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Unread 10-04-2019, 08:15 AM   #1
riccobo4
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Replacing Subfloor for Tile - Does This Make Sense?

I'm a long time reader, new poster. I already successfully did my two bathrooms with your help and now I need some advice preparing my kitchen subfloor for porcelain tile:

My subfloor is 3/4"x5.5" tongue and groove boards run perpendicular to the joists. My joists are overspanned for tile. What I did upstairs is sister the joists from below then screw down a layer of 1/2" plywood, followed by the ditra and tile.

The problem downstairs is that the adjacent rooms have cheap 5/16" thick top nailed oak flooring put over the subfloor. If I add extra reinforcing plywood to the kitchen there will be an almost 1" high transition between rooms. The rooms share a 10' long opening so its important.

Therefore, my plan is to remove the subfloor in the kitchen, sister the joists, then put 3/4" T&G plywood subfloor down, then ditra and tile. This will leave me with abut a 3/8" to 1/2" transition height between rooms which seems reasonable.

Does this plan make sense? Any suggestions? Thanks in advance!
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Unread 10-04-2019, 08:22 AM   #2
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Your plan appears sound.

Depending on structure, you may be able to add mid span support for joists as opposed to sistering, which might be simpler.
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Unread 10-04-2019, 09:03 AM   #3
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Glad you've gotten some good use from the forum. It's been a while, but I wanted to formally say, "Welcome to the forum, James!"

And I agree with Peter's confirmation on your plan.

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Unread 10-04-2019, 12:26 PM   #4
riccobo4
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Thanks for the replies. This is over a full unfinished basement (that I plan to finish someday) so I have easy access for sistering and I would rather not add more support.

Now the ditra instructions say you only need 5/8 plywood with 16" spacing but I imagine 3/4 is the best way to go to be safe.

I'm also open to other ideas. For example I'm considering just upgrading the adjacent rooms to real 3/4" hardwood to raise it up but that is really stretching my already exceeded budget.
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Unread 10-04-2019, 12:46 PM   #5
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Welcome, James.

I would personally never use a first layer of subflooring thinner than nominal 3/4", which is already thinner than I'd like.

And I would never tile over a single layer nominal 5/8ths" subfloor regardless the substrate manufacturer's recommendation.

My opinion; worth price charged.
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Unread 10-04-2019, 01:14 PM   #6
riccobo4
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Thanks I figured 5/8" was iffy.
3/4 is okay though? I think I'm just doing basic 12" or 13" square porcelain tile. nothing crazy.

Remember I am also sistering the joists which theoretically effectively reduces my spacing to 14.5". I'm not sure if that happens in practice though since the new joists will never line up exactly with the height of the old ones.
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Unread 10-05-2019, 01:32 PM   #7
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The size of the tiles makes no difference.

If you're sistering the joists, you could run a bead of contruction adhesive on the top side of the new pieces so that they support the underside of the new subsfloor, effectively reducing their "joist bay" span to 13".

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Unread 10-08-2019, 09:34 AM   #8
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Here's an update: I got a quote to get real hardwood installed everywhere else on the 1st floor and It was a bit cheaper than I was expecting so I am getting real 3/4" hardwood.

So for the kitchen the plan is:

-Tear up 3/4" plank subfloor
-Sister joists, while making everything as flat as possible.
-Cover with 3/4 t&g subfloor, glued and screwed.
-Lay standard ditra
-Lay large tiles

Can anyone confirm from experience that ditra + 3/8" thick large format tiles will end up level with a hardwood floor? My calculations say they will but there's a bit of guesswork with mortar thickness.
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Unread 10-18-2019, 01:49 PM   #9
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Can I use a Larger Notch Trowel to Slightly Increase Tile Height?

Lets say my 18"x18" tiles get nice 100% back coverage with a 1/2" square notch trowel. Is there anything wrong with using an even larger 3/4" trowel if I want the tiles a tiny bit higher to make them perfectly level with the adjacent floor? Is there anything I should be wary of?
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Unread 10-18-2019, 01:56 PM   #10
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In my experience the larger your notch is the more thinset tends to make its way up and out of the joints. 1/2 is perfect for most large tile I'm setting. You can put down a crack isolation membrane that'll raise it up about an eighth. Or set last row or two progressively higher to match up with a higher surface.

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Unread 10-18-2019, 02:09 PM   #11
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I'm talking a height differences of less than 1/8". Not a big deal but I figure if I can make it PERFECTLY level by simply adjusting my trowel, why not? The perfect trowel would probably end up being a geometry between 1/2 and 3/4 square or maybe a 3/4 held at a lower angle.

I already plan on using ditra anyway and ditra XL would be too much height
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Unread 10-18-2019, 03:08 PM   #12
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This is for your kitchen floor, James?

You can try your method, of course, but don't be shocked when you find it doesn't work quite as well as the theory. And give yourself some extra time for the process, I think you'll find it pretty slow going.

My opinion; worth price charged.
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Unread 10-18-2019, 06:38 PM   #13
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If it's less than 1/8th, set the tiles like usual and slightly ramp the last couple rows up to the other floor like Jeff said. Slightly ramping 1/8 won't be noticeable.
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Unread 10-18-2019, 07:44 PM   #14
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What you'll find is that as you add mortar, more than is needed, the incidence of tiles sinking will increase. Corners will dip in places more than you'll like.

And the mortar squeezing up through the joints is a pain. Trying to clean it without pushing the tile out of place, then trying to push the tile back in place, only to have more mortar squeeze up.
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Unread 10-18-2019, 10:45 PM   #15
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If you’re going with a big notch, make sure to not exceed the final compressed bed of mortar thickness recommended by the instruction for your given mortar. If you’re using a ‘large format’ mortar, the allowable final compressed bed thickness far exceeds ‘thinset mortar”.

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