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Unread 12-31-2021, 06:33 PM   #1
-Christopher
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Remodel to Kerdi Shower, Mud base

It's been a decade since I was last here... I have a new project that I'd like to share and get advice. My last shower was DIY decent, because of this forum, so thank you...

I was told by shewhomustbeobeyed that we need to update our house to be more current. I responded by ripping out tile flooring in an existing first floor hall bath, the 25' hallway, the pantry, the laundry, and at least two closets. In addition, I also ripped out the cultured marble shower and all plumbing, aluminum framed wavy-glass shower door, toilet, pedestal vanity. I really enjoyed the destructive power of my hitachi SDS-Max demo hammer and spade bit. Then I sat back and enjoyed a beer amongst the dust and destruction. In the glow of my satisfaction, she asked when we'd have a working toilet there again... well...

Since then, I've reinforced the shower framing + niche, installed a highflow valves + 3/4" copper, rerun the vanity and toilet water pipes through the wall instead of the tile floor. I'm just getting ready to roll on the shower. 48"x32"x8'

I'm planning Kerdi for the walls over Gold Bond Purple XP drywall, a custom mud pan (CustomFloat?) with a kerdi drain, a concrete curb using kirb-perfect, kerdi niche... I know I've forgotten something...

It's currently sitting on 3/4" flooring directly supported by an I-beam under the middle of the shower -- So not really concerned about the increased load.

First question -- why does everything hurt more than last time???

Second, since I have such support underneath, do I need to add another layer of plywood beneath a mud pan or am I good? I'll be putting down some plastic between the mud and the flooring.
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Last edited by -Christopher; 12-31-2021 at 07:12 PM. Reason: added dimensions
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Unread 12-31-2021, 09:00 PM   #2
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Welcome back, Christopher.

Reasonably increased dead load on the joist structure is seldom the limiting factor in engineered floor such as you have. And it appears that the joists are not separated over that support beam. Is that the case?

My first recommendation would be that you eliminate the purple material and use plain white (or gray, actually) gypsum drywall if you plan to attach Kerdi membrane.

As to your first question, when you hit about 75 you'll begin to understand.

As to your second question, what is the spacing between your joists?

My opinion; worth price charged.
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Unread 01-01-2022, 12:32 AM   #3
-Christopher
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Happy New Year! And thank you for the end of year quick response.

Correct, the joists are spaced 16"oc and they pass over the 8"x21' steel beam continuously. One side of the shower wall is on the end of the beam over the poured foundation wall. The beam is supported every 8' by steel pillars. The side walls of the 48" shower are relatively over joists, while two support the center of the shower while the beam is neatly centered under the whole shabang.

The shower framing is original first floor 18yr old house build, I just added additional glass/drywall blocking and nailers. Looking at it, I was thinking: "I don't need to stiffen this expanse with more subflooring just for the mud pan", but I'm certainly only a DIYer, and not the journeymen that I seek for confirmation.

I've few sheets of purple gypsum are left over from a recent basement bedroom addition as sunk cost -- or I'd take your advice on the cheaper stuff -- unless there's a mechanical reason to waste it.

I'm closer to 75 than I am away from it! Certainly, the trip from 40-50 begs for more stretching daily.
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Unread 01-01-2022, 09:17 AM   #4
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Some of the MR boards (Moisture Resistant) have face coatings that are not particularly friendly to thinset mortar, Christopher, which is the reason for avoiding them in your application. I've never used the purple kind and can't really testify as to the suitability. I do know the plain white boards will work well.

Your subfloor, if in good condition, is technically suitable for a mortar shower floor. If that hole is oversized for a Kerdi drain, I'd add a second layer of nominal half-inch plywood to correct that condition.

My opinion; worth price charged.
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