Expert: Bruce E. Johnson - 10/22/2009

Question
Bruce,
I am adding an addition onto the back of my house.  It will span 32' by 18'6".  I built 3 8" block walls and plan to tie into the existing house for the fouth.  My question is, would 2"x10"x20' beams running the length of the 18'6" span be OK?  I know they will support the weight fine, but I was not sure about bouncing.  This is going to be a mainly open area like a large family room/play room for the kids.  I thought of making one bedroom also, and thought the weight of the wall would easily be supported.  What do you think?  Safe to just run them with no support under them?

Answer
Hi Patrick, you need an intermediate support beam to cut the span of your joists in half..a 2x10 floor joist should only span 10 feet effectively, 18'-6' is too far.  The joists will sag under their own weight at that distance plus the bounce with be unbearable.  Install a carrier beam down the middle with  a row of 2x10 bridging between the joists right above the beam for a well built floor system..I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com