Pressure-treated plywood is a building product made of thin sheets, or plies, of natural wood that have been glued together. It is saturated with preservative chemicals infused into the plywood under high pressure. This rigid material is desirable for many outdoor construction projects because its preservative chemicals enable it to resist mold, decay and insect infestation. It lasts much longer than untreated plywood in an outdoor environment. Ordinary steel nails, screws and other fasteners shouldn’t be used with pressure-treated plywood because they corrode quickly. Instead, use hot-dipped galvanized or stainless steel fasteners.
Pressure-treated plywood typically comes in 4-foot-by-8-foot sheets in thicknesses ranging from 3/8 inch to 1 inch. The panels usually have a greenish tint from the preservative chemicals in them, but some preservative compounds impart a yellow tint to the wood. Pressure-treated plywood is typically marked as such by an ink stamp on its face or by a yellow tag on one end. A sheet of pressure-treated plywood is noticeably heavier than an untreated plywood sheet.
Until 2003, the vast majority of pressure-treated plywood used an arsenic-based preservative known by the initials CCA. But concerns over adverse environmental impacts from toxic arsenic led the plywood industry in 2003 to switch from CCA to less-toxic alternative preservatives, such as copper and boron, in plywood intended for use around homes and gardens. As of 2014, pressure-treated plywood is produced with 12 different levels of preservation for various outdoor environments ranging from sheltered open air to damp underground uses.
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