I have a small bathroom I would like to use ceramic tile on the floor.
The joists are 2x8 on 16 centers at 11.5 foot span with 5/8 exterior plywood sub floor.
The bathroom is straddling the main beam.
The tub and the whole room is 1/2 on one side of the beam and 1/2 on the other, so the whole bathroom is within 2-1/2 to 3 feet of the beam and the furthest part away from the beam is at the walls.
It seems to me that the deflection is minimal in this situation, but I am not sure and don't want to put it down if it will not be sound.
I was thinking of using 1/2" Hardi board on the floor.
Does that sound like it will fly?

If the span from the beam to the outside wall is the 11'6" then your floor is not even rated for ceramic tile. You could install sheet vinyl only as it is now.

Additional joists added to the existing ones or another support beam installed underneath are your only options.

Once you get the floor joists up to spec for tile, another layer of 1/2" plywood plus a tile underlayment should be installed.

1/2" Hardie is not necessary for the floor, 1/4" Hardie is fine.
FYI, no cement board or sheet membrane underlayments add any structural strength to a floor.

Thanks for your reply. I know what you say is true and probably, even though the floor joist system is sturdier over the beam, the scissor action of the joists meeting above the beam would cause problems. I was hoping it would be stiff enough for the tile.
The joists are below in an unfinished utility room, so access is not too much of a problem. I have open joist bays, so getting the sister joists in place shouldn't be too much of a problem.
I had basically the same problem in my kitchen. I doubled up the joists so now they are on 8" centers. Lots sturdier. One end sat on a ledger and the other was on a beam. The ledger was not a problem, but putting the joists up between the floor and the beam puzzled me until I found the solution. I made a tapered cut on the bottom of each joist that sat on the beam. It started about 3/4" from the bottom on the end and tapered down to nothing in about 8". That gave me room to put the joist in flat and rotate it upright over the beam, then slide it onto the ledger. Next, I used the piece I cut out as a shim to raise the joist up to the bottom of the floor, shimming on top of the beam. Glued, screwed and sturdy as can be.
I guess it's time to get some framing lumber.
Thanks again.
tenderfoot

If you're willing and able to add more joists, you should go ahead and do it cuz your floor will be stiffer. But, I'm not totally convinced that your framing doesn't meet minimum specs for ceramic and porcelain tiles. Just depends on the species and grade of your joists.

OK, so you have 2x8 on 16" o.c. that span 11.6" exactly between the face of the supports, right? Do you know the species and grade. If not, find out.

As I said, it's best to add some joists irrespective of what I'm telling you now, but you might be within specs.

Jaz

tenderfoot, your floor as specified in your post is real close to meeting a ceramic tile installation according to my calculations. But close is only to the bare minimum requirements. "More is better" as I say.
Jaz is right about knowing the lumber details to determine floor defection specifications. Other factors such as strapping, bridging or a combination of both can make substantial improvements in floor deflection ratings and span distances.

My joists are southern yellow pine in good shape. I looked closer and with the drain pipes and wiring, I could not get any sizable joist up beside the existing joists. I could get a couple, but not enough to help a lot.

Yes, I used the calculator and put in values only slightly different and passed for ceramic, but like you say, it's the bare minimum.
Like I said about my kitchen, I doubled up my joists to 8" oc. That made a nice sturdy floor, but I can't do that under the bath.
My wife likes some of the laminated flooring that goes well with the ceramic I'm using on the walls. She made an observation that takes some looking at. She slipped on the ceramic tile in the other bath and thinks the laminate will be less slippery.
We're 60 and 65, so not as agile as in our younger days.
Thanks.

I don't know how slippery the laminate would be, but I don't think you'll want to use it in your bathroom. Just the moisture from a hot shower can cause laminate to buckle and separate. Even leaving a wet towel on the floor for a couple of hours, or a small puddle of water that isn't promptly wiped up can cause the entire floor to be ruined. A leaking toilet is certain death to laminate.

I would strongly recommend that you consider something other than laminate or wood for your bathroom.