I have a home built in 1890. It is a 1.5 story plus 1 according to the city. I have a full size staircase that walks up to my attic. My house is 700sq feet on the 1st and 2nd floor. I have a Chimney running dead center of my attic though the whole house. The roof line is plus or X shaped depending on how you look at it with a very steap pitched room at roughly a 45 degree angle.

My attic floor boards are 1x8x10' or so laid out accross the attic floor. they aren't tongue and groove but they definately overlap.

I have no deflection in the attic dispite my jumping up and down up there. I started building half walls and ran electrical up to my attic. It is then that I discovered that my joists in the attic are only 2x4's spaced 16" apart. bottom side of joists have lathe and plaster boards nailed to them.

I have two questions. Is it possible this floor is strong enough to finish the remodel? If not, can I add additional joists to be safe? Can I perhaps buy wall to wall floor boards to add strength?

I called an engineering firm and a 300lb guy came over and looked at it with me. He said that he'd need his boss to look at it but to him it didn't seam like hte floor moved at all and he'd probably just finish off the rooms.

I'm looking for some second opinions between the now and when his boss comes over.