Deck building: What does 16 inches on center mean?

Answered by Jeffrey Anderson ~ August 29, 2013 ~ No Comments

What does 16in "on center" mean when placing floor joists for a deck?


Jeffrey Anderson

Kim, there are a few framing terms every DIYer should be familiar with before building a deck. Knowing the definitions not only makes it easier to follow your drawings and blueprints, it could help you avoid building inspection rejections.

As you should be able to see from the drawings, deck joists are installed with their narrow edges facing upward ready to support the deck floorboards. Most decks are designed to use joists that have 2 inch top edges, although in reality the width is actually about 1 ½ inches.

If the drawing calls for the joists to be installed on 16 inch centers, your initial measurement should be from the outside face of the first joist installed. The center of the next joist's top edge should be 16 inches from the outside face of that first joist. As 16 inches on center is a common design for joists and wall studs, many measuring tapes and rules have every 16 inch increment highlighted.

The 16 inches is based on the size of the joist, the length of its span, and how much weight the deck is designed to support.

A good method for laying out deck joists is to install the initial board and then mark the "ledgers" on their top edges with a framing pencil at the 16 inch increments. A framing square can then be used to mark the outside edges of the joists where they will meet the tops and faces of the ledger boards. The ledger boards are the framing members that run perpendicular to the joists. In most cases they will be the same size as the joists.

Another term to be familiar with when building a deck is "flashing." This is the material that is placed between the deck joist or ledger board that is flush up against the home. It is usually a thin metal sheet that helps prevent rain water and snow from leaking into the home at the location where siding has been removed to accept the deck. Your deck drawings should show how the flashing ought to be installed for your specific application.

One last term you should know prior to installing the deck joists is "hanger." Almost all deck designs require that the joists be secured to the ledger boards with metal hangers. Joist hangers are available at most home improvement stores. They are made in various sizes and should be matched to your joist size. The hangers can be installed after the joists are in place or to the ledger boards prior to setting the joists. Use joist hanger nails to secure the hangers to the joists.

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