Expert: Ted Barnhill (Principal, Design45 LLC) - 6/4/2008

Question
QUESTION: Ted,

I'm adding a pergola to my existing deck.  It is 12' x 14' with the rafters (2x4's) spanning 12' width.  I think I need a center support for my rafters and need help.  I'm using two 2x8's connected to either side of 4x4 posts on each side.  Could I create the ctr support by using joist hangers to connect the ends with 2x8s and then using joist hangers off these for a 2x8 ctr support?  Could I use 2x6s?  Correct to use joist hangers?

ANSWER: Tom,

A pergola such as yours will have little "live load" on it - at most some vegetation.  If you cantilever the ends of the 2x4s over your beams to help balance the dead load of the 2x4s, I wouldn't expect a great deal of sag.  You can look around at other pergolas, or just mock up a piece to see if it looks good to you.  If using 2x6s doesn't feel too heavy, they would certainly be stout enough.

If you do want to add a center beam, it is fine to use metal hangers to make the "figure-8" that you describe.  (However, I tend to find them unattractive and the center beam will look heavy.)  Check the Simpson-Strongtie website for the correct exterior product for your beam sizes.  Especially for the corner-corner hanger, you'll want a connector built for the geometry rather than using off-the shelf joist hangers.     Any home center can special order the "odd" hangers if they don't stock them.  

Good luck, and enjoy your shady deck this summer!  -Ted

---------- FOLLOW-UP ----------

QUESTION: You're giving me hope of not needing a ctr support.  If I attach my 2x4s to each of 4 2x8s (2 per end) and they span 12' in between, how much cantilever would I need on each end to avoid excessive bow and a ctr support?  Really appreciate your insight, and like the simpson website.

Tom

Answer
Tom,

I don't know a definitive reference for no-load spans.  In my experience, 12' is getting pretty long for 2x4s - depending on the design and how many grapes are growing on it, you will have some sag but maybe not a visually troublesome amount.   My "belt-and-suspenders" suggestion is to use 2x6s, or better still, paired 2x6s (space them with 2x blocking).  Good luck, Ted.