Mud bed floor in 40's bathroom

[MustangDave]

Hello all. I’m in the process of a full rehab on my main bathroom in a 1940’s brick house in Pennsylvania. The original floor cracked because the previous owner, genius that he was, removed a support from the center joist which I have now fixed. Original floor was mud bed that I removed completely.

The construction of the old floor was something I haven’t seen before. A wood subfloor was installed between the joists about 2 ¼” below the tops of the joists. The mud bed was installed on top of the subfloor to a depth of 3”, leaving ¾” above the tops of the joists. Then the tile was installed over top. All of the plumbing was embedded in the mud bed also. Floor dimensions are 10 ft by 4 ft.

Wife wants radiant heat in the new floor, and we’re going with hydronic / pex. Drawing attached. The only change from the old floor is the pex.

- I’m gathering there needs to be a moisture barrier between the subfloor and mud bed. Planning to use 4:1 deck mud unless you suggest otherwise.

- I’ve read in other threads that reinforcing mesh should also be used. Is this recommended and would it be applied on top of the joists?

- Anything else that needs to consider here? Better ways to do this job?

I’ve done a number of small, simple floor projects, but I feel this one warrants some serious advice to get it right.

Any help is greatly appreciated!

[ss3964spd]

Welcome, Dave,

My first question, given the notches in the joists tops, is what is the free span of those notched joists?

[MustangDave]

Thanks for the reply. The free span is about 12 or 13 feet. This goes from the centerline of the house to the outside wall. All of the houses in the neighborhood use the same construction, and there is a surprising number of notches in this construction. At least they're 3" beams.

[ss3964spd]

Not sure I understand - what beams?

The joists appear, from here anyway, to be typical 2X something joists. What I should have also asked is how tall are they. IOW, are they nominal 2X8's? 2X10's? If they are 2X8's and the free span is 12' or 13' they're over spanned without the notches, and if 2X10's they're right on the edge, depending on how close the notches are to either supported end.

If there is a 3" beam somewhere within that 12' or 13' span that's a different scenario.

[MustangDave]

Apologies, i meant to write joists not beams. The joists are 3 x 10's throughout the house. I've measured them and they are in fact 3 x 10's on 16" centers.

[Dave Gobis]

I think your going to get cracks on the tops of the joist due to changes in mass and thermal variation.

[MustangDave]

Hey Dave. Thanks for the reply. Thinking about this a bit and I can see your point. Its the heating that would change the dynamic here.

One of the potential methods I was thinking about is to minimize the depth of mud between the joists, bring the mud up to the top of the joists, then put 3/4 Kerdi board screwed to the joists and bonded with thinset to the mud.

Any thoughts on something along these lines?