Expert: Dan Griffin - 1/31/2008

Question
Getting ready to help daughter develope attic of 80 year old timber frame house. Current 1st floor ceiling joists are full 2" x 6" yellow pine on 16" centers supporting lathe and plaster ceiling. Spans vary from 13' to 16'. For a 30# live and 10# dead load, are current joists sufficent for floor joists? If not,can I fasten something on top to stiffen them or must I put in sister joists of a larger dimension? I think I intuitively know the costly answer put need to hear it from a Pro.  Thanks for your assistance. Found your site by chance while trying to research this problem.

Answer
Jason,

I wish I could tell you something different, but your intuition is really good.  Here is a pretty good span table:
<http://www.awc.org/calculators/span/reversecalc/reversecalc.asp>

The 13' span will need 2x8
The 16' span will need 2x10

You will also need to verify the bearing capacity of the foundation/floor girder/stud wall/header capacities for a second floor.  These requirements are not huge, but bear verification.  At some point I suspect your building department will require a structural engineer's stamp of approval.

As you develop other questions, please feel free to come back.