Raising tub subfloor 1.75"?

[C.H.H.]

We took out an old tub and found out the floor beneath the tiled area is 1.75" of concrete. So looks like the tub was installed and then the concrete was poured around it. We really don't want to smash all of the tile part up, so wondering what might be the best way to build up the subfloor beneath the tub. I don't think we will need a mortar layer to set the tub, so either a couple sheets of plywood/cement board, replace the 1" planks with 2" planks, add a layer of concrete, ...???

 

[Jadnashua]

It depends somewhat on what material the tub is made of. Some do want a mortar base to last and feel solid. That isn't a huge value, and you could do it with piles of deck mud mixed a little rich that would get smushed down when you set the tub and fill in some in between the piles. It's really hard to get it level if you try to make a solid surface of mortar underneath, and generally, it's really hard to pour in concrete. If it's a CI tub, you only need a ledger board(s) and maybe some shims underneath the feet.

 

[Sponsor]

 

[C.H.H.]

It's an acrylic tub with the leveling thing on the bottom, so I think I just need a little to keep it from rubbing and squeaking, but I don't want to put so much that it messes with that setup. So I might put a little down, but not like 1/4" or anything. And it has an attached apron, so I don't have any leeway on the tub bottom/apron levels.

 

[Cacher_Chick]

Only in a perfect world will the subfloor and the tub be perfectly level without making any adjustments, and very few if any acrylic tubs do not flex when not supported in mortar.

All tubs need to be leveled, using shims under the feet, or mortar, or a combination of both. If you want the tub to feel solid when you are done, you want to bed it in piles of mortar. Normally the finshed floor butts up against the edge of the apron, so any gap is unseen once the flooring is installed and caulked.

 

[Cacher_Chick]

If you want to raise the floor, use whatever combination of plywood subfloor thicknesses you need. It is also a good practice to waterproof the subflooring in the tub area to protect it from seepage.

 

[C.H.H.]

OK, so it's not a problem if I just stack up 3 sheets of plywood? I also just noticed that the old plywood is sitting on the joists, so I don't think there are any planks or anything in between. If I rip that up, could I do 2x8's to make it a bit thicker and then a sheet or two of plywood on top? Anything special I would need to do between sheets, or just screw them down?
Another question - The concrete tile area is level, but the height difference for the tub area is anywhere from 1" to 2" since the wood is not level. Is it better to have the subfloor uneven and make up the difference with mortar and shims, or should I try to get a level subfloor by shoring up the joists or anything?

 

[Jadnashua]

You need your tub to be level...if the subflooring underneath it isn't, probably the easiest way is with mortar. The first layer of subflooring should be applied with construction adhesive on the tops of the joists and either ring-shank nails or screws to hold it down. If you stack additional layers, I'd just screw them down. If you really want to glue them, you need to do it with something like Titebond wood glue, not a caulking tube construction adhesive and you need a full spread, then screw things together before it starts to set up. Is it stronger - yes, when done right, but it is a lot of work that is not generally needed. If you use a thick, viscous material like a construction adhesive, you'll generally have gaps and problems.

 

[C.H.H.]

OK, thanks, this helps a lot!

 

[Cacher_Chick]

You have to pay attention to the actual thickness of sheets of subfloor, as like most lumber, the actual is not what you might expect. The 1-1/4" ply will make the stiffest floor, but it is heavy to move. The floor joist spacing should be consistent throughout the structure.

If you plan on doing the entire room flooring, you might consider pouring self-leveling underlayment to flatten and level the subfloor, before laying the finished floor.