Hello, you will see that I'm new.
I am doing a complete renovation of our bathroom. It will be gutted and rebuilt new. I am working on a bill of materials. I am reading up on the subject, here and at Schluter's.

For now I expect to remove the flooring and bridging and double up the 2 x 8's where possible. They are on 16" centers spanning 9' now. Then install one layer of 3/4 plywood screwed and glued into the joists. Another layer of 3/4 ply offset 4" or so and screwed into the first ply layer. The second layer to have 1/8" gap at butt joints and 1/4" gap at perimeter. Then I plan on using regular Ditra as per Schluter's instructions with joints done with Kerdi band.

On top will be 12" porcelain tiles. I will install an acrylic tub shower, new store bought vanity and cupboards, toilet etc. It makes sense to me to just do the whole 6' x 12' room with the tile, and plunk the things down on top of it all. I believe that would reduce cutting and fitting, maybe look better, and be simpler to seal and waterproof.
Am I thinking straight here? Is there a reason to not do it this way?

I still have not read up on how to do penetrations such as toilet flanges, tub drain, etc.

If the Schluter level transition product will not work I can make something homemade out of oak.
The house has had additions and renovations before. We are use to differing floor level joints and they are not a big deal to us at this time.

Hello, Bluejay.

Any particular reason you're doubling up the joists? At nine feet of span, they're more than adequate for a tile installation, provided they're in good shape. Also, if you use tongue and groove 3/4" ply for your subfloor, there's really no need to add another layer.

To answer your question, yes, I would tile the open floor and add the vanity, etc. after the tile is installed, but I would install the tub/shower before the tile. The tub/shower can sit directly on the subfloor. Just cut out an area big enough for the drain.

As for the toilet flange, how high does it sit now? Is it right down on top of the subfloor?

Thanks Kman.
After reviewing the tub shower instructions I can see it will be advantageous to not tile under. This is partly because of the alcove you need to build beforehand. I do see how it will be good to tile before adding the vanity.

The toilet flange and piping will be moved and installed new. I do not remember what it looks like under it now. I presume the flange rests on the plywood and is screwed down, but I can't remember.

The doubling up of the plywood and some of the joists is to insure a rock solid base. I have seen examples of "overbuilt" floors in new construction. The joist spacing can be tightened up to 12", and the joist dimension can be increased beyond the code specification. I definitely notice a better "feel" in these examples. Part of it is a change in the way vibrations propagate.

Besides this, I have been known to travel in the winter. I drain the plumbing and leave the heat off. Not only do things get very cold, the relative humidity goes pretty wild as well. Doubling the wood in a 6' x 12' area will be the least expensive item on my whole "to do" list, and I only have this one chance.

My thought now would be to use tongue and groove for the base layer of ply, and cover with non tongue and groove, leaving the 1/8" expansion gaps. Is this correct or partly correct? I have seen things swell in this house after an extended absence.

You are on the right track. Tiling under the vanity allows for renovations later without having to re-do the tile.

Over building is fine and I'm not going to try and talk you out of it, but 3/4 ply plus Ditra is sufficient. Or 2 layers of 5/8.
Make sure it's fir plywood.

Pre-finish your oak transitions or add them after all grouting is done. Alkali water from concrete products makes oak go black.

I am going to re think the plywood question, seeing two experts are advising against. What got it in my head is, that is how my neighbor did his place.

Just for interests sake, would a 3/4" layer plus 1/2" layer be as good or better than two 5/8" layers?

And - assuming I try to get fir exterior plywood, would you recommend good both sides, good one side, knots on both sides?
If both layers don't need to be good 2 sides, which layer on top? I am thinking the bottom structural layer should have a higher quality level than the upper "underlay" layer. You can thinset into knots, oui?

Some of this is academic because I will have to choose from what they have in the warehouse. I know they have good 2 sides 3/4 but maybe not the other stuff?

You don't need any good sides on the ply. It all gets troweled over with thinset anyway. As far a s 3/4 plus 1/2 vs 2x5/8, the latter tends to be more economical because you can use up some of the cut offs from the first layer in the second.

Okay, assume for now I will use 2 layers of 5/8 exterior fir plywood. It has been suggested to use tongue and groove plywood.

Would I use 2 layers of T&G ?
Would spacing gaps exist only on the ends?
Should the gaps be 1/8" or 1/4" ?
Should the gaps be present in both layers?
Should the 2 layers be turned 90 degrees to each other?
Should the gaps have thinset pressed into them? I can't see how to avoid this when pressing the Ditra down into thinset.

The second layer is normally not t&g, but can be if the joints are not tight. both layers run at right angles to the joists with the joints offset on the second layer. 1/8th inch gap in all joints of second layer only is best but slightly bigger is ok too. the second layer is not screwed to the joists, just into the first layer.

The reason for the gaps is that the plywood absorbs some of the water from the thinset and swells a little bit. getting thinset into the gaps is fine, because this does not prevent panel movement.

Perfect, that explains a lot.