Preservative-treated wood for foundations

General information is presented about the use of preservativetreated wood in foundations. Introduction Wood is an important construction material because of its favorable cost , strength, working characteristics, and availa b i l i t y . Neverthe less , it has natural enemies, namely decay fungi , t ermites , and other insects . Where wood is at a high moisture content as a result of exposure to weather or contact wi th the so i l , or is used in important structural components such as poles, s i l ls , or foundations, protection from decay and insect attack is achieved through use of indepth, preservative treatments. Several levels of protection are available with preservative treatments. The main difference in protection levels is the amount of preservative chemical forced into the wood. Considerations of cost and difficulty of replacement led to the development of specific standards for wood in foundation materials that include a margin of safety. It is important that only wood which meets these standards be used in wood foundations. Treatment Standards Major building codes and the U.S. Department of Housing and Urban Development (FHA) Minimum Property Standards require that each piece of lumber and plywood in wood foundations be identified with the American Wood Preservers’ Bureau (AWPB) “FDN” grademark to indicate conformance with standards that were developed for wood foundations. A reproduction of this stamp is shown in f igure 1. The AWPB “FDN” grademark is applied only to wood treated under the quality control provisions of the American Wood Preservers’ Bureau, P.O. Box 6085, 2772 South Randolph Street, Arlington, VA 22206. This grademark provides assurance that the products meet the treatment level required by the standards of the AWPB, and the requirements of Federal Specification TT-W-571 (copies available from Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402). These standards require that wood for foundations be treated to l / Maintained at Madison, Wis., in cooperation with the University of Wisconsin. P r e s e r v a t i v e s T e s t e d F i r s t a r e t en t ion o f 0 .6 pound o f pe r se rva t ive pe r cub ic foo t o f wood. Adequate treatment of each veneer in plywood is essent i a l f o r l o n g t e r m s t r u c t u r a l d u r a b i l i t y . For th i s r eason , indus t ry s t andards r equ i re sample bor ings to check pene t ra t ion of each charge of plywood, and these borings must show all veneers to have been penetrated. The s t andards fu r the r s t ipu l a t e tha t add i t iona l p rese rva t ives be app l i ed to exposed ends and edges of lumber cut af ter t reatment and that the amount of hea r twood in t r ea ted mate r i a l s o f ce r t a in spec ies be res t r i c t ed to wi th in ce r t a in a l lowab le l imi t s . One research study, in i t i a t ed a t the Fores t P roduc t s Labora tory in 1937, exp lo red the u t i l i t y o f c reoso te t r ea ted wood in a house foundat ion. Tha t founda t ion gave exce l l en t se rv ice , bu t c reoso te t r ea ted wood i s no t accep ted today fo r use in r e s iden t i a l founda t ions . The only wood preservatives accepted for use in residential wood foundations are chromated copper arsenate (CCA) and ammoniacal copper arsenate (ACA). These p rese rva t ives a re o f t en re fe r red to as wa te rborne p re se rva t ive t r ea tmen t s because the wood i s t r ea ted wi th a so lu t ion o f chemica l s d i s so lved in wa te r . As the wood dries, these chemicals become permanently f ixed within the wood. Once the wood i s d ry , these t r ea tmen t s l eave a d ry , pa in tab le su r f a c e . Founda t ions bu i l t w i th wa te rborne p rese rva t ive t r ea ted wood have been used in p r iva te r e s iden t i a l cons t ruc t ion s ince 1969. Before a wood p rese rva t ive t r ea tment i s accep ted , i t i s ex tens i v e l y e v a l u a t e d f o r i t s e f f e c t i v e n e s s . A preservative must pass bo th l abora to ry and f i e ld t e s t s be fo re i t can be used in b u i l d i n g s . L a b o r a t o r y t e s t s a r e c o n d u c t e d f i r s t . T h e p r e se rva t ives tha t r e s i s t t e rmi te s and decay fung i in the l abora t o r y a r e s e l e c t e d f o r f i e l d t r i a l s . I n f i e l d t e s t s , 2 b y 4inch s t akes , 18 inches long , t r ea ted wi th d i f f e ren t amount s o f the t e s t p rese rva t ives a re ha l f -bur i ed in an upr igh t pos i t i o n i n t h e s o i l . These s t akes a re exposed in f i e lds fo r decades and examined pe r iod ica l ly . The Fores t P roduc t s Labora to ry ma in ta ins f i e ld p lo t s in Wiscons in and Miss i s s ipp i . Wood in southern Mississippi is exposed to severe decay and t e r m i t e a t t a c k . Unt rea ted (con t ro l ) 2 by 4 inch s t akes o f southern pine sapwood last about 2 to 4 years in the Miss i s s ipp i p lo t and approx ima te ly 4 to 6 yea r s in the t e s t plot near Madison, Wis. Stakes of southern pine sapwood treated with waterborne prese rva t ives a t l eve l s comparab le to those spec i f i ed fo r wood foundations have been exposed in the soi l in southern Mississippi for more than 30 years with no fai lure. S p e c i f i cal ly, 2by 4-inch stakes of southern pine sapwood treated with ACA at retentions of 0.24 to 1.25 pounds per cubic foot of wood have been in test for 32 years, while comparable stakes treated with CCA-I at a retention of 0.15 to 0.44 pound per cubic foot have been in test for 31 years . S takes t r ea ted wi th CCA-II at retentions of 0.26 to 1.04 pounds per cubic foot have been in t e s t fo r 29 yea r s . No fa i lu res have occur red in t e s t stakes treated with ACA at retentions of 0.51 or more pound per cub ic foo t ; in t e s t s t akes t r ea ted wi th CCA-I a t r e t en t ions