Best Way to Deal with this Subfloor issue in a bathroom?

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  #1  
Old 02-16-13, 07:29 AM
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Best Way to Deal with this Subfloor issue in a bathroom?

Hello,

They say a picture is worth a thousand words... I have uploaded 3!!! I hope they help explain the situation...

I have stripped the bathroom down to what you see on the floor. The board on the floor is 1/2" thick and rests on 2x8 joists. The joists are not uniformly spaced, but are between 14 and 18" apart. I have easy access to most of them from below and can shore them up (all except the part below where the vanity will end up; shown in the second picture).

This floor used to support about 1" of mud and 2 layers of tiles, all of which have been removed.

I have about 7/8" between the top of the board and the top of the adjoining hall floor to work with. The toilet flange is sitting about 1.8" above the top of the board. Previously, there was a threshold under the door and the tiles were flush with the top of the threshold. I still would like to have a threshold, but would like to keep it within reason and keep the tiles below the top of the threshold.

First question. Can anyone tell what kind of board we are looking at? Is it Plywood?

From what I have read on the board, a minimum of 5/8" (ideal 3/4") plywood is recommended as subfloor for tiled bathroom. It will be covered by ¼” backer board, sandwiched between two layers of ¼” cement. I am stuck and can’t decide what to do. I don’t want to go overboard and do a lot of unnecessary novice work, but like to do it right, so it does not fail within a few years. The options I have thought of:

Option 1: Build right on top of the current board. Tile, Backerboard, and cements will add about 1”, meaning I will be about ¼” over adjoining floor, which the threshold will nicely hide.

Option 2: Remove the existing layer of board and replace with ¾” plywood. If I build on top of that, as in option 1, I will end up about ½” above adjoining floor. Still probably threshold will cover.

Option 3: Put 1/3” plywood on top of existing board and then build from there. This will raise the floor 7/12 over the adjoining floor. How will it look with threshold…

Anyhow, I hope I have explained the problem. Your help is greatly appreciated and needed...
 
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  #2  
Old 02-16-13, 12:42 PM
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Andi - If your toilet drain is cast iron and you don't want to mess with it then you should build so that the flange rests on the finished floor. Personally, I would look at this as an opportunity to get rid of a cast iron flange/drain.

IMO concern about floor height should be secondary to support for the tile. Assuming you've done your due diligence and the supporting joists are adequate, then I would put down 3/4" T&G plywood and 1/4" cement board over the existing plywood. That will give you a floor height of about 1 3/8" and a 1/2" transition. The 1/4" "cement" you mentioned will not be that thick. It is applied using a 1/4" notched trowel and it will knock down to around 1/8" or less when the tile and cement board are installed.

I will not get agreement here, but another option is to put down a 3/4" ply underlayment atop the existing subfloor and skip the cement board. The plywood will have to be smooth and defect free. Use a quality Type 1 mastic instead of thinset to adhere the tile directly to the plywood. That will lower the transition by 1/4". This arrangement is acceptable for anything other than the shower floor.

Option 1 is not an option. The subfloor/underlayment will be inadequate to support the tile. Option 3 is also not an option. Besides I don't think you can find 1/3" plywood.
 
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Old 02-16-13, 02:33 PM
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I agree,

#1 is no good. In #3 I guess you meant to say 3/8" not 1/3". Substitute with 5/8" and you'd have an adequate subfloor to start with. Probably the easiest method since you're not removing the existing subfloor.

#2 gets you to today's specs if you glue and fasten well and also install blocking under the short ends where there's no support.

1/4" backer is about 5/16" total installed. Some "1/4" CBU's (Durock) is 5/16" plus thinset, so about 3/8" total. Some other CBU's are 3/8".

Thinset for tiles add about 3/32" - 1/8" max using 1/4 and 3/8" trowel respectively.

Tile doesn't care about your desire for the floor heights to be close to the next. Forget the thickness and do what's needed. The threshold will help you along with shoe molding and carpet shims if applicable.

Now tell us about the joists; We know they're 2x8, do you know the species and grade, o.c spacing, and the longest span to the inch?

Jaz
 
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Old 02-17-13, 04:51 PM
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Thank you all for the replies and the insightful comments. I have attached another picture that helps further explain the situation. The pink lines represent the area I can easily access from the unfinished laundry below.... The brown lines represent floor joists below the bathroom. They span from the top of concrete wall to the top of a metal Joist running the length of the house, in the other direction. The longest joist is 13' long. However, at about 9 feet (that's 1 foot beyond the pink line closest to the green closet), I have a 2x4 wall perpendicular to the joists in the basement. That's the wall of the laundry right below the bathroom. Than k you.
 
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Old 02-21-13, 03:50 AM
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Hi. This is what I have come up with, thus far:
1. Screw existing 1/2" plywood to joists, using 1.5” deck screws;
2. Screw 5/8" plywood (Grade A, B, or C) to current plywood (not to joists), using 1.5" deck screws;
3. Use unmodified thinset and screws to connect backerboard on top of the 5/8" plywood. It is OK if the backerboard get screwed to both layers of plywood. However, it won’t be screwed to the joists. Stop backerboard about 1/8” from the skirt of the tub;
4. Use modified thinset between the seams of the backerboard. Seams will also get covered by alkali-resistant mesh tape;
5. Use thinset to install tiles, stopping 1/8” from the tub. Grout between the tiles;
6. Use grout to fill the 1/8” space between the tile/backerboard and tile.
Does anyone see any issues with this plan?
This leaves two issues to work-out.
1. I am going to roughly end up with: Plywood (5/8) + thinset (1/16) + backerboard (1/4)+ thinset (1/16) + Tile (1/4) = 11/8" elevation. This is 3/8" above the hallway floor elevation, which I expect threshold will hide. However, I will be 1/2" below the bottom of the toilet flange in the middle of the room. Any recommendations on how to deal with that?
2. I am not certain whether you can make this from the pictures, but the hardwood floor protrudes 2/3rd of the way, under the door. Previously, they had put the threshold on top of the hardwood and adjoining mud. Should I cut some of the hardwood out and build the floor below it at the same time I am building the rest of the bathroom? Then, rest the threshold on that? What is a good tool to do that with?
Thank you
 
  #6  
Old 02-21-13, 05:40 PM
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1-6 sounds fine to me. You're using modified thinset to set the tiles too.

The thickness of the thinset will be a little thicker, but not much gained there. May as well go with thicker ply then. Or get a plumber to cut the waste pipe and lower the flange. The ideal height is when the bottom of the flange sits on the finished floor.

Remove the strip of hardwood and put ply there then the CBU. Tool? Router, chisel?

Jaz
 
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