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Unread 02-14-2020, 05:17 AM   #1
Surfnsail
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Zero entry framing with mud bed

I am remodeling my upstairs bathroom and would like to drop the subfloor down, flush with my joists (~16 OC), all of the joists have been sistered previously. My subfloor is 5/8 with 3/4 on top in the areas around the shower. I plan to put the same between the joists. I am then planning to make a 4:1 mud bed with lath stapled to the floor over 15lb roofing felt. The mud bed will end at a center drain that is 3/4" thick and goes up to 1 1/2" at the edges. That gives me a 1/4" per foot slope in the long direction of the pan (6'x4'). Lastly, I plan to install schluter kerdi membrane over it and the walls with ditra outside the shower. Is this mud bed sufficient and OK over the subfloor flush with joists? Should I add 1/2 plywood over my joists and framing above for a mud bed and raise my floor too? I'd prefer not to raise the floor.
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Unread 02-14-2020, 09:38 AM   #2
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Welcome back, Spencer.

If memory serves correctly, the mud bed over a subfloor like yours should be a minimum of 1 1/4" thick. So you would need to lower the subfloor more to get the additional thickness.

But I wouldn't set a mud floor like you have planned over a subfloor that is between the joists only. I'd want some plywood on top of the joists.

Also, the 1/4" per foot rule on the shower floor is a bare minimum. I usually set mine at about 3/8", and sometimes more depending on the tile that is to be installed.

Do you have enough height on the floor joists to cut out 2"?
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Unread 02-14-2020, 02:23 PM   #3
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I am able to get the shower subfloor 2" below the other subfloor which would give me 1 1/4 mud bed but It would still be over subfloor level with the joists. I've read you can do mud beds at 3/4" thick, I've never seen a mud bed over subfloor level with joists. Is that typically not done?
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Unread 02-14-2020, 06:06 PM   #4
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I'll do a 3/4" thick mud bed over concrete, but I've always set it at 1 1/4" at the drain when going over a framed subfloor.

If it were mine, I'd cut the joists down 2". If I was trying to save some height, I'd set the 3/4" or 5/8" subfloor between the joists like you had planned, then cover the whole thing with a layer of 1/2" plywood. Then I'd put the mud bed in like you described.

My advice assumes that your joists will still be stiff enough after being cut down 2", which we haven't addressed yet, and also that your shower is no more than 36" from the drain to the furthest point, something else we haven't addressed. My advice might change if I found out differently on either of those two points.

Are you setting tile outside the shower, or is tile already there? If you're setting tile over a backer board, that's even more height that you'll be able to gain inside the shower, since the tile on the shower floor will be at the same height as the tile outside the shower.

Now, someone smarter than me may come along and correct me on some of my advice, particularly about the mud bed minimum thickness, but that's the way I've always done it, and I know that it meets or exceeds the minimum requirements.
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Unread 02-14-2020, 09:21 PM   #5
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I’d probably go to a foam pan over mud to avoid cutting any joists down.

HoustonRemodeler has branched our into that market; check out the builtwithfoam.com site for details.
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Unread 02-15-2020, 07:01 AM   #6
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I, too, would look into a foam pan, I think you'd have a devil of a time cutting joists that have already been sistered, never mind getting those cuts reasonably accurate.

Consider recessing the entire bathroom floor; for doing so will allow you to make it both flat and level. Using a foam pan will require the floor to be flat and level. That's how I approached the build for my curbless shower.
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Unread 02-17-2020, 08:18 AM   #7
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That's for the suggestions. I cannot cut my joists, currently that are double/triple 2x8s over an 11 ft span. I didn't realize custom foam pans existed, I think this will be my route. Are those ok to go over subfloor flush with joists? Do I need to put 3/8 or 1/2 inch over the joists for less movement? Also, can I fill in any inconsistencies in flatness with thinset before thinsetting the pan in? I'm estimating about 1/16 variation before subfloor and joist tops.
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Unread 02-17-2020, 08:56 AM   #8
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Spencer,

Someone here posted Schluter literature showing their foam pan installed over a recessed subfloor that was dead even with the tops of single joists, so theoretically it can be done.

Achieving that ideal, though, will be a challenge IMO, and unachievable if you need to adjust the height of the recessed subfloor in relation to the joists in order to make the floor close enough to flat and level so that your foam pan will work. And in your case with sistered joists, if portions of the recessed subfloor need to be raised above the tops then you have 2 or 3 joists (3-4.5 inches) that the foam would have bridge over. What I don't know is if it would be acceptable to fill that void with mortar to support the foam pan, even if the foam pan was only bridging 1 joist. If filling the void of 1 joist with mortar is unacceptable to Schluter then a sheet of 1/2" would be best, but I don't think it would do the trick for 2 or 3 joists, but you could install shims between the joist tops and the plywood to fill the void.

Start by checking your existing subfloor for flat and level. The results will dictate what options you have.
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