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Unread 11-17-2008, 12:27 PM   #1
james.walters
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Join Date: Feb 2007
Posts: 10
Master Bath Remodel - Poured Concrete on Traditional Foundation - Need Advice!!

Master bath remodel; searched the forum but couldn't locate a thread with my particular situation and don't want to get the fundamentals wrong, please help!

Current State
- 53 year old house
- master bath gutted doun to studs
- 8' x 8' (man did they cram 'em in in those days)
- tub removed; space to be converted to shower (5' x 3 1/2')
- Floor joists 2x8 SYP under bathroom floor (2x10 SYP in remainder of home including where the tub was sitting), 4' is longest span, 16" OC
- 1x8 subfloor on 2x8 floor joists
- 1 1/4" concrete bed poured on top of 1x8 subfloor
- 1 3/4 mud bed laid on top of concrete which has been removed; on top of the mud bed was 1/4" tile

Tile selection
- Natural Stone
-12x12 bathroom floor, 3/8 " thick
- 2x2 shower floor, 3/8 " thick
- 3x6 shower and bathroom walls, 3/8" thick

I have made the appropriate plumbing updates to accomodate the change from tub to shower by cutting out sections of the cast iron drain and vent and repositioning with PVC. The unions between the cast iron and PVC were made with those 1/4" thick rubber collars with steel bands. The hot and cold water rough-ins, changed over from 1/2" copper to 3/4", are in place in a new framed-in shower wall in order to expose the old bathroom wall making space for a pocket door.

My next step is to place the new subfloor in place in order to accept tile. I first have to even out the poured concrete floor a little. Originally the builders knew thay were putting down a thick mud bed and wern't too careful about leveling out the concrete. It's not off by much so I plan on using the self leveling concrete (SLC?) commonly found at the big box stores.

After the concrete is leveled my intent is to put 2 layers of 3/4" ply, but not over the entire floor. The first layer will cover the entire bathroom floor. Screwed to the 2x10 floor joists in the area where the tub was removed. Over the existing concrete bed I can drill through the concrete into the 2x8 floor joists and attach with screws here as well. The second layer of 3/4 ply I only intend to put in the shower area, where there is no underlying concrete bed, screwed only into the first layer of ply.

On top of the second layer of ply, located in the shower area only, will be a traditional 3 deep, 2x4 shower curb, mud bed and shower pan. In the bathroom outside the shower I now have 2/8 floor joists, 3/4" 1x8 subfloor, 1 1/4" concrete bed and 3/4" ply screwed into the floor joists. Thinset, 1/2" CBU, screws (only into first and only layer of ply) tape and more thinset will complete the floor preparation.

Will this make for an adequate foundation? I'd really hate to have to take a jack hammer to that floor!

Thanks for your help, this is a great website!
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Unread 11-17-2008, 02:10 PM   #2
bbcamp
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Jim, you should take advantage of the recessed area where the tub was to build a shower with either no curb or a low curb. The you could have the shower floor slightly lower than the rest of the bathroom and a correspondingly lower curb.

You do not want to attach plywood directly to your existing mud/concrete floor. The concrete won't be perfectly flat (even with SLC), so your plywood will have gaps under it. If you have areas outside the shower that need filling, I think the easiest way to accomplish this will be to remove the old concrete, and start over with a new mud bed or build up with plywood and install backerboard or a membrane. The reason is that if you must join an old bed with new work, you will have to use an antifracture membrane and use a soft joint in the tile work to accomodate potential movement at the cold joint.
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Unread 11-17-2008, 03:23 PM   #3
james.walters
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Thanks for the response, Bob (and fellow Tennesseean - though your on the nice side where I live in the flat muddy side called Memphis!).

I was afraid my description of the layout wasn't adequate to accurately reflect the layout and likely should post a pic or two. The top of the 2x10 floor joists where the tub sat and the top of the concrete, which was poured on 3/4" 1x8s on top of 2x8 floor joists, are level. The bottom of the 2x10s and 2x8s are also level.

Having said that, are there any viable alternatives which would allow me to using 3/4 ply and CBU which would NOT require me to break out that poured concrete?
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Unread 11-18-2008, 02:38 PM   #4
james.walters
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So what if I leveled the floor, installed two sheets of 3/4 ply over the entire bathroom and came across that with a product such as ditra. Would that make for a sufficient tile base?

Thanks,
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Unread 11-18-2008, 06:28 PM   #5
bbcamp
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No, you probably come closer to having a successful installation by butting your backerboard (and plywood subfloor) to the concrete surface, then using an antifracture membrane across the joint. Of course, this will mean soft joints (grout lines with caulk in them instead of thinset) on either side of any tile that spans the concrete/backerboard interface.

Again, putting plywood directly on top of a concrete surface invites problems due to the inevitable gaps between the two.

Bite the bullet. Remove the concrete, level the joists (sisters, etc) and install plywood and backerboard over the entire area.
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