How do I replace a sub-floor that goes under a wall before reaching the nearest I-joist?

Question

In a bathroom, due to rotten sub-floor and remodelling, I have to remove the sub-floor and put a new one (we're changing the location of the shower and the sink; the plumber's going to make Swiss cheese out of it).

The challenge is that most of the sides of the sub-floor go under the walls. Some of these walls are right over the I-joist, some are not.

Where it's possible, I'm thinking of doing something like suggested here:

3. If the couple of inches of flooring you have not removed at the wall is solid, you can screw through it and into a piece of wood below it that the new floor can also be screwed to in order to make the edge supported, in a somewhat easier fashion. It's crudely equivalent to having a T&G joint in that the floor acts as a single piece.

But I'm less inspired about a proper way to fix it where the floor is completely rotten and unusable.

I've thought of doing something like this:

The plan:

1. Since I have I-joists there, I thought about reinforcing the I-joists by putting in there 1" x 8", held by construction adhesive and 1 1/2" screws (those pieces are in green in my graphic). The plan is to make it fit snugly in there.

2. Then, with joist hangers, adding in 2" x 6" in-between the reinforced joists and use them as a kind of bridge (displayed in blue in the graphic; the joist hangers are not shown). The top of these bridges would arrive at the same height than the I-joists.

3. Finally, still using joist hangers, adding a 2" x 4" in-between the two bridges, right under where the end of the new sub-floor would arrive. (This part is the yellow one in the graphic.)

A couple of things to keep in mind:

• I will not have access to the sides of the I-joists that are not under the bathroom sub-floors, so I can't reinforce the I-joists on both sides.
• If that matters, I'll be installing ceramic tiles over the new sub-floor (after putting on there 1/4" cement boards).

The questions:

1. Is the idea suggested by the other post is the most appropriate?
2. Is the plan suggested here makes any sense? And if it does are the planks sizes appropriate and should I use screws or nails to attach joist hangers (and what size)?
3. And if it does not, what should I do instead?

Thanks!

Answer 1

Your plan, as diagrammed seems pretty reasonable.

Does the 3/4" thickness of the 1/8 that you were proposing to glue to the center web of the I-beam bridge out the whole width of the top flange of the I-beam? If not you may want to consider using 2-by material instead. Your joist hangers will nail or screw much better into the 2-by construction lumber than they will the soft pine of the 1x8 board and the OSB of the I-beam webbing.

As you suggested you really do want to screw that filler piece in after applying the glue because you will not have access to add clamps to hold the pieces in place until the glue dries. You will definitely want to pre-drill clearance holes for the screws in the filler pieces to that the screws can pull them up snug to the I-beam webbing. Make sure to deploy screws of a type and thread size so that they bite into the OSB webbing securely without stripping out.