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Modified or unmodified thinset?

Post in 'DIY and General non-hearth advice' started by LLigetfa, Mar 14, 2010.

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  1. LLigetfa

    LLigetfa Minister of Fire

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    I'm renovating a bathroom which includes retiling the bathroom floor. There is nothing wrong with the existing porcelain tile except for the colour and texture. It is set in a 3/8" bed of regular unmodifed thinset. I had stapled diamond stretch mesh metal lath to the 3/4" OSB subfloor and laid electric heating cable into the mortar.

    There are two schools of thought WRT to modifed versus unmodified and then there is the crack isolation membrane debate as well.

    Modified is usually called for over plywood or OSB. One school of thought is the wood wicks out the water from the thinset reducing the ability for it to wet cure resulting in a weaker bond. The other school says that the latex in modified thinset is a bonding agent. In reality I think both are true, the latex is a bonding agent and the latex is also an anti-dessicant slowing down the loss of water.

    For some reason that I don't fully understand, porcelain tile manufacturers call for modified thinset. Porcelain tile does not wick away water so that blows that school of thought. The DITRA/KIRDI makers say DON'T use modified for bonding sealed surfaces. Both the DITRA and the porcelain tile are sealed meaning they don't absorb water so all the water in the thinset has to leave via the grout joints. They say the latex in modified thinset slows down the loss of water.

    DITRA or cement backer board is usually prescribed for crack isolation.

    I did not want too good of a bond to the OSB so that if there was movement due to humidity changes, the mortar bond would give and the metal diamond lath would prevent cracks telegraphing to the tile or grout. It essentially was a trowel-in-place cement backer board and has worked well as there are no cracks in more than 11 years of use. The problem now though, is that I cannot chisel up the old porcelain tile without taking the mortar with it, damaging the heating cable in the process. I tested it by removing a tile from under the old shower base that I removed as there is no heating cable there to damage. The unmodified mortar is not releasing from the underside of the porcelain tile but it is releasing from the OSB taking up the diamond metal lath with it.

    My plan now is to lay new porcelain tile over top of the exsisting porelain tile. The old porcelain tile has a slate look so the face of it is textured which should help with the bond. Most of my search results say to use modified but that goes against what they say about DITRA. To further complicate this, I will be using epoxy grout so the water is less likely to leave through the grout. That means I need to wait longer before grouting so that means I have to go without my shower. Our other bathroom has a soaker tub but no shower. I hate bathing.

    There is one other school of thought, to use a whitewash of latex and water. When bonding new concrete to old, the old concrete gets a coat of watered down latex first. This is also prescribed for thinset mortar over plywood or OSB. The latex whitewash wets the wood so that the wood doesn't wick away as much water from the thinset and it also acts as a bonding agent. Being that both wood and concrete are porous, the water would not be trapped as it is on porcelain.

    Thoughts?

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  2. LLigetfa

    LLigetfa Minister of Fire

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    Did I stump the experts? Who would have thought there are no tile setters on a hearth forum. Surely someone has laid tile over tile on a hearth? Did the fact I'm reno'ing a bathroom and not a hearth throw everyone off?
  3. ChillyGator

    ChillyGator New Member

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    For what it's worth:

    http://www.ehow.com/how_2071956_install-tile-over-tile.html

    Tile Over Tile Floors

    Step 1 Inspect the tile floor and make sure there are no loose tiles; the existing tile must be in good shape for re-tiling.

    Step 2 Clean the tile thoroughly to remove any wax or build-ups on the tile floor. Scuff or sand the tile to make the surface a little rough; this helps the new tile bond to the old tile. Rinse the floor after it has been sanded to remove any dusty particles.

    Step 3 Apply a floor leveler to the whole surface of the floor. If you have a raised area, begin there and work your way outwards in a circular motion. The floor leveler will fill in previous grout lines and make the floor level for the tile over tile application. Floor leveler is usually latex-based cement.

    Step 4 Use a screed to level the floor; a screed is usually a long and very straight board or level that will move the excess floor leveler to lower levels and will help you level the floor. Allow the floor leveler to cure according to directions, usually 24 hours.

    Step 5 Buy thin-set mortar that has a latex additive in it or replace the amount of water with a latex additive. Latex or epoxy type mortars are best for installing tile over tile. Mix and apply according to manufacturer's directions.

    Step 6 Set the new tile in small enough sections as the thin-set mortar directions call for. After the whole floor has been tiled, install the grout according to instructions. Seal the grout if directions call for it.
  4. LLigetfa

    LLigetfa Minister of Fire

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    Ja, I found that page already, thanks. My old tile is not glossy so I don't plan to scuff it up. I was going to skip the floor leveler as I don't want too high of a transition. The thinset mortar will fill all the voids anyway. Was thinking of priming the floor with liquid latex instead of the leveler to aid bonding.

    Not sure what they mean by small enough sections.
  5. ChipTam

    ChipTam Burning Hunk

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    Don't think I can help with your specific question but I have done a Schluter shower. As you probably know, that's the same company which makes Kerdi and Ditra products. Yes, for their shower, they do specify a non-modified thin-set. I had some difficulty finding non-modified thin-set in rural Newfoundland and finally had to have a bag shipped out from St. John's. By the way, the folks at the Schluter 800 number were very knowledgeable and helpful when I was doing the shower. You might want to give them a call.
    ChipTam
  6. ChillyGator

    ChillyGator New Member

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    I had my shower redone this past summer using KERDI as the waterproof layer. I did the demolition and reconstruction up to the point of installing the KERDI and a professional tile layer came and did the remainder.

    Small Section: I assume they mean doing a small section at a time while working with the thin-set so it doesn't dry out too much before the tile install?
  7. LLigetfa

    LLigetfa Minister of Fire

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    I laid all the full tiles last night. Ended up prep'ing the old tile with latex and then skim coated it with modified thinset. Still need to cut and set the perimeter tiles tomorrow.

    Next weekend I get to tackle the shower. The modified/unmodified mortar requirement will be there too. Some say to use modified between the concrete tile backer and KERDI and unmodified between the KERDI and the tile. I plan to use modified behind the KERDI because it will give me stronger corners.

    The porcelain tile I will put over the KERDI calls for modified but given the 10" x 16" size tile, it will be unmodified. I am also putting in three stripes of glass tile accents and they too call for modified thinset. Since the glass tiles are only 5/8ths of an inch wide and I'm imbedding a metal divider, it will be done with modified.

    On another note, since I am only a guest in my wife's bathroom, I haven't shaved in a week and at this rate it will be more than another week unless I take my shaving kit to work when I go back on Monday.
  8. purplereign

    purplereign Member

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  9. ChillyGator

    ChillyGator New Member

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    Post a picture when you can....sounds like a nice project.
  10. LLigetfa

    LLigetfa Minister of Fire

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    Here is a progress pic of the shower stall. I've since finished the tiling but need to give it a few days before grouting.
    [​IMG]

    The white field tile was set with unmodified but the glass accent tiles were done with modified.

    [​IMG]

    The wife took the pics and they didn't turn out well. I'll take some pics after it's all grouted and the fixtures installed.
  11. LLigetfa

    LLigetfa Minister of Fire

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    OK, I have another question. As you can see from the pic, I have white field tile and darker glass tile feature strips. The whilte tile gets white grout and the glass tile gets a darker matching grout. There is a brushed nickle metal edge in between that will be separating the two different coloured grouts.

    The unsanded white grout was mixed with a liquid emulsion called Grout Boost to make it stain resistant. The presently ungrouted glass tile and metal edge were masked off during the white tile grouting. SCB, the makers of Grout Boost specify a 10 day cure before starting routine cleaning. Obviously, the white tile will be masked off before the coloured grout is applied to the glass inserts but I'm concerned that during the application and cleanup of the coloured grout, some will bleed through the masking and stain the white grout. The metal divider edge is only 1/8th of an inch wide so it's impossible to get a perfect seal.

    I want to use clear packaging tape to mask off the white tile but the wife doesn't. She thinks it will stick too much and leave a gummy residue and wants to use regular masking tape instead. During the application of the white grout, the regular masking tape did let go in some places. I'm concerned that the emulsion in the coloured grout will reactivate the emulsion in the white grout and permanently stain it. I don't know if waiting 10 days will make much difference but the wife doesn't want to wait.
  12. LLigetfa

    LLigetfa Minister of Fire

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    Here is the finished shower. Maybe I should have chosen clear glass to show off the tile work.

    [​IMG]

    Here's a couple of pics with the shower door open. The grout on the glass tile looks uneven because of the flash. Some of the tiles are clear glass and others are matt and the flash affects the clear tiles differently.

    [​IMG]

    [​IMG]

    The clear tape did leave some gummy residue but the white grout didn't stain. Still waiting for the granite counter top and vanity to be delivered so I can finish the job.
  13. begreen

    begreen Mooderator Staff Member

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    Looks pretty sharp there Lig. Nice work. Is that a Grohe relaxa shower? The shower head looks just like ours. Though I mated it with an American Standard valve set.
  14. LLigetfa

    LLigetfa Minister of Fire

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    The shower base and doors are made by MAAX, the control is a Delta Monitor 17 series, and the wall union and shower bar are JL Baril. Seeing as I hate to have water flow restrictions imposed on me by some government body, I found and defeated the flow restriction device in the head and connected to the unrestricted tub filler side of the control instead of the restricted shower side. I have yet to run any water through it to see if it can peel skin.

    You cannot see it in the pictures, but I had to make a 1/2 inch thick escutcheon for the top mount of the shower bar since the wall tile comes out 1/2 inch from the plane of the wall above the tile. I found a stainless steel measuring cup that matched in style and finish and cut the bottom off with a skinny disk. It looks just like it's a part of the fixture.

    I got word today that the vanity parts will be another week and then I still need to custom fabricate much of it so it will be a while before the bathroom is complete. I should not have mentioned the 10 day cure for the Grout Boost cuz now the wife says I have to wait a week to try out the shower.
  15. ChillyGator

    ChillyGator New Member

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    VERY NICE!

    Really like the contrast of the white tile and glass......good job!
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