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Unread 03-02-2021, 05:51 PM   #1
openhand
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substrate for diagonal subfloor

Hi all- I'm doing a bathroom and will set 8" tiles on the floor. I removed existing 13/16" maple strip flooring and want to get the finished tile floor as close to existing (hallway) as possible but realize the tile floor will have to be a bit higher. Joists are 7.5"x2" actual size, most likely hemlock. Subfloor is diagonal 3/4" actual thickness, random widths and not t&g. Joists feel very solid but some of the subfloor boards are only about 5" and deflect under foot. I plan to use 1/8" Ditra over whatever underlayment I fasten to the original subfloor. Lots of screws to the diagonals then pl400 under the new underlayment. What's the thinest I can get away with? Thanks!
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Unread 03-02-2021, 06:07 PM   #2
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You could cut out the t&g and replace with 3/4" plywood and if you want the minimum thickness. This will allow you to beef up the joists withsistered joist if they don't meet the deflection minimum. Otherwise 1/2" minimum on top is what schluter recommends
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Unread 03-02-2021, 06:50 PM   #3
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Out of level floor under tub and tile layout and ditra layering

Hi All- I have one other concern on the tile job with the diagonal subfloor I just posted about. I'm installing a new tub and the floor under the tub is out of level by 7/16", and not in a great direction- the foot of the tub is lower. This really isn't so bad for an old New England house. I'm thinking of putting the tub in slightly out of level and bringing Ditra and 1/4 thick tiles up to its edge to cover the bottom of the tub apron. That means I lift the foot of the tub 1/4" or so, but not the full 7/16", set the ledger board on the walls accordingly, and set the tub in a mortar bed so the foot end of the tub apron is up 1/4". Then bring 1/4" Ditra to it's edge and seal with silicone. So the Ditra doesn't run underneath. Then when the tile goes down it closes on the tub and the discrepancy is covered, and this is also caulked with grout- color matching caulk. I'll have to taper the first course of wall tile coming off the tub but I don't think anyone will see the imperfection. Can anyone make suggestions of a better way to do this? Also, Is Ditra typically installed so it runs under the apron of the tub for moisture control? Will a tub drain ok when it loses about 1/4" of its pitch? Thanks!
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Unread 03-02-2021, 07:39 PM   #4
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Welcome back, Clark. Please keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. A moderator can give it a more generic title any time you'd like to suggest one.

Without knowing the joist spacing and unsupported span, we can't help determine if the joist structure is suitable without improvement.

As Hai points out, the absolute minimum the industry accepts on top of a sawn board subfloor is nominal 1/2" exterior glue plywood with no face of grade lower than C. But that presumes the boards to be perpendicular to the joists and have T&G edges. If you want to use the plywood over what you've got, that's entirely up to you. Please don't try to glue the plywood to the board floor, 'specially not with a gun-applied construction adhesive. Mechanical fasteners only and only into the boards, not the joists.

Your tub was probably only required to have a slope of 1/8th" per foot when built. Being mounted out of level in the wrong direction will certainly affect that, but you'll just hafta set it and put a level on the bottom to determine how much. Leveling the floor would, of course, be the better plan.


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Unread 03-02-2021, 10:03 PM   #5
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Hi, Thanks for the response. The 2x7.5” joist span is 16” o/c why do you recommend no glue? And why no fasteners thru to the joists? I thought that would stiffen the whole assembly even more? Would you recommend ring shank nails or screws? Thanks!
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Unread 03-02-2021, 10:17 PM   #6
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The heavy-bodied ribbons of construction adhesive lift the plywood a little bit from from its substrate, creating little gaps between these ribbons. These gaps allow vertical deflection to occur with live and dead loads before the plywood "bottoms out" on the substrate where it is supported. You don't want gaps because this type of vertical movement is murder on stiff, brittle tile.

As far as avoiding fasteners into the joists: House structures expand and contract a bit with changes to temperature and moisture. Stiff, brittle tile doesn't like too much movement, as it is not inherently strong. By only securing your plywood to the 1x boards, you're allowing for a small amount of this structural movement to dissipate instead of being transferred into the top layer of plywood. Make sure to fasten your 1x boards to the joists so as to make them flat and not creaky.
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Unread 03-02-2021, 10:19 PM   #7
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That gives us the joist spacing, Clark, but we still need to know the unsupported span of the joists to make any judgement on the design deflection.

Trying to glue sheet goods such as plywood to individual board subfloors sounds like a good idea until you try to do it successfully. And successfully would require a full spread of wood glue rather than anything from a gun tube. The gun beads can actually create voids that end up being a negative rather than a major benefit. Good mechanical fastening, which for me is always screws in a subfloor application, to those boards with a pretty close schedule will be about as sound as you can expect to make it.

The reason the tile industry recommends avoiding fastening to the joists with the second layer is to prevent movement of the joists from telegraphing through the top layer. No, I don't know how it does that, but I'm told it can happen. But that's a whole different discussion.

Refasten those boards. Make the surface of the boards as flat as you can. Install a layer of plywood. Install a tile substrate of your choice. Install your tiles. Or remove the boards, sister or otherwise bring the joist tops into plane and level, install minimum nominal 3/4" plywood and the tile substrate of your choice and tile.

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