find the correct deck joist spacing and span before you start building. see calculations for 2x6, 2x8, 2x10 and 2x12 deck joists and common types of wood.
the proper deck joist span (distance between supporting beams below the joists) for your deck should be known before you start building and is needed to draft your deck plan. the spans of joists varies based upon several factors: joist size , spacing between joists , decking material and wood species.
deck joist maximum spans no cantilever with cantilever* *joists may cantilever up to ¼ of the actual adjacent span. for example, a 2 x 12 joist placed 16” oc. spanning 16’-6” between supports may cantilever up to an additional 4’-1 ½” for a total maximum length of 20’ -7 ½” southern pine deck beam maximum spans*
a 2x8 up to 12 feet; 2x10 to 15 feet and 2x12 to 18 feet. the larger the deck, the larger the joists. in typical deck construction, with a ledger on one side of the joist and beam on the other, the size of the joists is driven by the size of the deck and based on the general maximum spans mentioned above.
the cantilever (overhang) can be 1/4 the joist length, but again i would keep that to 2'-6 max. for 2x8. deck loading has a tendency for the people to gather at the railing. so i typically don't go beyond that.
position a piece of 2x8 lumber against the outside face of the posts and lift it up until the top edge touches the bottom edges of the joists you attached to the posts. use lag bolts to fasten the 2x8 to the post faces, two bolts in each post. repeat steps 5 and 6 with a second 2x8 on the opposite face of the posts to create the double beam.
the appropriate spans for deck beams and joists are determined in part by the weight loads that the deck will have to bear. the so-called dead load is the weight of the deck structure itself .
my personal deck is a large "l" shape that i built on top of beams and used my rail posts (all of which go to ground and are 6' or less o/c) for the perimeter rim joists. my spans are 7' and 12' respectively (between the beams) and with 2x8 installed 12" o/c and blocking, even with 35+ people on the deck there is zero bounce.
deck cantilevers. section r507.5 notes that deck joists can cantilever up to 1/4 of the joist span. but you’ll notice when you look at the portion of the span table for joists with cantilevers that the joist span for decks with a cantilever is often less than the span for the same sized joist without cantilevers.
when the deck joists are 8 ft. long, the beam can span 7-7. when building a deck with 7-ft. joists, you know the beam can span somewhere between 7-7 and 8-0, but you don’t know exactly where. check with your local code official, or simply drop back to the shorter value in the table and assume the allowable span is 7-7.
for instance, joists that are placed 24 inches apart must be wider than joists placed 16 inches apart. see the deck joist calculations below and study your local code. deck beam spans: distance a joist spans between a beam and a ledger or between beams, using no. 2 and better southern pine or douglas fir
re: 25x12 deck what size beams/joists? i put together this little image to make sure i am understanding correctly. 25' beam(s) size should be 2-2x10 or 2-2x8 sandwiched? joists 2x8 12" oc or 16 oc? 4 or 3 6x6 posts per beam? again, the decking top will be 36 above the ground. thank you so much for your help here.
floor joists: in general, "2x" (two-by) lumber with an actual thickness of 1-1/2" is used for floor joists. probably the most common floor joist size is 2x8 lumber - actual size is 1-1/2"x7-1/4". the species of lumber, the spacing of the joists, and expected design loads all determine what size floor joist is required.
recommended or allowable joist & beam spans: this article explains typical spans for deck joists, deck beams, and deck flooring, giving both standard span tables and a quick "rule of thumb" that works pretty well. our page top photo shows a small deck addition to a 1920's bungalow in poughkeepsie ny.
the distance your joists can safely cantilever or overhang a drop beam is determined by the size of the joists, the wood type and grade of the lumber and the spacing between joists. many building departments limit joist cantilevers to 24" maximum. you must use at least five framing nails to attach the rim joist to the ends of your joists.
joist hangers are sized by joist depth, and run about 1⁄2 inch shorter than the nominal size of the joist (you can size deck joists by referring to table r507.6 in the 2018 irc, which lists spans for deck joists sized from 2x6 to 2x12). for example, a 2x8 joist hanger is about 6 5⁄8 inches tall.
that all depends on the length of the clear (unsupported) span the joists have to make. without that number, this question cannot be answered as asked. when i was taking a course on building codes to get my contractor's license, i was taught a fai.
irc in the most restrictive condition allows a maximum span of 8'4" for a 2x8 joist made of redwood, western cedar, ponderosa pine, or red pine. since you did not mention which way you were going to run the joists, if you need a 10 foot span you would need either a 2x8 or a 2x10, depending on the species of lumber you use.