I recently opened up the ceiling below my bathroom and was surprised by what I found. The floor joists are 2 1/2" x 8" x 15.5' and 16" on center. This seems way too small for the span. Also, some of the joists have been cut to accommodate drain pipes. I'm going to be re-doing the bathroom, adding concrete board and tile, and I want to make sure the load, along with the load of the tub, can be properly supported. I'm contemplating sistering the existing joists with 2 x 8 x 16' joists. Will this be sufficient to handle the load? The bathtub is about 5 feet in from one wall. To complicate things, the house is balloon framed, so I will have to attach a ribbon or ledger to the 2 x 4s to support the new beams, but I'm concerned about what kind of fasteners to use (the sheer strength of the fasteners). Any info would be great.

Thanks

First - Are you sure that your existing joists are 2-1/2" x 8"? I only ask because that is an unusual size - more common is 1-1/2" x 8.

Moving on:

Sistering on another 2x8 alongside the existing ones will help, but not that much. Especially when you factor in the tub and the tile floors. Tile floors need to be pretty stiff so any 8" wood framing, even double 2x8's will certainly flex over a 15-1/2' span.

What it really needs is a larger joist - 2x10's or 2x12's would work much better. Even at that though, a 2x10 floor 16" OC is maxed out at 15' so it cannot support a tile floor. And I understand that you can't add the larger framing as it sounds like the bath is upstairs, with finished living space below.

That does limit your options.

I think the best was for the stiffness you need would be to add a few steel filch plates that are sistered alongside the existing 2x8's. One plate on every other joist should do it. The down side is that you'll have to get a steel fab shop to make them us and install is difficult as they are very heavy. But, it would do the trick.

Given the house is balloon framed, I'd assume it is fairly old, correct?

Got any pictures?
Not sure where your thinking of adding 2 X 4's.
I hope that's not what your thinking of using for added supports.