Expert: Bruce E. Johnson - 5/22/2010

Question
QUESTION: I'm having a 14' x 11' pergola built in the back yard. The posts and beams are 6x6. The rafters are 2x6. I think it's a pretty simple design but I am concerned about the 14' span on the 6x6 beams. I fear that without a post in the middle of that 14' span there will be dipping and warping. Can you recommend a method to engineer the beams so they don't dip without putting a post in between?  Also, please note how the beams and rafters will be attached (to the face of the posts/beams rather than as a top load)

Please see the link below for design specs:

http://pergola15.shutterfly.com/pictures

ANSWER: Hi Donna, first of all, after looking at the photos you attached, the cabanas shown are not the same as the pergola design.  The cabanas only have a couple of joists and some kind of lattice work on top.  The pergola design has what appear to be 2x6's at 12" apart which increases the bearing dead load on the beams.  It appears that the cabanas shown are made of larger material also, like 6x8 posts and 6x10 beams.  But to answer your question, 14' is a long span for a 6x6.  Me personally?  I would either upgrade my beam size to a 6x10 or use knee braces in the corners that go down two feet and out two feet from the post to the beam.  This will cut the span to 10 feet and give the structure some sway bracing. I am attaching a drawing to illustrate.  The drawing I am attaching shows the knee braces set 4' each direction but I'm sure you will get the idea of what I am talking about.  I hope this information helps, feel free to write again regarding this or other matter, sincerely bruce e johnson.

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QUESTION: Thanks so much for the fast response!
The specs for my design are the cartoon-like images.  The photographs of the cabanas are to illustrate the contemporary nature of the design.  Because I'm going for a "minimalist design" (like the cabanas) I'm not really a fan of the knee braces.  Is a 6x10x14 beam readily available to purchase or does this need to be built up from 2x10's?

Answer
Hi again Donna, it really depends on where you live and the quality and quantity of good lumber yards in your are.  But both methods are acceptable. I hope this information helps, feel free to write again regarding this or other matter, sincerely bruce e johnson.