Sister floor joists or build support wall?

Bcoleman6

I have two projects in the works. The first is building a bedroom in the basement below the kitchen and the second is to tile the kitchen floor. The joists are true 2 x 8, 16" OC with a 12' span. According to John Bridge's deflection calculator this is borderline for ceramic tile. Currently the sub floor is 1 x 8 boards which run at an angle.
So what to do? When I build the bedroom it will include a wall perpendicular to the joists, is it possible to build this wall to be load bearing? If so it would make the span 10' or should I just sister the joists? If i do either of the above can I just use 1/4" hardibacker on top of the current plank sub floor, or do I need to rip out the planks and put down 3/4" ply and 1/4" hardibacker or leave the planks and put 5/8" ply and hardibacker. Not doing the kitchen floor until next year but if I need to sister the joists, now is the time.

Sorry for the long post.
Thanks,

Ben

Rehabber

If you are going to build that wall anaway, that will be sufficient support to reduce the deflection

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Bcoleman6

Thanks for the quick reply.

Will a 2 x 4 header be enough?

Rehabber

If you line up the studs below the joists, that should be fine

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If you think you can, or if you think you can't, you're probably right.

ASE certified master auto tech, Retired.