Joists are the repeated structural members that are used to build a deck frame. Much like the ribs of a skeleton, joists maintain the shape and strength of the deck. Joists generally run perpendicular to the house and are suspended between the ledger board attached to the house and a beam or between 2 self supporting beams. Joist hangers are used to attach the ends of joists to the face of a beam or a ledgerboard. Joists are installed parallel to each other for ease of construction and to distribute weight evenly for structural integrity.
The layout term for joists is “On Center” which is the center to center measurement from one joist to the next. Most decks use 16" on center spacing for joists. Most decking is not strong enough to support longer spans than 16". Some builders reduce joist spacing to 12" on center to strengthen the deck frame or to increase maximum allowbale joist spans.
The materials used for joists sizes are most often 2x6, 2x8, 2 x10 or 2x12. They are the most widely used, economical and easiest to find/purchase. The larger the joist size the longer the allowable joist span. Other factors including the type of wood you are using will affect how far a joist can safely span. Decks.com recommends using 2x10 Southern Pine for most deck projects.
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Ponderosa Pine |
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Southern Pine |
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Western Cedar |
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12" O.C. |
16" O.C. |
24" O.C. |
12" O.C. |
16" O.C. |
24" O.C. |
12" O.C. |
16" O.C. |
24" O.C. |
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2x6 |
9'-2" |
8'-4" |
7' |
10'-9" |
9'-9" |
8'-6" |
9'-2" |
8'-4" |
7'-3" |
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2x8 |
12'-1" |
10'-10" |
8'-10" |
14'-2" |
12'-10" |
11' |
12'-1" |
11' |
9'-2" |
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2x10 |
15'-4" |
13'-3" |
10'-10" |
18' |
16'-1" |
13'-5" |
15'-5" |
13'-9" |
11'-3" |
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2x12 |
17'-9" |
15'-5" |
12'-7" |
21'-9" |
19' |
15'-4" |
18'-5" |
16' |
13' |
When installing your joists you should carefully examine the board for defects. If you identify a crown in the board you should always be install it upwards. The crown will eventually settle after completing construction and should stiffen in the proper position after drying.
Another defect to look out for would be a large knot at one side of the joist. If you intend to use a joist with a knot like this be sure that it is at the top side after installation. The topside of a joist is always under compression and the bottom is always under tension. If you have 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. You want all your joists as perpendicular as possible.
