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Old 11-29-2014, 10:01 PM   #1
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Bathroom Subfloor & Porcelain Tile Questions


Hi all!

I am in the process of ripping out a (pretty nasty) half-bath and ran across some issues with the subfloor. I plan to tile the floor using porcelain tile.

The bathroom is about 5' x 4' (including the 1/2" drywall) and has a hodge podge of subflooring. It looks like the previous owners have added at least one layer of plywood over the original subfloor with a newer section around the toilet flange.

The floor had linoleum on a luan sheet stapled to plywood. There's also some kind of felt paper stuff in one section and roofing paper under plywood on another section.

Bathroom Subfloor & Porcelain Tile Questions-2014-11-29-13.28.00.jpg

In the image above you can see the newer section of plywood, but it is damp and moldy looking in the back corner.

I pulled up the top layer of subfloor in one section and it looked like this:

Bathroom Subfloor & Porcelain Tile Questions-2014-11-29-15.11.38.jpg

The whitish looking part is the original subfloor (I think), the black part on the right is the roofing paper.

I would like to remove all of the subfloor and replace to give the new tile floor a better surface.

My plan is to use 23/32" T&G plywood screwed and glued to the floor joists. Lay down modified thin set with Ditra on top of the plywood followed by unmodified thin set for the 12" x 12" porcelain tiles. I think since it's only a 20 sq ft room that I could lay the new plywood down in one piece, but it would be much easier to lay down in two pieces.

My questions are:

First, does this sound like a proper plan? Is the one layer of T&G enough? Should I lay it down in one piece or is two ok?

What's the best way to cut the existing subfloor(s) next to the walls and how close should I try to get to the wall with my cut and plywood replacement?

I have read that there should be a 1/16" - 1/8" gap between the plywood joints for expansion, but shouldn't the T&G joint be tight?

I want the room to be as air sealed as possible, can I caulk or spray foam the joints, around the toilet flange, and around the bottom of the wall 2 x 4's, would that do any good/harm?

I am planning on using 2-1/2" T-25 screws. The box says Deck Screws, is that ok?

I know it's a bunch of questions and thanks for your time!
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Old 11-29-2014, 10:30 PM   #2
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Your plan sounds good

I would cut between the joists with circular saw then pry the plywood off that way.
Give us a pic and size of the joists once you get that far.
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Old 11-29-2014, 10:39 PM   #3
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Thanks JetSwet!

The joists are 2 x 8. Not sure of the spacing but I think they are 24". I'll know more once I start pulling the plywood up.
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Old 11-29-2014, 10:48 PM   #4
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Quote:
Originally Posted by TeeRiddle View Post
Thanks JetSwet!

The joists are 2 x 8. Not sure of the spacing but I think they are 24". I'll know more once I start pulling the plywood up.

NP!
You don't want to butt the ends of your new plywood too the stud walls because any expansion will cause even slight buckle T&G or not... This will crack the thinset and cause may issues even though it's really only caused by moisture or high heat don't want to chance it.
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Old 11-30-2014, 04:36 PM   #5
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I pulled up a chunk of the original subfloor (which looks to be 5/8") and the floor joists are spaced 16" centers.

So now my question is how close should I cut the existing plywood to the walls? I'm thinking I need to get as close as possible. If I leave say 1-1/2" or 2" there will be a height difference between the new 3/4" plywood and the existing 5/8". Or does that really matter?
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Old 11-30-2014, 08:25 PM   #6
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You need to take then entire old plywood out right to the wall studs
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