Expert: Bruce E. Johnson - 2/16/2011

Question
We (husband and wife team) plan to install an attached 16x20 pergola this spring; the 20' section attached to the back of the garage and extending out 16' with a paver patio beneath (pavers already in place). Because of various landscaping and waterfeature currently in place, we would like to NOT use more than 2 corner posts.  I've seen numerous kits online for 16x20 attached but we're unsure as to what size lumber to use for ledger, posts, double beams and rafters. Foundation for corner posts is 42" below grade. Your suggestions?

Answer
Hi WJ, the sixteen foot length can be cut down to 12 feet by moving your outside beam in four feet thus enabling you to use 2x8's for ledger and rafters and letting them cantilever over the beam four feet at the front.  The twenty foot beam is another story unless you put knee bracing from each post to cut the span down somewhat.  Even then you are looking at a triple 2x12 on 6x6 posts.  If you go much smaller than that you risk the chance of the beam sagging under its own weight.  If your knee bracing extends out three feet on each side your beam span is decreased to 14' which you might be able to squeak by with a triple 2x10 for your outside support beam.  You have to remember that the longer the span the easier it is for the wood to sag over time.  By putting the crown of the lumber up you are helping to resist the gravitational pull.  I say 2x8 ledger if the rafters are butted into it and joist hangers are used to attach the joists to the wall of the garage.  If you are setting the rafters (joists) on top of the ledger you can get by with a 2x4 ledger if it is attached at each stud location at the garage. I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com