I’ve been asked to review the deck in these pictures to see if t’s safe. I’ve had a friend run the deck dimensions through the Simpson Strong Tie Deck application and the Sketchup drawing is what we have come up with. It’s recommends 3-2×10’s for the beam and 2×8’s for the joists.
My question is could I double up (sister together) on the 2×6’s to maintain the height underneath. If it can’t could someone explain why?
Thanks, Dennis
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The stiffness of a solid wood beam is (in theory) proprotional to the square of the vertical dimension of the beam. Thus if a 2x6 has a stiffness of 36, a 2x8 has a stiffness of 64, and a 2x10 has a stiffness of 100. Thus two 2x6s would roughly equal a single 2x8, but you'd need three to equal a 2x10.
out of luck
Prescriptive code for deck joist and beam spans can easily be reviewed online and is published with safety being paramount. Simpson's app is probably based on this code. If it is not allowing your framing option to fly, it's probably for good reason. Safety would be why. Keep in mind that if the deck you're attempting to analyze or restructure is outside of prescriptvie code perameters (which appears to be the case) than that structure is questionable and should be analyzed by a professional engineer if safety is of concern. Clearly Dan is not even close. Nor, does it seem, are you.
BTW this forum, to my knowledge, has never proven to be a legitimate proxy for a P.E's. analysis. Good luck finding it here.
no you can not double up an undersized joist in an attempt to gain the maximum span for that size lumber.
a #2 southern pine 2x6 (most common decking material around here) can only span 10'6".
bottom line it doesnt meet minimum standards.
Although some need an engineer to tell them how to build anything (even a simple deck) all you really need is a little understanding of minimum requirements (so that you can exceed them) and a permit so that your work can be verified by someone who more than understands what those requirements are.
if you are hung up on using 2x6's then add a beam in the middle to break the span.
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Thanks for all the great replies.