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Treated Wood Decking Done Right

Construction How-To, Decks, Decks February 4, 2016 Sonia










DIY Installation Advice

The most widely used material for deck boards is treated lumber. There is, however, a right way and wrong way to install treated deck boards to achieve the best results. The experts at Wolmanized Wood offers the following tips to get the most from your material.

During installation, separate the deck boards as follows to allow for expansion and contraction. If they’re heavy and wet, separate boards no more than 1/16 inch as some shrinkage will occur, and you don’t want the gaps to appear to large and unsightly. If the deck boards are light and dry, separate boards about 1/4 inch to allow for swelling.

Shorter spans between joists will help to minimize warping and twisting of deck boards as they dry. Also avoid designs with long cantilevers unsecured at one end; check with your local building code department on maximum cantilever allowed. On decks with unsecured cantilevers, consider fastening a rim joist along the ends or blocking between them to prevent the joists from twisting.

Lumber wider than six inches should not be used as a flat surface. Wide, flat boards are subject to ponding of rain water, which can lead to cupping problems.

Always install the deck boards bark side up. This side will be more likely to have treated sapwood on the exposed face. To determine which side is the bark side, inspect the ends of the boards and judge by the direction of the growth rings.

If a board is bowed, install it with the crown up. Gravity and the weight of people and furniture will flatten it.

If a deck board is bowed, install it with a crown on top. Gravity combined with the weight of people and furniture will flatten the board and prevent sagging.

If a deck board is bowed, install it with a crown on top. Gravity combined with the weight of people and furniture will flatten the board and prevent sagging.

For above-ground treated wood, make sure there is good underdeck ventilation, allowing airflow around the entire deck so the wood can dry, which inhibits mold growth. All above-ground treated members must remain off the ground and free of leaves or other debris.

Proper flashing or spacers should be used between all adjacent structures and the deck. The ledger connection to the house requires especially careful attention to prevent water intrusion, because it is the most common cause of deck collapse.


Cover upper ends of railing posts with post caps or cut them at angles to shed water. Before capping or after cutting, apply topical preservative solution to all ends, holes, or other intrusions in the wood. Exposed end-grain that is left unprotected will absorb water and contribute to rot.

Use two fasteners across a 2×4 and three across a 2×6. Drive nails at a slight angle toward each other. Use 3-1/4-in. long nails on nominal decking and 3-in. nails for 5/4-in. decking. Check fastener boxes to make sure you are buying fasteners that meet standards. Fasteners should be hot-dipped galvanized meeting ASTM A 153 or equally protected material. Connectors should be made from galvanized steel sheet conforming to ASTM A 653 class G185.