Pressure Treated Lumber Sizes for Decks


Pressure treated lumber utilizes chemicals applied under pressure to preserve wood and retard decay. Pressure treated lumber is categorized by the pounds of chemical preservative it retains for each cubic foot of lumber. The lowest classification is UC2, which retains only a quarter-pound of preservative per cubic foot and is classified for indoor use. UC3B is classified for decking and contains 0.25 to 0.4 pounds of preservative per cubic foot of wood. Lumber of this classification is available in a variety of sizes.


  • Square posts of 4 or 6 inches commonly serve as supports for decks. The size of the deck is not the controlling factor, but decks with posts placed less than 8 feet apart can use the smaller posts and lighter joists and frame work. Cutting posts, especially 6-inch posts, requires reciprocating or small chain saws.

Dimensional Lumber

  • Dimensional lumber, nominally 2 inches thick but in various widths, are used for deck framing and joists. The dimensions of the board, stated in inches, are before planning. A 2-inch thick board is actually 1 1/2-inches thick. The 6-inch wide board is actually 5 1/2-inches wide. These boards provide the strength of the deck.


  • The top or surface of the deck often consists of 5/4-inch boards. These boards, sometimes called five/quarter lumber, actually measure 1 1/4-inch thick. Attach the decking boards to the joists to serve as the top surface. Heavier duty decks sometimes use dimension lumber as decking. In those cases, space the joists 24 inches apart rather than 16 inches.


  • Specialized milled lumber is available for hand rails and other elements of the deck. These boards are shaped into profiles specific to the use. These materials also are pressure treated to preserve the lumber in outdoor conditions. Other specialized lumber includes posts turned to serve as decorative uprights supporting the rails.

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