Cross laminated timber (CLT) has become a popular wood composite in large building applications that were previously limited to concrete and steel. To extend service and prevent biological attack, chemical protectants are often applied to wood. Cross laminated timber panels are not typically treated, in part due to a lack of data on efficient and effective methods for treating large panels. There are also limited long term field data regarding the exposure of CLT to biological attack. In this study, commercially prepared CLT panels consisting of either 5-ply Norway spruce or 3-ply southern pine were obtained and treated via a double-vacuum preservative treatment with borates and placed in an above ground covered field test. Samples measuring230mm x 300mm x 100 mm were cut, edge-sealed, and double vacuum-treated with a with a 10% disodium octaborate tetrahydrate (DOT)/glycol solution. Target loadings were a low (~1kg/m3 SBX) and high (~2.5 kg/m3) borate retention. There were also untreated samples of either wood species employed in the test. All treatments employed 10 samples for a total of 60 samples in test. Samples were placed in an above ground AWPA E21-18 covered field test in Saucier, Mississippi in 2023. One-year ratings and moisture contents indicated an increase in sample moisture content and negligible biological attack. The presence of termites was noted on feeder stakes in the test subplots, but termite attack on samples had not occurred at this point. As this is the first year of a long-term test, further evaluations will continue.

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