Is the floor of that bath (joists and subfloor) strong enough to hold that tub when full of water and you?

How much will it deflect - and will your tile and grout crumble from the flexing.

If you have space around the tub, you need to figure out how to deal with splashed water ... some sort of drain back there or under the tub.

Thanks very much for the responses.

The subfloor is 3/4" t+g ply atop 9.5" open web engineered joists, new construction as of 2016. Would you consider that adequate support to go right to the Ditra/tile? If so, it sounds like the right solution for us in this case.

You start w/ deflection and tile material rating

You make the decision based on deflection score or rating..... Open web refers to trusses typically and trusses usualy have a stamp with mfg rating (most of the time i find trusses mfg deflection rating around L440 and a second layer subfloor is needed)

If you have 9.5" joist you will measure deflection via span and supporting members with the size and condition of joist and spacing factored in. This will equate to a deflection rating (IE L720)

with this deflection rating and the tile material itself accounted for you will determine if yourr 3/4 tg ply is adequate and further select a membrane and if needed additional plywood layer.

Remember cemend board even when embedded in thinset properly is NOT adding structural strength to the floor , I NEVER use it unless i need it as a bonding medium or to raise the floor install elevation and ONLY if the subfloor already meets deflection for the tile material (IE stone porcelain ceramic )

The best tile substrate over joist/truss structures is plywood and membrane (when installed properly) IMHO - best

Ok, that is good to know, thank you. I have the joist specs at home somewhere, with the other beam reports from the engineer.

From what I recall (I am out of town and it's been some months since installation) the TJIs are Trus Joist (Weyerhaeuser) brand, 9.5" wide, open web in the middle and OSB-filled webs on one end.

The span of the TJIs comprising the bathroom floor is a continuous 12', with one end sitting on the upper rim of the exterior 2x6 wall, and the other sitting halfway across the thickness of a central LVL beam, which I believe is 16" wide. TJIs are on 19.2" (or whatever black diamond spacing is) centers. Full 3/4" thick t+g plywood sheathing lays on top, liquid-nailed and mechanically nailed to the tops of each TJI.

The only thing I can't remember (and therefore am having trouble using the manufacturer's load charts) is how wide the flange is on the TJI. I think it was the smallest, 1 3/4", making them the TJI 110 variety.

Also unknown at present is the capacity of the tub (and therefore the load it would introduce when filled with water and people).

Also unknown is the deflection tolerance of the ceramic tile, as we have not picked it out yet. Our plan is to lay down the small octagonal tiles, which I am hoping I can find locally in rolls :)

I will also have to look into what effect the Ditra material has on decreasing deflection stresses on the tile.

So many things to calculate!

Yep!

That is a big span and only 9.5". Try and find a LXXX deflection number or a telephone number for the MFG stamped on truss. Typical dimensional lumber joist calculations are more straight fwd but A second layer of EXP1 5/8 ply would help regardless.

Just a note, Ditra itself only alots for 2" or larger tile so no 3/4 or 1" mosaics. I have used ditra on a TGI + 3/4 ply floor before but there were supporting members and an architect taking liability, Perhaps take a look at NoblesealTS as a thinner membrane option that allows small mosaics in combination with a second layer ply IMHO. - best

Thanks very much for the followup! I will gather that information and move forward, most likely with another layer of ply and something like Nobleseal.

Hi y'all. I realize this thread is a year old so am hoping it'll still be read. Curious what Benjamin ended up doing.

I'm actually doing something very similar. I am renovating my master bath and took out the carpet floor. I'm in the process of tiling my master bath which is roughly 12'L and 10' W. As you enter, along the left side are the toilet (untouched), 48" one-piece shower stall which I replaced and garden tub tucked in the corner which has been removed. I re-piped the plumbing to accommodate a new freestanding 60X32 freestanding tub to be placed close to were the garden tub was.

The subfloor is 3/4" ply with 10" wood joists, 16" OC. I laid Schluter Ditra for the membrane. I am using 12X24 ceramic tiles. The new freestanding tub is a 2-piece acrylic so its fairly light. It has 4 adjustable legs hidden behind the apron.

I have the same question about whether to tile all the way thru and set the tub legs on the tile.

As far as deflection, the tub will be very closed to the walls so am thinking there won't be much deflection if any. I would think deflection would be a concern if the weight is in the middle of the floor. I also ran my plans by the local building inspector who is familiar with the construction of the builder of my house and he said its not a problem. Plus the fact that there used to be a garden tub practically in the same spot. Not having laid tile before my concern is just the weight of the tub when filled and its effect on the tiles.

Tundra, thanks for response. My bottom line question is whether to set tub on tile.

I cant help you without the full equation Best..