On Apr 19, 8:07*am, cj wrote:
greetings, i am building a deck that will be 17 feet wide by 9 foot
deep. i will use 4x4's dug three feet into the ground, with a bag a
cement in each hole. my question is how much support(4x4)will i need for
the 17 foot span? obviously one 4x4 on each end but will one 4x4
centered be enough or should i break the 17 into thirds? so 4 columns
instead of three?
cj

it depends on how big your rim joists or outside girders are. or if
you double them. I'm working on a deck now, that is 22' by 14' and we
used treated 2" x 8", doubled. then the rim joists are through
bolted(galvanized 10" bolts), and just for kicks we installed support
blocks under the joists on the posts. On the 22' side, there are 4
posts (including the corner). All joists in between are attached with
hangars and approved nails. Don't use deck screws to attach the
joists to the posts and expect that to be good enough. No shear
strength.
hth
perry