In Florida ,the climate index varies from approximately 90-140 with the wood decay potential increasing from northwest to southeast (Figure 2). Peninsular Florida has exceptionally high decay potentials. When left untreated, wood exposed to moisture (rain, condensation, high humidity, soil moisture, and sea water) in a warm environment is readily attacked and rapidly degraded by naturally occurring organisms. These organisms (fungi, bacteria, termites, carpenter ants and several beetles and borers) are ubiquitous and abundant in our atmosphere, forests, soils and waterways. They are beneficial as they decompose and recycle woody debris, but when they do their thing in our wood structures they become pests. Unfortunately, the most destructive of these pests (decay fungi and termites) can destroy wood from the inside; often they go unnoticed until significant damage has occurred. The biology and ecology of wood destroying organisms are varied and complex, but fortunately this detailed knowledge is not required to prevent their damage to wood. It is only necessary to know that wood which is frequently or continuously subjected to moisture must be pressure treated for durability. Wood exposed to such conditions in Florida include decks, poles or posts in the soil, foundations in contact with the soil and dock pilings. Wood sills in buildings in contact with soil and wood siding subject to continuous moisture should also be treated. The rule is simple keep wood dry or use PT wood. Some termites and decay fungi transport moisture via visible tunnels or tubes from the soil to normally dry wood above, resulting in subsequent deterioration of the wood. This deterioration can be avoided with soil barriers (chemical pesticides) or with PT wood. Dry wood safe from most organisms can be attacked by drywood termites and powder post beetles. Screens and paint films can minimize these problems. Mold and mildew fungi grow on the surface of wood or wood coatings, and while unsightly, do not significantly decompose or weaken wood. These typically dark-colored stains can be washed or bleached from most wood surfaces. Water repellents containing a fungicide mentioned previously provide some short-term protection (less than 2 yr) against mold and mildew. Figure 1. Above ground wood decay potential. Decay hazard zones (map adapted from Cassens et al. 1995 and AWPA).
How To Select And Use Pressure-Treated Wood in Florida 3 Figure 2. Climate index for wood decay in Florida (adapted from Scheffer 1971). How to Select And Use Pressure-treated Wood The most common PT wood sold in Florida is southern pine; however, softwoods and hardwoods from other regions are also pressure-treated and sold in Florida. Proper selection of PT wood depends on its intended use. Wood used in higher risk conditions (soil contact and/or exposed to frequent moisture) requires higher levels of retention (amount of preservative retained in the treated wood). Retention level is measured in pounds of preservative per cubic foot of wood (lbs/ft3 or pcf) and this number appears on labels affixed to the PT wood product (Figure 3). Wood-use categories and conditions, typical construction applications and appropriate retention levels, as specified by the American Wood-Preservers' Association (AWPA), for the most commonly available preservative in Florida (chromated copper arsenate or CCA Type C) are listed in Table 1. This information provides critical guidelines for the selection and use of PT wood in Florida and elsewhere. For example, CCA treated wood with a retention of 0.25 pcf (UC1 and 2) is sufficient for interior construction protected from the weather. The same retention level (0.25 pcf) is appropriate for interior or exterior construction where wood is not in contact with the ground (UC3A and 3B). Wood in contact with the ground or in fresh water (UC4A, 4B and 4C) must have retention levels of 0.40 0.80 pcf, depending on their structural use. The higher levels of retention are used for critical structural materials. Wood for piling in saltwater (UC5B and 5C) requires 2.5 pcf of CCA. Figure 3. Examples of the important information appearing on typical labels on pressure-treated wood (A) PT wood suitable for ground contact; (B) PT wood not suitable for ground contact: (a, suitability for ground contact; b, chemical preservative (shown here is the commonly used CCA Type C, but there are several other effective preservatives); c, AWPA wood-use category standard [refer to table 1]; d, retention level). When this circular was printed some manufacturers used the older system of user category, eg., C1, 2, 3, etc. Untreated wood is generally used for interior, protected construction, however, recent guidelines suggest that added protection is provided by borate. Borate-treated wood is commercially available as a pressure treatment. An alternative is to spray a borate product on the wood during construction, but this treatment is less effective than PT wood. The untreated mature heartwood of several tree species native to Florida is naturally decay-resistant, e.g. longleaf pine, eastern red cedar and cypress, and was valued for its durability. However, the sapwood (wood cut from the outer portion of a tree) of these species is not decay resistant and must be treated if exposed to moisture. This is important since much of the timber and lumber now sawn contains some, if not all, sapwood. For example, cypress logs made from young trees, unlike the old-growth mature cypress, contain a high proportion of sapwood and must be treated if exposed to moisture. Treating logs with borate or a water repellent with fungicide following construction may provide limited protection for exposed sapwood which was not treated prior to use. Precautions in Using Pressure-treated Wood Some chemicals used in PT wood are restricted-use pesticides. The wood treated with these chemicals (PT wood) is not a restricted-use product, but care must be taken during handling and disposal. Gloves should be worn to handle or mill wet PT wood and hands should be washed afterward. Do not eat, drink or smoke when working with PT wood. Sawdust and wood chips should be disposed of properly. Pressure-treated wood should not be
How To Select And Use Pressure-Treated Wood in Florida 4 burned in fireplaces or stoves in order to avoid exposure to smoke. Landfills which accept PT wood (Class A lined landfills) should be utilized for disposal. Pressure-treated wood should be kept from yard waste and construction materials destined for unlined landfills. Pressure-treated wood must not be used where it could come in contact with food. Directions on product labels and on customer information sheets, available where PT wood is sold, should be read and followed. Often-asked questions, especially those dealing with human health and the environment are answered in the 1995 publication from the American Wood-Preservers Institute, listed in the References Cited section of this publication. Similar to untreated wood, pressure-treated wood is subject to warp, shrinkage and checking caused by physical agents, e.g., sun and rain. Water repellents provide some protection and extend the appearance and useful life of the exposed PT wood on decks, etc. These protectants are usually applied annually although some may be incorporated in the pressure treatment. Newly installed wet PT wood does not hold paint well, but may be stained and/or treated with a water repellent when dry. Pressure-treated wood selected and used properly is a useful, attractive and durable product. References Cited American Wood-Preservers Institute. 1995. Answers to often-asked questions about treated wood. Amer. Wood Preserv. Instit. Vienna, VA. [http://www.awpi.org/] American Wood-Preservers' Association. 1999. Standards 1999. Amer. Wood Preserv. Assoc. Granbury, Tx. [http://www.awpa.com/ucs.htm] Cassens, D.L., Feist, W.C., Johnson, B.R. and DeGhoot, R.C. 1995. Selection and use of preservative-treated wood. For. Prod. Soc. Madison, WI. Publ. No. 7299. 104 p. Scheffer, T.C. 1971. A climate index for estimating potential for decay in wood structures above ground. For. Prod. J. 21(10)25-31.
How To Select And Use Pressure-Treated Wood in Florida 5 Table 1. Guidelines for selection and use of chromated copper arsenate (CCA) pressure-treated wood for protection against wood deterioration. Reproduced from AWPA Standards 1999. CCA is the wood preservative most often found in PT wood in retail stores. However, there are other effective wood preservatives. Use category Service conditions Use environment Common agents of deterioration Typical applications CCA retention level p/cf Kg/m3 UC1 Interior construction, dry, above ground Continuously protected from weather or other sources of moisture Insects only Interior construction and furnishings 0.25 4.0 UC2 Interior construction, damp, above ground Protected from weather, but subject to sources of moisture Decay fungi and insects Interior construction 0.25 4.0 UC3A Exterior construction, coated, above ground Coated, exposed to all weather cycles, rapid water runoff Decay fungi and insects Coated millwork 0.25 4.0 UC3B Exterior construction, above ground Exposed to all weather cycles and prolonged wetting Decay fungi and insects Decking, deck joists, railings, fence pickets 0.25** 4.0** UC4A Ground contact or fresh water For normal ground or fresh water contact, exposed to all weather cycles Decay fungi and insects Privacy fence posts, structural lumber & timbers, guardrail posts, utility poles in regions of low decay potential 0.40 6.4
How To Select And Use Pressure-Treated Wood in Florida 6 Table 1. Guidelines for selection and use of chromated copper arsenate (CCA) pressure-treated wood for protection against wood deterioration. Reproduced from AWPA Standards 1999. CCA is the wood preservative most often found in PT wood in retail stores. However, there are other effective wood preservatives. Use category Service conditions Use environment Common agents of deterioration Typical applications CCA retention level p/cf Kg/m3 UC4B Ground contact or fresh water or important construction components Severe ground contact or salt water splash, difficult replacement, exposed to all weather cycles Decay fungi and insects with increased potential for biodeterioration Permanent wood foundations, utility poles in regions of moderate to severe potential for decay or economic loss, building poles, horticultural posts 0.60 9.6 UC4C Ground contact or fresh water or critical structural components Very severe ground contact, exposed to all weather cycles, critical structural components Decay fungi and insects with high potential for biodeterioration Land or fresh water piling, foundation piling, utility poles with a severe potential for decay 0.80 12.8 UC5A*** Salt or brackish water and adjacent mud zone Continuous marine (salt water) exposure Salt water organisms, Limonia quadripunctata Piling, bulkheads, bracing 1.25 *** 20.0*** UC5B Salt or brackish water and adjacent mud zone Continuous marine (salt water) exposure Salt water organisms, Limonia quadripunctata Piling, bulkheads, bracing 2.5 40.0 UC5C Salt or brackish water and adjacent mud zone Continuous marine (salt water) exposure Salt water organisms, Martesia, Sphaeroma Piling, bulkheads, bracing 2.5 40.0 Retention levels used currently on labels are in pounds/cubic foot (p/cf), however, in the near future the equivalent metric units of kilograms/cubic meter (kg/m3) may be used (kg/m3 = p/cf x 16). ** Retention level 0.40 (6.4 kg/m3) will give added protection in Florida and elsewhere in the Southeast where termite and decay potential are very high *** Not for use in Florida.