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I've been convinced to use Ditra with my powder room/half bath tile installation project. Should I consider the addition of Kerdi tape as optional or mandatory? I don't want to overkill for a half-bath but I do want waterproofing protection.

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"Schluter®-KERDI is a pliable sheet-applied waterproofing membrane and vapor-retarder designed for the direct application of tile. KERDI is ideal for use in tiled showers, bathtub surrounds, residential steam showers, and other tile applications in wet areas." Schluter Kerdi

I would consider Kerdi tape or membrane to be overkill anywhere outside a shower. Ditra is indeed waterproof, but its primary purpose is to serve as an uncoupling membrane, which prevents movements in the subfloor from cracking tiles. The tile and grout are intended to create a water resistant barrier sufficient for keeping drips and spilled water on the surface long enough to be addressed with a mop or towel. If you want to be thorough with your bathroom install, make sure you seal your grout or invest a little more in a urethane grout like CBP Fusion Pro.

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It is really unclear what your motives are for this bathroom. Installing a Kerdi membrane does not waterproof your bathroom. It will waterproof the area underneath the area it is installed.

However water flows downhill. In almost all cases one of your bathroom walls will be the lowest point in the bathroom - not the middle of the bathroom floor. So that means that if you dump a gallon of water on the floor a good amount of that water will flow to wall X and into the subfloor and maybe whatever is below it.

To make a bathroom waterproof yes you would need to use Kerdi tape. You could test out your floor and just install the taping on the 1-2 walls that are the lowest or do it right and install it around every wall. Honestly it is a really quick process if the membrane is being installed. It is normal to go 6" up the wall to for a sort of bathroom pool.

Why would you install it on a half bath? Basically in case the toilet stops up and leaks/overflows you don't have sewage traveling underneath. Do you have to do it? No. Is it normally done? No. The main thing is don't kid yourself about the membrane as it will not do much better to hold water out below than the tile on top of it - however it may keep the tile from cracking.

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