Expert: Dan Griffin - 10/19/2009

Question
QUESTION: Sir,

A friend of mine is remodeling an old home, in which he is rennovating an attic to make it into a living space, complete with a full bath. Unfortunately the joists are 2x6's, spaced 24" apart. The span length is about 13' 6". The existing joists are made out of hemlock. Despite that, I calculate that the maximum horizontal span to be just over 8'. Even calculating putting 2x6 joists every 12" apart, doesn't seem to do the trick either. I can only get a 10'  span out of it. My recommendation was to make the joists out of 2x10's. Hovever, he's already too far along putting in the plumming and rough wiring and such. The housing inspector has already failed the structual inspection. The qusstion is, how much more strength will my friend achieve is he sisters the joists? The Span tables give no guidance on this, although I suspect what is needed is board depth, not width.

Please advise.

Thansk,

Mark Hoffman

ANSWER: Mark, you already know the answer to this one, but let's look at a few other possibilities.  Whatever he does now will probably require and engineer's stamp as he has already red flagged the inspector.

It might be possible to go in the room under the remodel and run a beam under the ceiling/floor joists. to reduce the span on the existing system.  The beam will require proper sizing, support, and bearing.  It can be boxed in gyp or made to look like a real wood beam.  It may require adding more than the one to make it look intentional.  Look up a coffered ceiling for a visual.

I doubt that  the situation can be handled with any sistering of 2x6, though flitch plates could make the span.  This would involve double lumber sandwiched on each side of a steel plate.  I suspect this would need to done on every joist.

YOur friend got off to  a real bad start, 24" centers won't carry many typec of flooring.

I hope that one these ideas might help.

---------- FOLLOW-UP ----------

QUESTION: Thanks, that was helpful. However, being an electrical engineer and a physics teacher I would like to know how the span length is calculated so I can play around with the variables; though my mecvhanical statics is a bit rusty in this area. If you could point me to any technical documentation that specifies how to calculate the span length, I would be obliged. I suspect is a set of equations using deflection, moment of inertial,  as well as the physics properties of the lumber.

Thanks,

Mark Hoffman

Answer
Mark, the formulae for spans is readily available.  I prefer using the organized span tables such as:
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

Another that will work with flitch plates is here:
http://www.toolbase.org/Design-Construction-Guides/Exterior-Walls/flitch-plate-b

If you really want to work the math:
http://www.efunda.com/formulae/formula_index.cfm