Expert: Bruce E. Johnson - 11/17/2008

Question
I am planning a pergola that will have 6 x 6 posts on the corners of a 10' x 16' area. I plan on using double 2 x 10 across the 16' span and using 6 x 6 bracing at a 45 degree angle for support. the total height will be 10' and the supports will be 4' in. I then plan on using either 2 x 4 or 2 x 6 across the 10' span, and tying these to the 2 x 10s. Will I need further support to prevent either 2x4 or 2x6 from sagging? What would be the max span for a 2x4 or 2x6 and still have no visible sagging?

Answer
Hi Drew, for non structural joists like Pergola joists I usually will accept a 6 foot span with 2x4 and a 10' span with 2x6.  All dimensional lumber has a crown.  The leading edge will have a hump or a sag when you sight down it.  Always install the joist, rafter or beam with the hump or crown up.  The crown will resist sagging.I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com