Expert: Bruce E. Johnson - 6/19/2011

Question
Hi, I was wondering if you could help me? I live in Chicago area and want build a garage 24wx30dx18h. I was planning on a Cape Cod style and now looking at a Gambrel for more room upstairs. Typical monolithic foundation, 2x6x8 lower walls. I have a quote to run Gambrel trusses 2x4/2x10, at 2'o.c. with a girder truss for the stair way. It has a 25psf snow load. but at 2'oc is this going to be a bouncy loft, they did not give me LL/DL. I was thinking about installing a 30' I-beam to carry the second floor and run 2x10 for the floor joists and run them into the wide flange I-beam so I get the 8' clearance on lower level and beam is hidden. Then 3/4 T&G glued and screwed. If I stick build gambrel, how far in off the 24'w or outside walls can I come in for the Purlin walls?  Is there a formula for that?  I'm looking for a LL of 40/10 or 20 for DL?

Answer
Hi George, a 24' span for 2x10 is going to be a bit bouncy at 24" centers.  By going to a 12" or 16" center layout will help along with three rows of bridging blocks, one at the center of the span and six feet each direction from center.  I'm not sure what you are calling purlin walls? unless you mean a vertical wall above the attic floor?  Normally a short stem wall of this type is 1/3 the distance to the center of the span so in your case this would be 4', however gambrel roofs don't normally require this wall for support.  An I-beam that will run 30' will be a tough install since a beam of that length really isn't accomplishing much with your design.  If the beam ran under your joists it won't be required to support the entire weight of the attic but your design would make the I-beam the only support for those joists so now you need to calculate not only the beam span capable of holding up its own weight but also capable of bearing the entire floor load.  I would opt for the engineered truss system and insist that the truss manufacturer design a floor to suit your needs.  They have the engineering software for this.  Another option would be to add corresponding lvls in between the two foot truss layout for additional support..I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce johnson..bejohnsonconsulting.com