Efficacy of alternative copper-based preservatives in protecting decking from biodegradation

Abstract: The above-ground performance of decking treated with two alternative copper-based preservative formulations is being evaluated at a test site near Madison, Wisconsin, USA. Southern pine sapwood lumber specimens (38 mm by 140 mm by 910 mm) were pressure treated with 0,93 %, 1,40 % or 2,34 % (oxide basis) actives concentrations of a boron-copper formulation (BC) composed of 7,2 % copper hydroxide and 92,8 % sodium tetraborate decahydrate. Similar specimens were pressure-treated with 0,66 % or 1,32 % actives concentrations of a copper-zinc formulation (CZDP) composed of 18 % copper (CuO basis), 12 % zinc (ZnO basis), 14 % dimethylcocoamine and 56 % propanoic acid. In both cases untreated specimens and specimens treated with a 1% concentration of chromated copper arsenate Type C (CCA-C) were included for comparison. The specimens were installed on racks approximately 760 mm above the ground and periodically evaluated for extent of fungal decay and surface microbial growth. After 18 years in test specimens treated with the lowest solution concentration of BC (0,93 %) suffered substantial degradation and all but three replicates have failed. Obvious decay has not yet been detected in specimens treated to the highest BC concentration (2,34 %), but decay is suspected in one of these specimens. Decking specimens treated with CZDP exhibited no evidence of decay until year 17 when a fruiting body was observed on one specimen treated with a 0,66 % solution concentration. There has been no evidence of decay in specimens treated with 1,32 % CZDP or in either set of specimens treated with 1 % CCA-C. Both BC and CZDP-treated specimens were at least as effective as 1 % CCA-C in minimizing noticeable surface microbial growth. These decking studies confirm that relatively low copper concentrations can provide substantial protection for decking exposed in a moderate climate, and that the CZDP formulation is potentially more effective than the BC formulation. However, caution is warranted in extrapolating these findings to more severe climates and to construction designs that are more likely to trap moisture.