I forgot about that thread.

As to the 2x4s on the flat or on edge, you'll have a more rigid top by putting them on edge. Not sure why I drew it the other way!

Yes, you can build the deck on a 3/4" subfloor.

I always have built room walls over the subfloor and then installed underlayment where needed, between the walls. You can put underlayment in the shower area and build the pan and curb on top of the underlayment.

Install a "slip sheet" of tar paper or plastic on top of the subfloor so the deck mud can move independent of the plywood.

Don't install tar paper as the slip sheet over radiant floor heat. Use plastic instead.

Thanks so much for your reply, Mongo!

... so the 3/8" ply is considered underlayment, right? So: we build the tub deck and the shower partition wall right on the 3/4" advantech subfloor, then lay the 3/8 underlayment everywhere else and build the shower curb on top of that. I think I got it!

I'm still not totally clear on the tub framing though. Do you build the "wall" parts 3.5" shorter than the entire height and then make the " mini floor" part set on top of those? Do you think its really necessary to do that when the deck is only 2" on either side and 8" on the end of the tub, recessed into a 3-wall alcove? I will do so if its best, just wanted to be sure.... I was going to do two long "walls" and then four short ones -two on each end- between the "walls" to support the 3/4 ply top.

Oh- another question I just thought of!! Given my dimensions described above, is it important to use a whole sheet of ply and cut the recess out of it, or can it be pieced together from smaller chunks?

I'd build something like this:

That drawing just depicts the top frame made of 2x4s on edge. 3-1/2" thick.

Add legs under it to get the height you need.

On top of this you'll have 3/4" advantech, then 1/2" cement board, then what, say 1/8" of thinset and 3/8" tile? that's 1-3/4" of topping over the 3-1/2" of 2x4, for a total thickness of 5-1/4".

So if you wanted the top of your tile to be 20" above the bathroom subfloor, your legs would be 14-3/4" tall.

Note that I doubled up the sides.

You could cover it as drawn with advantech pieces instead of cutting the top from one full sheet.

Don't have cement board seams fall over advantech seams.

Now if you were to build it with a single 2x4 on each side instead of the doubled up 2x4s on each side, then I'd say cut the top out of a single sheet of advantech.

Does that make sense?

wow, that's great, Mongo, thank you!!!

It does make sense:)

One clarification: the legs on the back and sides (i.e. against the existing walls)... do they need a "sole plate" under them or can they just be like legs sitting on the subfloor?

I assume that the FRONT legs need a sole plate, so that there's something to fasten the bottom edge of the cement board to???

This is very timely, I think this is getting framed tonight or tomorrow, and I can print it out and show my handyman who's helping with framing.

Thank you SOOOOO much!

Stacey

Correct, you don't need a sole plate on the back or side walls. The front apron framing would need a sole plate for cement board support, and the front apron legs would have to be positioned to fall behind any seams in the cement board.

Not sure what type of tub this is, but keep in mind any needed access panels for motors, etc. Either through the front apron wall or one of the other walls. Your tub installation manual will have that info.

You're welcome!

Mongo

Thank you so much, Mongo. You are awesome :)

It's just a plain soaking tub (Kohler Archer) with no jets or anything. So we just have the plumbing access panel on the back side of one of the sort ends (in an adjacent closet).