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Unread 05-20-2020, 09:43 PM   #1
jimmartz
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Underlayment over plank subfloor

Hello,

I am wanting to tile a kitchen.
The house is late 1800s with 7/8" plank T&G subfloor,
each about 5 to 6" wide and perpendicular to the joist.
There is to much involved to replace the subfloor, so I
am going to place the underpayment over it.
My plan is to first stiffen the joist(another post) then
add 2 screw per plank per joist. At that point I have
a stiff enough and tight plank floor that needs flattened.
The planks are cupped up to 1/16" each and the floor overall
has dips up to 3/8".
Here are the options I am thinking of:
1. Do nothing to the plank floor and screw 1/2" plywood to it
with some PL Premium. Then screw 1/4 cement board to the
plywood. Then use a cement based motor to flatten the floor
before installing the tiles.

2. Do nothing to the plank floor and screw 1/2" plywood to it
with some PL Premium. Then use a cement based motor to
flatten the floor. Then install Ditra before installing the tiles.

3. Remove the high spots on the cupped planks. Then screw 1/2"
plywood to it with some PL Premium. Then use a cement based
motor to flatten the floor. Then install Ditra before installing the tiles.

4. Remove the high spots on the cupped planks. Then fill in the dips with
shimming maternal. Then screw 1/2" plywood to it with some PL Premium.
Then install Ditra or cement board, before installing the tiles.

Please let me know your opinions of each option.
I would like to just use a leveler over the plank floor, put all the levelers
I have found are cement based and will probably crack when installing
the plywood. I seen some liquid rubber underpayments, but non of them
list as being used as a leveler on planks, and are pricey.

Thanks for any help.
Jim
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Unread 05-20-2020, 11:00 PM   #2
cx
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Welcome, Jim.

You want to flatten the existing subfloor as much as possible. Renting a floor sander might be involved (can't see it from over here).

You must install the nominal half-inch plywood before you do anything else and you do not want to use any PL Premium or other construction adhesive.

Once the plywood is installed with lots of mechanical fasteners, you'll be able to decide how much and what type of flattening still needs to be done.

If you elect to use the CBU, you must install that before you do any non-structural flattening.

If you elect to use Ditra or similar membrane, you must do the flattening before installing the membrane.

My opinion; worth price charged.
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Unread 05-21-2020, 06:17 PM   #3
jimmartz
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CX,

Thank you for the reply.
Let me repeat what I think you are suggesting to make sure
I got it right. Adding a few details of my own.
1. Sand down the high spots on the existing plank subfloor.
2. Install 1/2" or thicker plywood, no glue, and screw every 6"
each direction with construction screws (GRK R4 maybe).
3A. If using CBU, install the CBU and then flatten, using a motor
based leveler. Then install the tile.
3B. If using Ditra, flatten over the plywood, using a motor
based leveler. Then install the Ditra. Then install the tile.

Would this be correct?

A few questions.
I see a lot of people shimming the low spots in the existing subfloor
with door skins, shingles, and roof felt. before installing plywood.
I think you are suggesting just sanding before adding the plywood?
If so, any reason for not shimming before the plywood?
Any reason to use CBU vs Ditra?
I do not have any preference/experience with either, but the
Ditra looks quicker/easier.

Thanks,
Jim
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Unread 05-21-2020, 06:34 PM   #4
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The issue with shimming the joists is that unless it's giving continuous support, and plywood doesn't have the same stiffness in both directions, you could end up with movement you don't want. If you did remove the planks, you could sister shorter 2x material to the sides to account for any low spots, and plane down any high spots. I did that for about 500 sqft on the first floor of my condo when I remodeled...it was way out of whack! I wasn't tiling, but the specs for the floating floor were as tight on flatness.

If you decide on a floor sander, make sure to run around first with a nail set and make sure the screws you install don't stick up, or you'll be buying lots of sandpaper!
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Unread 05-21-2020, 07:35 PM   #5
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jadnashua,

If I shim on the existing plank subfloor, I was assuming
I would shim everywhere, not just at the joist.
Screwing every 6" would probably pull the plywood into
a dip if there was any gap under the plywood.
I have a 4" belt sander that I can use to take off the
high spots.

Thanks,
Jim
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Unread 05-21-2020, 08:02 PM   #6
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How many square feet will be tiled?
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Unread 05-21-2020, 08:51 PM   #7
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Kman,

About 175 ft^2

Jim,
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Unread 05-21-2020, 09:15 PM   #8
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I would go with a floor sander unless you just have a small area to sand down. Working that floor down with a belt sander will wear you down very quickly.

I'll also echo Jim's suggestions about sinking the nail and screw heads before you start.
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Unread 05-22-2020, 08:48 PM   #9
jimmartz
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I think I had a different idea of sanding when I was talking about
sanding the high spots. I was thinking of just sanding the high spots
mostly caused by the cupping in the planks, maybe 1/16 max.
The floor dips down about 3/8", that would be a LOT of sanding to
take 3/8" off most of the floor.
I was thinking of filling in the low spots with shim material, then
putting the plywood down. Or just putting the plywood down and
then filling in the low spots on top of the plywood with a motor
based leveler. I am guessing that either of these ways would work.

Thanks,
Jim
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Unread 05-22-2020, 10:41 PM   #10
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If you're taking off just 1/16", a belt sander would work. I imagine you're just hitting the joints, not the center of the boards.

I wouldn't use shims. Just secure the plywood with screws and use a leveling compound on the plywood if using a tile membrane, or on the cement board if that's your choice of tiling substrate.
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Unread 05-23-2020, 11:27 AM   #11
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If you follow the fastener spacing recommended, that will generally pull the plywood down to the surface beneath, so if it isn't flat, that will follow. If things are way off, a thicker ply may work out better as there are limits. You're trying to do two things: isolate the solid lumber from the tile, and help add strength. You don't want a point load to be an issue if it's not supported underneath unless it's strong enough on its own to enable tiling.

If the planks are cupped, they may split or bend back once you add the screws...see what it looks like once you've refastened everything before adding the new ply.
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Unread 05-25-2020, 11:13 PM   #12
jimmartz
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I am starting to figure out what I need/want to do with
my floor. After more research and influence from Sal DiBlasi's videos,
I am going with a decoupling membrane, probably Ditra.
And not going the cement board route.

Now back to the plywood, I got several questions, and searching
the internet I feel I got information overload and sometimes it
is hard to tell what is the most correct thing to do. It seems that
there is really convincing information and videos that have less then
a 10% change of lasting more than a year. Then there is a lot of
information and videos that have around 80% chance of lasting,
but with many variables. I have a hard time finding that information
that has a 99% chance of lasting for years to come.

Anyway, should I post me plywood questions here, or start a new thread?

Thanks,
Jim
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Unread 05-25-2020, 11:18 PM   #13
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Please keep all the project questions on this thread, Jim.
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Unread 05-26-2020, 09:25 PM   #14
jimmartz
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The current plane is this
1. put 1/2" plywood over the plank subfloor.
2. flatten the floor with a cement based leveler.
3. apply a decoupling membrane(eg. Dirta).
4. Apply tile.

1. Since I am covering most of the plywood with
a leveler and then a decoupling membrane.
Will CDX 4 ply 1/2" be ok? Or do I really need BCX?

2. No adhesive between plank subfloor and plywood?

3. Space between sheet, I read nothing, 1/32, 1/16, 1/8,
what should the space between plywood sheets be?

4. Perimeter spacing, how much if any space should be left
between the plywood and walls?

5. Put the seam over the joist, or make sure the seam between
plywood sheets is off the joist a few inches?

Thanks for any help.
Jim
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Unread 05-26-2020, 09:49 PM   #15
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1. No, you do not want to use any ply with a D-face...that means it can have voids...you don't want any weak points.
2. No adhesive, but lots of screws not into the joists.
3. In a subfloor, spacing is needed as the building is likely open to the weather. Some carry that forward with underlayment, but the plywood institute says no, but a gap won't hurt.
4. 1/4-1/2" is fine...whatever you need to still let the baseboard cover it...same with the tile on top.
5. Ply ends at 1/4-span the space between the joists. That helps to isolate any jacking that may occur over them at plank ends from transferring through the underlayment.
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