Old Subfloor Under Cast Iron Tub

[Elliot Coard]

Hey everyone, this is my first DIY renovation. I'm mechanically inclined (Auto tech by trade), and I pride myself on research, just so I have a general idea and am not completely ignorant to what's going on. With that being said, My home was built in 1924, I just gutted the bathroom, and decided to remove the the tub to modernize the space, as it's only 5x5. After smashing the cast iron tub to pieces, I noticed that the sub-floor underneath is uneven.

It looks as if it mortar, or sand even, compacted between wooden beams (Joists? Forgive me for use of incorrect terminology if any). I Plan on using a KBRS tile basin (30x60). Manufacturers specs calls for an application of thinset on plywood to set the base. Am I able to smooth out the existing subfloor by chiseling out the higher pieces? Would I then be able to add a piece of plywood, ensuring it's leveled of course, to set the base? Or would more of the mortar have to be poured to ensure a leveled surface before adding the plywood? Any insight is greatly appreciated!

Attached are pictures of the subfloor. Don't mind the galvanized piping, as that's getting upgraded too.

 

[wwhitney]

If you look closely at the area around the tub shoe (the drain that was on the bottom of the tub), I think you will see that there are boards running between the joists at level a 1-2" below the top of the joists. In between would likely be what's called a mud bed, which is a lean mixture of sand and Portland cement and is a setting bed for tile. So I assume the mud bed extends under the tile, and you should be able to see that at the edge of the tiled area.

Often the mud bed is set on a normal subfloor that is on top of the joists, which would then make the tiled area much higher than an adjoining wood floor. So I'm guessing that the mud bed was recessed into the floor system to avoid that, although I would think it would cause the opposite problem of the tile being lower than an adjoining wood floor. Or maybe the mud bed is only partially recessed, and in the tiled area there is a thin layer of mud bed on top of the joists?

If the existing tile is staying, then you probably should not disturb the mud bed. If the area to get plywood is bounded by joists on both ends, I suggest cleaning off the tops of the joists, adding thin shims as required to bring the joist tops into a level plane that is unobstructed by the mud bed, and then installing 3/4" subfloor plywood (not CDX). Assuming the tile is not 3/4" thick, this would leave the edge of the plywood exposed. So you'd want some sort of finish transition material to cover that.

If the existing tile is to be removed, then just remove the mud bed everywhere and put in a new subfloor in the whole room.

Cheers, Wayne

 

[Sponsor]

 

[Elliot Coard]

If you look closely at the area around the tub shoe (the drain that was on the bottom of the tub), I think you will see that there are boards running between the joists at level a 1-2" below the top of the joists. In between would likely be what's called a mud bed, which is a lean mixture of sand and Portland cement and is a setting bed for tile. So I assume the mud bed extends under the tile, and you should be able to see that at the edge of the tiled area.

Often the mud bed is set on a normal subfloor that is on top of the joists, which would then make the tiled area much higher than an adjoining wood floor. So I'm guessing that the mud bed was recessed into the floor system to avoid that, although I would think it would cause the opposite problem of the tile being lower than an adjoining wood floor. Or maybe the mud bed is only partially recessed, and in the tiled area there is a thin layer of mud bed on top of the joists?

If the existing tile is staying, then you probably should not disturb the mud bed. If the area to get plywood is bounded by joists on both ends, I suggest cleaning off the tops of the joists, adding thin shims as required to bring the joist tops into a level plane that is unobstructed by the mud bed, and then installing 3/4" subfloor plywood (not CDX). Assuming the tile is not 3/4" thick, this would leave the edge of the plywood exposed. So you'd want some sort of finish transition material to cover that.

If the existing tile is to be removed, then just remove the mud bed everywhere and put in a new subfloor in the whole room.

Cheers, Wayne

Appreciate the clarification Wayne, I do see the the boards underneath the tub shoe, so it's safe to say that there is definitely a mud bed there. The existing tile is staying, and it looks as if the mud bed is consistent underneath, there seems to be 2'' of concrete poured on top, which is where the tiles sit. Luckily the space is bounded by joist on both ends, so I'll prep the surface for the plywood application. Thanks a lot!