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Unread 10-19-2019, 12:13 PM   #1
W01F
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Do I need to level my subfloor?

First off, this entire forum is wealth of information, it has seriously helped me through my first time tile installing tile around my bathtub/shower!

I am now moving onto preparing the subfloor for floor tile, and was planning to use Ditra membrane + QEP's lash system.

I have however hit a dilemma which I am not certain how to approach:

I noticed a bit of a dip in the floor, so I placed a 4ft level perpendicular to the joists and found it dips a bit approaching the tub (abt 3/16").
There is also a slight dip on the other side of the room (abt. 1/8").
The subfloor is original from over 35yrs ago.
Width of the room from bathtub to wall is 66 inches.

- Is this short dip (3/16") something to be concerned with?

- Would the two levels of thinset + ditra be enough to level on it's own?

- Or do I need to use a levelling compound to make the floor more level?
I'd rather not increase floor thickness if I can avoid it to match the hardwood floor level at the threshold.

I'd link or post some pics but I don't think I am permitted to with this account yet. Otherwise if anyone know's how to I'd be glad to hear about it!

Thanks in advance for any advice!!
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Unread 10-19-2019, 12:46 PM   #2
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Welcome.

First, your floor does not need to be level unless you just want it to be. Your ceramic tile installation (I'm guessing?) doesn't care a whit about level, tiles only want your substrate should be flat. Just how flat will depend somewhat upon the size of the tiles, which we do not know.

I would first like to know the structure of your subfloor before we continue. Have you evaluated the joist structure to determine if that is suitable for a ceramic tile installation? What is the subfloor material and thickness?

Installing Ditra should not change the flatness of the floor at all as it's flexible and bonded directly to the subflooring. Can you accommodate some imperfections with the thinset mortar you use to set the tiles? Maybe, but you really want to flatten the substrate before you start setting the tiles.

You can attach photos from storage on your computer at any time using the paper-clip icon above the Reply dialog box.

For links (don't link to photos, attach them) you'll need to wait 'till you have a couple more posts.

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Unread 10-19-2019, 01:19 PM   #3
W01F
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Thanks CX!

My apologies, I should have though about including this info with my earlier post:

Joist size : 10x2.
Joist spacing 16 oc.

Subfloor material : 5/8 tongue and groove plywood (note: there are a couple patches but they are from joist to joist with furring strip under the edge between the joists.

Now the tile is a sore subject for me.
Originally I wanted to use 24x24 porcelain tile but the floor not being flat I was weary about doing so.
Now we are looking at 10x10 Hex tiles, 12x12, or maybe even 2x2 hex... all depending on what the floor could accommodate I guess.

Unfortunately I still cannot upload pictures.
They are 3MB jpg, and the upload manager says that it will auto scale if under 6MB but it keeps giving me an error... I'll try to find a way to shrink it manually i guess...
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Unread 10-19-2019, 02:05 PM   #4
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Joist size doesn't tell us much without knowing the material and at least the unsupported length of the joists, Matt.

While a single layer of nominal 5/8ths" plywood subfloor over 16" joist spacing technically meets the substrate manufacturer's minimum requirements, you would not get me to install ceramic tile over such a subfloor even were it new and in perfect condition. An old, apparently damaged, patched subfloor of that material? No way could I recommend that. But it's your floor and your tile and your choice and you can tile over anything you're comfortable with.

The ceramic tile industry flatness requirement for tiles under 15" on a side is no deviation from intended plane of more than 1/4" in ten feet nor 1/16th" in one foot. For tiles larger than 15" on one side, it's 1/8th" in ten feet and 1/16th" in two feet. The first is a pretty flat floor. The second is a very flat floor.

Don't know what to tell you about the photo loading. We had a major server failure a couple weeks back and I have not tried to upload any large photos since that event, but our webmaster thinks we're back to normal. Perhaps there's still something amiss and I'll point this out to him.

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Unread 10-19-2019, 05:08 PM   #5
W01F
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Thank you CX,

For whatever it is worth, I was successfully able to upload the pictures after shrinking the file size a bit!

I did not build the place I am in, so I couldn't tell you 100% what the joists are made of; however the wood is in good condition (very likely softwood lumber), and unsupported length not more than 10ft.

When I checked the deflection calculator it said OK for ceramic but no go for natural stone. The previous tile laid (2x2) did not show any signs of cracking within the tile nor within the grout.

As far as the flatness this is what concerns me the most. You may note that the difference between high and low points are within a foot as they appear to be dipping between the joists.

Judging by the flatness requirements you list below, it does not look good for me.
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Unread 10-19-2019, 06:42 PM   #6
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The "dipping between the joists" would be a characteristic of the thin plywood you have along with it being old and no longer in perfect condition. I'd very strongly suggest you add a second layer of nominal 1/2" exterior glue plywood before installing your tiling substrate and tile.

In the alternative, I'd suggest you at least remove the subflooring you have and replace it with a first layer of nominal 3/4" T&G, exterior glue plywood, which would be somewhat better as a single layer subfloor for your tile.

But that's all up to you.

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Unread 10-19-2019, 08:09 PM   #7
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Although it would be some work, I'd go with removing what you have and installing 3/4" plywood. I imagine you'll solve two problems with that, one being the substandard subfloor, and the other being the flatness issue.

If you do go that route, I'd recommend checking across the joists with a string to make sure they're all in line. It's a perfect time to sister or shave joists to get them where they need to be.
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Unread 10-20-2019, 06:52 AM   #8
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That dip at tub, as well as the other dip, pretty much precludes using a large format tile over the existing sub floor. You could use a small mosaic, which will follow the dips, but going over 5/8" ply of questionable integrity is a gamble I'd not be willing to take in light of the amount of work it would take to fix it should it fail.

The recessed detail of the tub might also pose a challenge. It appears to start roughly 5/8" above the subfloor. If you were to add 1/2" ply, then some type of leveling compound to eliminate that dip, then mortar and typical 3/8" floor tile it looks like you're going to be well into that recess.

If I read correctly you've already done the wall tiling in the tub alcove, which suggests removing the tub to properly address the subfloor may not be an option.
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