Effect of species composition of preservative-treated Douglas fir plywood on its decay resistance
R S Smith, P Balcaen
Plywood is now commonly used for constructional purposes in environmental situations where decay by wood-destroying fungi can be a considerable risk. However there seems to be a lack of understanding of the relative durability of plywood, both untreated and when treated with wood preservatives. Part of the reason for this situation has been the difficulty of carrying out laboratory decay tests on plywood, since active fungal growth in small containers in the presence of unleached plywood is almost impossible. It has been suggested by Savory (1969), that inhibitory factors in the glue used for the manufacture of plywood are toxic to fungi when incubated together under standard conditions (ASTM: D1413-76; BS 838 - 61). Another reason for the confusion surrounding the durability of treated plywood comes from the belief that its treatment in terms of loading and retention can be described in terms similar to those used for solid wood. But plywood is generally heterogeneous in species composition, considerably variable in the distribution and amount of sapwood and heartwood and variable with respect to the degree of lathe checking in the plies. Consequently, all of these factors will affect its ability to take up wood preservatives and therefore its resistance to wood-destroying fungi. In an attempt to clarify the influence of some of these factors on plywood durability, the effect of species composition of preservative treated Douglas-fir plywood on its resistance to attack by wood-destroying fungi was examined.