Deck Floor Joist Sizes and Spacing

Deck Joist

Introduction: The deck floor joists are the repeated structural members that are used to build a deck frame. They act like the ribs of a skeleton. The joists maintain the shape and strength of the deck. They generally run perpendicular to the house and are suspended between the ledger board that is attached to the house and rest on a beam or between more than one self support beam.

Deck joist hangers are usually used to attach the ends of joists to the face of a beam or the ledger board. The joists are installed parallel to each other for ease of construction and to distribute weight evenly to maintain the structural integrity of the deck frame. The ledger board is bolted to the house outside box beam to insure maximum structural integrity of the deck. Deck floor joist sizes and spacing is critical to the strength of the deck box frame.

The layout of the joists is usually16 inches on center Deck frame 3 for most decks. Most surfacing decking material is not strong enough to support longer spans that are larger than 16 inches between the joists. Some deck builders reduce joist spacing to 12 inches on center to strengthen the deck frame or to increase maximum allowable joist spans. This gives the deck much more strength and prevents deck platform bounce when walked on.

The materials used for joists are most often 2x6, 2x8, 2 x10 or 2x12. These are the stand sizes of structural building lumber and are most widely used in deck construction. The larger the joist sizes the longer the allowable joist span between beams. Other factors such as the type of wood will affect the distance a joist can safely span. Usually these structural members are pressure treated to prevent rot.

Before installing any joists you should carefully examine the material for defects. If you identify the crown in the board you should always install it upwards. The crown will eventually settle after the structure is completed and will stiffen in the proper position after drying.

Also be aware of any large knots on one side of the joists. If you intend to use a joist with a knot be sure that it is at the top side of the frame. The topside of a joist is always under compression and the bottom is always under tension. If there is a knot on the bottom of a joist it will not hold under tension and it will fail.

If you notice your board has a twist in it after attaching it to the ledger, you should try to straighten it before nailing on the header. Insure that all floor joists are as perpendicular as possible.

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