Dock building, and spans for 5/4 cedar

[MTScott]

I wasn't sure where else to put this, so it ended up here. 

I've finally sold my house, collected the cash and moved my family into my parents small lake house (okay, it's an old fixed up trailer home, but it is on the lake!).  This puts us a big step closer to buying land and starting our own owner-build.

In order to thank my dad for his graciousness, I'm going to build him a new dock while I'm there, and we need one bad.  All the past docks have been built out of left over barn scraps and ended up on the burn pile last winter.  I want this one to be decent and last awhile, but I'm still in a budget.

I'd like to build the whole thing out of cedar, but I can't find much for cedar dimension lumber around here for the framing (and when I do, it is exorbitantly priced).  I could make a road trip to Spokane and find some there, but for now this is my plan:

4' x 10' panels

2 x 6 x 10 (3) Doug fir joists.  I realize this is a long span for a 2x6, but this is what the commercially sold ones are made with.  Weight is also a concern, as we have to drag the panels back up on shore every fall and reinstall them every spring.
2 x 6 x 10 (1) end (rim?) joist (4' each end), and blocking (2 pieces at 1' 10.5", near center)
5/4 x 6 cedar decking.

This will leave the joists at 24" off center which is where my question really comes in - Will that be too wide for 5/4 cedar decking?  It seems most sources say a max of 16" oc.  Keep in mind this just needs to function, it doesn't need to be perfect and it doesn't need to meet any codes.  I just don't want it to be dangerous, and totally flimsy.

I'll probably use some products from a company called "tommy docks" that sells through home depot.  They join the panels, and also hold the posts.  Tommy docks sells glav posts, but they are very expensive, so I may give some 1.5" EMT conduit a shot and see how it works.  For the first 4 panels or so the water is < 4' deep

Any input is appreciated.  Thanks!

Scott

[Don_P]

Look at it another way, I doubt one more joist is going to break the bank or change the weight appreciably.

[Ernest T. Bass]

..or go w/ 2x8--same weight difference, but might be stiffer. It would have to be pretty lousy 5/4 cedar to not be perfectly stiff on a 2' span, imo..

[MTScott]

Look at it another way, I doubt one more joist is going to break the bank or change the weight appreciably.

I've thought about that.  A 10' 2x6 weighs about 20 lbs, so it is 3 at 60 lbs or 4 at 80 lbs.  We've gotta carry these things by hand so it is a consideration.  You're probably right though, and the whole panel would be stiffer.

[Don_P]

Ernest T brings up a better point.
for bending strength the combined section modulus of 4 2x6's is 30.25 "
For 3 2x8's it is 39.42... more than the equivalent of 5 2x6's strengthwise.

[tigrr]

Might want to check out the dock with 45 gallon plastic drums under it.  Light and can be towed out with a car.  The videos I watched it was built upside down and flipped over and pushed in the water.  I want to use this for a party barge idea with an outboard motor.