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Unread 01-06-2020, 06:33 PM   #16
dappelha
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The subfloor is 90's era 13/16 OSB tongue and groove with a subfloor glue applied between the subfloor and the joists.

I've never really understood the deflection requirement. Doesn't a floor move more in the middle of the span? So doesn't the location of the floor relative to the span matter for determining if grout and tile are going to crack?

If you tiled a 10x10 area directly over a supporting wall, you would only have 5 feet in each direction. Isn't this going to flex less and experience less stress than the same 10x10 area in the middle of a span?
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Unread 01-06-2020, 06:40 PM   #17
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Marble or not, I'm leaning toward dropping the subfloor in the shower for a curbless install. What do you normally use for cutting out the subfloor flush to the studs? I've cut some of this subfloor before with a oscillating multitool talk about a slow pain in the .

Sawzall with a long blade? Circular saw in the joist bays, followed by cleaning with the multitool?
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Unread 01-06-2020, 07:24 PM   #18
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David, the most common point of failure of tile installations over wood framed floors is over the joist tops. That's the point of most subfloor deformity when point loads are applied between the joists. And when the subfloor panels meet over a joist, the movement is greater. It's to counteract that problem that the stone industry requires a double layer of subflooring installed such that no subfloor joint on a joist top extends all the way through the subfloor.

Your example of a room with joists meeting in the center over a support structure is similar. You need to consider the unsupported joist span on each side of that support structure because a load applied in the center of the unsupported joist span, even though it may not be even in the same room as the tile installation, can result in tremendous forces applied under the subflooring at the top of the support structure.

As for cutting flooring or subflooring close to a wall or other obstruction, a toe-kick saw is generally the weapon of choice. But you can do it with a sawzall as you suggest so long as you're careful not to cut the joists or any wiring or plumbing below the subfloor.

Are you absolutely sure that OSB didn't begin life as a nominal 3/4" panel? That stuff has a habit of swelling when exposed to moisture and it never recovers its shape or thickness even when the moisture is no longer present. OSB that has suffered such moisture exposure is not as strong or rigid as when new or in good condition.

My opinion; worth price charged.
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Unread 01-07-2020, 07:34 AM   #19
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Quote:
Originally Posted by David
I have an 11 foot span, then a floating basement wall, then a 3 foot span
Since that floating basement wall isn't load bearing your free span is 14'. You could, of course, make that wall supporting - if not technically load bearing, which would help.

I used a Sawzall to cut out my subfloor. A long blade helped me keep it at a shallow angle, and only allow the blade to go as deep as necessary. Know where the joists are so you can carefully skim the tops.
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Unread 01-15-2020, 04:42 PM   #20
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I've lowered the subfloor to the top of the joists under the shower and fixed the duct work.

The subfloor really was 3/4 T G osb. I'm thinking I'll add a 1/4 plywood + 1/4 ditra plus thinset on the main area. To meet up with the 1 foam pan and give one surface to the bottom of the pan, I planned to add 1/4 ply under the pan as well.

The joist are purposely a little lower than the surrounding OSB. Before putting down the 1/4 ply under the pan, do you think I should skim with thinset or something to fill joints and voids?
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Unread 01-15-2020, 06:41 PM   #21
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David, the absolute minimum you can use for any structural subfloor layer is nominal 3/8th of an inch and I would not use that in your application. Minimum I'd recommend for your application is nominal 1/2-inch plywood.

My opinion; worth price charged.
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Unread 01-15-2020, 08:29 PM   #22
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So if I put 1/2 plywood down under the shower area, would you put it down as is or put something to fill the small voids between joists and OSB?
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Unread 01-15-2020, 09:27 PM   #23
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I wish you had made the subfloor flush with the joist tops, but that ship has sailed. Not sure what you could fill those gaps successfully with, but filling them would be a good idea.

Maybe something like water putty would work. You'll not be able to mechanically fasten to the joists no matter what you might use, but that's OK on accounta you don't wanna fasten your second layer of subflooring to anything but the first layer.

My opinion; worth price charged.
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Unread 01-16-2020, 08:21 AM   #24
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Drywall shims, David, drywall shims. They're 1/16th thick and 1.5" ish wide.

But, if you're using 1/2" ply over all of it I believe the ply will bridge the 1.5" joist gaps without issue, except for the points were two sheets of ply meet over those gaps.

Still, if ya gots ya a whole box of drywall shims might as well use them.
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Unread 01-16-2020, 09:25 AM   #25
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I'm gonna disagree with Dan on that. You absolutely do not want any joints in the second layer of subflooring to fall on top of those joists. And even more adamantly not on top of the gaps above the joists, drywall shims or no drywall shims.

My opinion; worth price charged.
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Unread 01-16-2020, 04:43 PM   #26
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Given what I see in your last photo, David, it appears you'll be able to cover the foot print of the shower floor with a single sheet of 1/2" ply, no seams/joints.

I see no reason at all that DW shims won't work, unless the difference between the tops of the joists and tops of the recessed ply is less than the thickness of the shims. Even without shims I believe the 1/2" ply will easily bridge those 1.5" joists.

What are you going to do with these openings?
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Unread 01-21-2020, 09:53 PM   #27
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Sorry for the delayed response, I just had a baby boy!

Any plywood in the shower area would cover all the joists with no seams because the area is small enough. The gaps of the joists themselves are slightly shallower than drywall shims, plus I don't love the idea of putting cardboard drywall shims on a floor since they are basically just cardboard. I guess the slight void is fine anyway?

I'm really wondering if I need any kind of second layer on top of the subfloor at all. I mean, what is the point of recessing the subfloor in the shower by 3/4 inch to then add a 1/2 inch back on it? All that work for a 1/4 of an inch? Schluter seems to permit ditra and their shower pans on single layer 3/4 OSB...so why add 1/2 ply if I go with ceramic tile?

A year ago I did a laundry room that is right next to this bathroom with the same deflection and 3/4 OSB subfloor, and there I put 1/4 plywood + Ditra + 8x8 porcelain tile + cementious grout and the grout and tiles have no cracking. Seems like you are saying the 1/4 plywood didn't really do anything structural . So maybe I would have also been fine just laying Ditra directly on the OSB?

Those holes in the picture are the return air ducts and I have sheet metal pieces that basically sit with 1 inch on the subfloor and 12 inches up the wall and they get installed behind the drywall. I was planning to put them back in once the final substrate is finished but before the pan and the drywall are installed.
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Unread 01-21-2020, 10:04 PM   #28
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Congratulations! You get any help from your wife or girlfriend on that?

You're free to tile or build your shower over anything you want, David, we can only tell you what the ceramic tile industry recommends and where the smart money is betting. We can't guarantee failures any more than we can guarantee success.

You currently don't have a 3/4" OSB subfloor, you have a subfloor patched together between your joists using 3/4" OSB. Not at all the same thing as having continuous pieces spanning 3 or more joists. If you want to tile or set a foam shower tray over what you've got, I suggest you contact Schluter and see what they think of your plan. If they like it, go for it. May be they're fine with your setting their foam piece over what you've got and in that case it matters not a whit whether I like it or not, eh?

Your using that 1/4" plywood in your other installation likely did more harm than good for your subfloor, but if it's working for you and you're happy with it, good on you.

My opinion; worth price charged.
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Unread 02-05-2020, 10:56 AM   #29
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Yeah, my wife did most of the work (ok, all of it)! I have a new shower valve in, new lights in the shower and have set the pan, drain, and the ditra.

Since the 1/4 plywood didn't actually help, I decided to go without a second layer of plywood and just lay the ditra on my 3/4 T & G OSB subfloor (which meets the porcelain requirements in the Ditra manual).

I also put the pan directly on my lowered subfloor which is also shown as acceptable by Schluter. I used Schluter ALL-Set for both.

Hopefully this works out long term. I guess I'm willing to see and the schluter manuals seem to approve of this.

Some issues:

I have a hump and low spot in my ditra--it looks like it was caused by my knees or stepping on it while still wet. The hump is about 3/8 raised and I plan to use large format 1x2' porcelain tile.

Think I should worry about this hump and redo it? All the ditra is well adhered, but I'm thinking about cutting out a 2'x3' section where the hump is and redoing this section.

The floor tiles I have selected are porcelain, but they have a matte finish (https://www.flooranddecor.com/porcel...100340819.html). Floor and Decor website says they are water proof but they have kind of a chalky feel to them. Any special tips about working with matte tiles, like is grout haze harder to remove or do they get water stains easily?

I have extra studs behind where I would want to put a glass sliding shower door, but I have not picked out the door yet. I was planning to just finish the tile work and worry about the door later (no special schluter profiles for the glass, etc). Is it a bad assumption to think I will just be using silicon to hold frame to the floor (and of course screws through the tile into the walls)? Here is an idea of the door: https://www.build.com/vigo-vg6041487...76?uid=1350601
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Unread 02-05-2020, 01:54 PM   #30
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A 3/8" hump is going to be an issue, David, 'specially with those 12X24's. Do you think the hump is only in the Ditra, and not in the subfloor?

I have matte finish 12X24's on the main floor in my bathroom, though mine are mostly white/off white. I did not find the finish especially challenging when it came time to clean off grout. As for water stains, I'd venture a guess and say yes, they will be more visible, and I base that on my very dark grey counter tops. If you're sufficiently tall, and/or have sufficiently poor eye sight maybe water spots on the floor aren't much of an issue? Towel off in the shower, use a bath mat, and spots shouldn't be too numerous.

I am inclined to think that particular door style is going to need the fixed panel securely anchored to the floor, and there will be a guide in the center for the sliding door. You can seal the bottom gap of the fixed panel with silicone. The center guide will need to be anchored, but you can get by, I think, with a shallow hole only the depth of the tile, a short SS screw to act as a pin, and clear epoxy.
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