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Old 12-19-2013, 08:48 PM   #1
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Bathroom Floor Demo - Mastic (?) Under Tile


I started some selected demo today in preparation for my bathroom gutting that will start next week. I took out a little of the 1x1 floor tile and found a somewhat strange material. On the plywood subfloor, there is a brown mastic. It is ductile enough to gouge with a screwdriver. It does look like there is a cement based material stuck to the back of the tile, so I don't think it was the primary adhesion material for the tile. There is next to no adhesion between the material on the back of the tile and this brown stuff. Leveling material maybe?

Picture is attached. The light brown area to the left is plain plywood. The darker brown is the mystery mastic. The loose tiles in the middle are upside down exposing the mortar backing.

I was planning on taking the floor out to the joists anyway - there are diagonal 1x6 boards under the plywood and a lot of squeaking.

I guess this is a bonus since the tile will come up very easily, but do I need to worry about asbestos in this stuff if I cut through it? I'm guessing the bathroom was done sometime in the 1970s.
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Old 12-19-2013, 09:03 PM   #2
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Bathroom Floor Demo - Mastic (?) Under Tile


Typically the floor mosaics were set in a solvent based mastic--which gets brown with age----is it possible the cement like substance is grout that got under the tiles?

I never heard of asbestos in the old tile mastic.

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Old 12-20-2013, 09:45 PM   #3
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Bathroom Floor Demo - Mastic (?) Under Tile


Mike is right on.

The '70's were the early days of DIY and we sold people mastic to install almost all tiles on plywood floors. There were no modified thin sets yet. Wonderboard was being used in the mid-70's on a few DIY installs, but not many. ($$$$).

Shouldn't be any asbestos in what you've got there. Could you be mistaking the mounting paper for the "cement based material on the back of the tiles? The tiles were mounted to either perforated paper or cloth mesh.

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Old 12-21-2013, 02:41 PM   #4
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Bathroom Floor Demo - Mastic (?) Under Tile


Thanks for the feedback. I knew asbestos was used back then in some mastic, so figured it would be safest to ask. It could be paper backing or the grout.

Since I am taking the subfloor out, what is the best to build the new subfloor with? I'm leaning towards a 24x12 glazed porcelain tile. I was also thinking of trying Ditra for the first time - it should be cost competitive with cement/hardi board since I only am dealing with a 4 x 8.5" floor. Is there someone in the northern Detroit suburbs that sells it by the foot for a reasonable price?

I am going to do what I can to sister some joists under half the room that has marginal support. The other half is over the main beam and a basement wall, so it is only a 4' span with 2x8 joists and max spacing of 16".
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Old 12-21-2013, 11:55 PM   #5
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Bathroom Floor Demo - Mastic (?) Under Tile


If you keep the 1x6 subfloor I would go with 5/8" underlayment grade ply, then Ditra. If you rip the planks, you need 3/4" t&g subfloor min. You can add a layer of min. 3/8" underlayment too, then Ditra.

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Originally Posted by tev999
I was also thinking of trying Ditra for the first time. Is there someone in the northern Detroit suburbs that sells it by the foot for a reasonable price?
Gee.....that would be me of course.

You need just under 11 linear ft. or 36 sq. ft. OR ..... Most of the orange stores carry the 54 sq. ft. rolls for about $84 +

PM if I can help.

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Old 12-26-2013, 06:02 PM   #6
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Bathroom Floor Demo - Mastic (?) Under Tile


Discovered the tile was actually on 3/4 particle board. Guess thats another blessing since it broke up pretty easily. There is still a layer of 3/4 plywood, then the planks. I am pulling everything since the planks are in pretty bad shape.


The room being tiled measures 49.5 x 102.5 between the stud faces. The tub alcove is 30 x 61. Joists are perpendicular to the 102.5" dimension. I was going to rent a toe kick saw to cut out the planks flush to the sole plate.

What is the best way to layout the first layer of subfloor? The current install has a 1x1 furring strip on both of the long walls to frame the sheet of plywood. It is all tight to the sole plates. Looks like they put a double layer of particle board right inside the door to fill the gap that a single sheet of plywood could not.

Should I cut the first piece of plywood to an L shape to go into the alcove? That would leave me with a 31.5" strip behind that. I would then have a 8.5" gap to fill. I might have to make some adjustments based on some odd joist spacing, but I think you get the idea.

Or would it be better to decouple the tub subfloor from the tiled room subfloor and just use on sheet plus some filler in the main section, and the tub on its own subfloor?

Do I need an expansion gap along the walls?
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Old 12-26-2013, 08:21 PM   #7
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Bathroom Floor Demo - Mastic (?) Under Tile


The ideal situation? A full sheet for the field (floor area) with the seam under the tub--

Be careful with that toe kick saw---they are extremely dangerous---I use a Sawsall with the blade in upside down--and make a shallow skim cut along the wall.
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Old 12-28-2013, 08:38 AM   #8
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I know the toe kick saws have a tendancy to kick. Still seems safer to me than the recip saw since I cant cut through a power or gas line with it as easily. $16 for two hours rental should be enough.

My latest dilemma is the transition at the door (see pic). The last joist in the room is about 4" inside the door - you can see where the planks on the right meet over it. Directly below the doorway I have a heat duct running the entire length of the house, so it is pretty much impossible to add blocking or an additional joist to support a transition from 3/4" planks to 23/32" plywood.

Obviously I don't want to leave planks and plywood hanging in space, so I guess I will have to cut the planks over that last joist and deal with the height difference. What is the best way to deal with that 1/16 lesser thickness of today's "3/4" plywood?
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Old 12-28-2013, 08:41 AM   #9
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Bathroom Floor Demo - Mastic (?) Under Tile


Forgot the pic.
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Old 01-10-2014, 11:07 AM   #10
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Bathroom Floor Demo - Mastic (?) Under Tile


I did end up renting the toe kick saw, but it was probably not necessary. Some of the planks were so warped I had a hard time keeping the blade deep into the wood - and it does have a nasty kick. Even when flat it would not cut all the way through, so I had cleanup to do. My Rockwell oscillating tool did a great job cutting where the recip saw could have caused damage. It was only $20 for two hours and I still have all of my appendages, so it was not an expensive lesson learned.

I've got 3/4 T&G plywood down with a 1/2" layer on top of that. The transition from plank to ply was not as big of a deal as I expected. I just put the 1/2 sheet up to the door frame to cover the transition. I'm going to go with 1/2" Durock as my tile substrate. That plus the thicker tile I am using and thinset should get me same height as the old floor so I don't have to mess with my cast iron toilet flange. I'm going to go with a wooden threshold in the doorway - I don't trust those old planks I can't replace. I did lift them, shot some liquid nails underneath, and screwed them back down.

I'm surprised this bathroom held up so well over the years. I also found the shower tile was adhered directly to greenboard with no vapor barrier. Never had any tile failure on the walls, and no signs of mold in the shower. Lots of mouse poop though.
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Old 01-10-2014, 08:40 PM   #11
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Unless you want the extra height--just use 1/4" wonder board or durrock---set in a bed of fresh thinset (like a giant tile) and nail it down with roofing nails---

Glad the demolition went well for you----
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Old 01-12-2014, 09:27 AM   #12
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I want the extra height. With 1/2" only about 50 cents a sheet more than the 1/4", that $1.50 is much cheaper and easier than trying to adjust the cast iron toilet flange to a lower floor.

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