Hi,

We’re doing an attic conversion for a house built around 1890.  We have 2 x 6 joists 16 O.C. in the attic currently and obviously this isn’t enough.  Wisconsin requires a 40 Lb live load and the span at the widest is 20 feet. 

I found some I- joists at a lumber center here that are rated for the span, the problem is when we did a dry run with 2 – 2 x 10s attached end to end we found out that due to the extreme roof slope, we can’t get them in!  We’re several inches too long, the walls have true 2 x 4s (the studs) and we only have about 20 – 24″ clearance between the floor to where the roof starts.

We might be able to wedge 2 x 6’s in, sistering them up to the existing joists.  I also heard that Sturdi floor can be added into my calculations…but I can’t find anything that gives me spans when you use more than 1 piece of lumber, and then adding in the Sturdi floor…I’m going to have to convince the inspector with some kind of calculations.  We also investigated putting some I-joists perpendicular to where the joists will go, and then using shorter lumber, but we had a couple of issues with that too (large spans, no structural walls that go down to basement, plus my general worries about cutting the existing joists to fit the beam in there).

The inspector mentioned that he has seen calcs using 2 x 6s and approved projects, but said I have to come up with numbers for my specific project and didn’t have any ideas for how to go about getting them.

Has anyone done this?  I can’t believe I’m the only one who’s run into this, but after checking at several building centers and lumber companies and starting to wonder…

Thanks for any advice you can give!