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Old 11-23-2015, 03:46 PM   #1
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Underlayment and toilet flange


Howdy,

I am working on a powder room remodel and just removed all of the tile. I was expecting to find backerboard under the tile, but I think it was just a bed of mortar. There was mesh sitting on top of the OSB subfloor and about 1 in thick mortar on top of that.

I am not necessarily looking to make the new tiled floor the same height as the old one, and in fact don't mind it being a little lower. I am going to use a Durock Tile Membrane kit which supposedly will act as a crack isolation membrane as long as you use their adhesive (included in the kit). It should be a lot easier for me to install than the typical backerboard or even ditra approach. Anyway, I need to add an underlayment on top of the OSB subfloor, and I am wondering what grade of plywood is typical for this purpose. Also, is there any issue going with a thick plywood (3/4in) since I do need to get some height back?

My understanding with regards to the underlayment is that I should glue it to the OSB subfloor and screw it with deck screws but avoiding any studs as I go, and also make sure the edges do not coincide with the edges of the subfloor. Are 1/8 in expansion gaps sufficient for the underlayment? How about along the walls? Do I need to add silicone to these gaps?

Also, my toilet flange is now likely going to sit too high. It is screwed to the existing subfloor and glued to the drain pipe. I will probably need to replace it. Should I remove it first, then add the underlayment such that the new flange will attach to the underlayment, or do I leave enough space around the pipe so that it will actually screw into the subfloor? The old one is screwed to the subfloor.

Sorry for the long post, but to summarize my questions:

1) What grade plywood for underlayment?

2) Can I use whatever thickness I want or is there a maximum?

3) How large should the gaps between plywood boards be?

4) How large should the gaps to the wall be?

5) Should I add silicone to these gaps?

6) Do I orient the plywood the same direction as subfloor, or perpendicular?

7) Should the toilet flange screw into the underlayment and/or subfloor?

Thanks!
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Old 11-23-2015, 09:50 PM   #2
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Hi Voodoo,

1 & 2. It should be underlayment grade with no face lower than "C", B/C is probably the easiest to find. The plies should be bonded with waterproof glue. Min. thickness is ⅜", the thicker the better.

3. ⅛"

4. " from all solid vertical objects including door jambs.

5. No silicone, but some people like to caulk, or do as the adhesive manufacturer tells you.

6. Yes, ply always goes perpendicular to the joists, just like the subfloor, and of course offset both ways.

7. The bottom of the flange should ideally sit on the finished floor. So if it's being removed and re set, you can install it after the tiles. Of course leave enough tiles out so there's no tile where the screws go.

Jaz
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Old 11-23-2015, 11:24 PM   #3
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Jaz,

Thanks for all of the input!

I have a couple of follow-up questions - the room in question is a rectangle but is at 45 degrees to the joists. The subfloor is indeed placed perpendicular to the joists - thus the subfloor runs at 45 degrees to the sides of the room if that makes sense. Is it ok to add the underlayment at 45 degrees to the subfloor so that I can simply add rectangular pieces of plywood, or is it really critical to run the underlayment perpendicular to the joists as well? Either way is doable, it's just that the 45 degree layout makes for a few more cuts.

Do you think PL375 is a good adhesive to bond the underlayment to the subfloor with?

Thanks!
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Old 11-23-2015, 11:42 PM   #4
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Quote:
Do you think PL375 is a good adhesive to bond the underlayment to the subfloor with?
No, it's a bad idea. That glue may be good, but you should never glue underlayment to the subfloor, fasten only and do not use long fasteners to try to hit the joists. Do not fasten into the joists, just the subfloor.

Ply and OSB is stiffer when the face grain is oriented across the joists. At a 45 it's a little weaker. How much? I have no idea. Either make a few extra cuts, or use even thicker ply. I say make more cuts.

Jaz
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Old 12-04-2015, 07:21 PM   #5
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Why the Durock kit is easier than Ditra?
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Old 12-04-2015, 08:44 PM   #6
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Quote:
Originally Posted by french_guy View Post
Why the Durock kit is easier than Ditra?

Durock will slightly conform to minor bumps and "unlevelness". I've never worked with Ditra, maybe someone else can help with that.
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