Expert: Bruce E. Johnson - 4/8/2009

Question
i need to build a pergola on my deck. it faces south with no shade.my deck is 18'wide by 20' deep can i span with post just on the perimeter ?

Answer
Hi angus, 18-20 feet is a long span for nominal lumber.  You might be able to run a hefty beam mid way to cut one of the spans in half.  A 6x12 maybe?  The problem with these long spans is that unsupported lumber will beging to sag under its own weight over time.  Plus the bigger the joist the heavier it is so if you get bigger wood to make the spans then your supporting wood needs to be heftier.  I can't think of any reasonable way to build this pergola using minimal lumber sizes.  If you can create a beam capable of spanning the 18 or 20 feet and split your joist span down to 9-10 feet then you can use 2x6 or 2x8 for your rafters/ joists.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com