Deck Floor Joist Sizes and Spacing
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
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
The layout of the joists is usually16 inches on center
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
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