Expert: Bruce E. Johnson - 6/26/2011

Question
I want to put a patio roof on the back of my home. If I want to span 16 feet between posts is this possible. Can I use double  2 by 12s with plywood in between to make a beam? I am trying to eliminate a 3rd post to carry this beam.

Answer
Hii Bob, Southern Yellow Pine or Douglas fir could make this span.  16' is pretty much the limit of what I would call a "sensible span" for this type of beam but it should work.  LVL or laminated beams are good alternatives as well.  A 12" LVL is 1 3/4" thick and two of them together combine to make up the 3 1/2" as well.  If you stick to your original design be sure to use a good exterior construction adhesive (Liquid Nail) and stagger your coated screws or galvanized nails in a 4" pattern no closer than 1 1/2" from the edge of the beam..in other words a row of nails/screws 8" apart at the top of the beam 1 1/2" from the edge and a row of nails/screws 8" apart at the bottom of the beam 1 1/2" from the edge but offset so that they are in between the nails at the top of the beam.  Some people like to follow up with a row of nails/screws down the center of the beam as well but this may tend to split the wood if the nails are too close together along an individual grain.  Make sure all the wood is installed with the "crown" up.  If you sight down the edge of 2x12 you will notice that there is a high spot on one side and a corresponding low spot on the opposite side.  By putting the high side up you, in essence are giving the beam a chance to "settle" down and support it's own weight without sagging.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce johnson..bejohnsonconsulting.com