How should I build a raised subfloor for my basement bathroom?

Question

I've ripped out the bathroom in my basement that was finished by the previous homeowner. I'm now rebuilding, and curious to the most correct way to go about this.

The whole basement was finished using 2x4's running over the basement concrete, fiberglass insulation between them, (paper side up) plywood on top, tiles directly on plywood. No vapor barrier above or below the slab. (The fiberglass in the rest of the floors WILL eventually be replaced and maybe the floor lowered, I know the moisture plus fiberglass will be bad)

The bathroom floor was previously raised about 10 extra inches relative to the rest of the floor, for drain pipes that were run above the slab. There was no headroom. Now I've cut up the concrete floor, had a plumber run pipes, re-filled with bagged concrete, and leveled the cutout with self leveling cement.

Due to the way my house sits, the main drainage pipe was only 1" deep in the slab, meaning that as the pipes go across the bathroom, sloping down into the main drain, parts of pipe are only half-under the concrete, and parts are completely over the concrete. At the highest point, one pipe is 2.5" over. This means I'll have to raise the subfloor to at least that, and that's fine because it'll match the rest of the basement. I'll have to notch the floor joists for pipes, which is why I did self-leveling underlayment - I want everything to be supported well so tiles don't crack.

Now that the scenario is described, I just don't know where to start. I'd like to just put pressure treated 2x4's down, laying flat, ramsetted into the concrete. I'm not sure how that will stick to my fresh concrete/self-leveled substrate, and I don't know if I need a vapor barrier under that? Directly on top of those 2x4's, I'd like to lay down regular 1x4 and just screw them together, giving me a floor height of 2.25". I'd like to put 2" foam board between the supports, but I don't know if that's the right type of insulation and I'm not sure what type of board to use.

At this height, the floor will clear everything except for a patch directly in front of the toilet, where the thickness of the 90 degree elbow will be 1/4" above the top of the floor joists. I figure I can just take a router to the floor board there to allow it to clear the pipe, as nobody will ever step directly there... the toilet is on top of it, and the toilet will be well supported in other areas.

On top of that would be 1/2" cement board, ditra, thinset and tiles.. I'd end up with roughly 4" tall floors, which is actually the same height as the rest of the basement!

Now I just realized what another problem is.. if I do it this way, the 90 degree PVC bend that's intended for the toilet will be about 1/4" above the tile. Is that too high to work with? Could I take an oscillating tool and cut it flush with the flooring so I can install a closet flange? Is there a certain depth that 3" pvc fittings need to mate?

Answer 1

Lots of questions (related to this Meta discussion). I will just tackle a few points I feel confident about answering.

Subfloor

The Schluter DITRA does not need to be on CBU so I would skip the 1/2" layer of cement board you are planning on using and make your plywood subfloor thicker (or just skip the CBU and keep your 23/32" OSB). CBU is not "stiff" enough to be a subfloor, it can be used instead of DITRA but it must be secured to a subfloor. It rarely makes sense to use 1/2" on the floor unless you are trying to make the finished floor come out to some specific height (for example to help the transition to another room with a different floor). If you are not goin gto use DITRA, you will want some CBU on the floor but the 1/4" stuff is fine.

Apply the modified thinset and DITRA directly to the plywood subfloor. Checkout Schluter's DITRA Installation Instructions for details on the subfloor prep and how to secure to DITRA to the plywood.

Toilet

Without a picture or diagram it is not really clear to me exactly what the issue is with the toilet drain, but that won't stop me from taking a guess :)

You may be right about getting away with cutting a larger hole in the subfloor to handle the bend of the toilet drain pipe. Are you sure the PVC drain will stick above the finished floor? Realize that the thinset between both the subfloor/DITRA (not very much) and DITRA/tile (more so) will add a little to your height, maybe between 1/8" to 1/4".

Have you measured the height of the toilet drain with the flange installed (just dry fit it to take a measurement)? The flange can be installed on the top of the finished floor (tile), on top of just the DITRA, or on top of the plywood subfloor. In your case you would probably want it as high as you can, so install the tile first and then glue the flange to the PVC closet bend. I think once you dry-fit the toilet flange you will see you are OK here.