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I'm re-tiling for the first time. It's just a small powder room. After I took out the tile, I discovered that who ever had laid the tile had done it directly over vinyl. I removed the vinyl and it looks like there is a layer of plywood in most of the bathroom except for around the toilet area,which has a small section of Ditra underlayment.

My question is, should I remove the underlayment and replace it with cement board (or something) or is it possible to reuse the Ditra underlayment if I clean it out properly (clean out the thinset from the holes and reattach it to the subloor). The Ditra was attached to the subfloor, but the corners came out as I was cleaning the corners. Here is a pic of the area.

ditra around toilet

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I regularly use Ditra. You can reuse it if you can get the corners down flat and co-planar.

Its not necessary to remove the old thinset, if it's well adhered. A new coat of thinset will bond to the old. A coat of concrete admix or bonding liquid (latex) the day before, will ensure old and new will adhere.

Schluter doesn't recommend using latex modified thinsets over Ditra and under porcelain tile. This is due to Ditra being impervious, porcelain being impervious and latex modified needing to 'dry'. Straight Portland cement thinset will chemically cure faster. If you have small format tiles (4x4 or smaller), you can use modified.

Ditra, in addition to being a water/vapor barrier, is an excellent crack isolation membrane. If you are using natural stone tiles or large format tiles (> 12x12), you should consider filling the whole power room area with Ditra.

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I think I might have damaged some of the notches of the ditra in the process of trying to clean out the old tile concrete that was there already on the top so I might have messed it up already. It is very strange to me that they would use ditra in just that little corner while leaving the rest of the room with plywood. I would have liked to replace the sheet with a new ditra sheet but this stuff is pricey! I think I might just take out this piece and replace it with some ironply for that corner. These are large tiles (13 x 13). Do you think this is a good idea? –  ceiling cat Aug 4 '13 at 19:08
    
Ditra in this case was being used as a vapor barrier for repeated small leaks. I am biased for Ditra in particular and uncoupling membranes in general (Laticrete makes Strata mat, for example laticrete.com/architects/products/surface_preparation/…). –  HerrBag Aug 6 '13 at 21:00
    
The larger the tile, the flatter the surface must be, to avoid bridging (which will break tiles). The larger the tile, the greater the flex forces of bending will affect the tile. This is where the uncoupling part comes in, relieving the in-plane stresses that arise with the bending. For stone tile, the floor must be twice as stiff (L/720) as for ceramic (L/360). –  HerrBag Aug 6 '13 at 21:01
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