When using Kerdi for your shower floor you need to make a hole in it for the drain. If you don’t then you simply have a shallow swimming pool rather than a shower. This hole must be the exact size and it must be in the correct spot. There are two ways to do this.
The first way is to install your membrane on the floor then cut out your hole for the drain. While this method works, I’m not a fan. It leaves open the possibility of cutting more than just your membrane (where it needs to be cut), slipping and damaging the drain or messing up the bond of the membrane around the drain where you need it to be bonded (accidentally debonding a portion of the membrane while you’re cutting the hole).
It works, but there’s a better way. I’m assuming here that you’ve already cut your hole in the floor, installed the drain and mud deck and cut your kerdi membrane to the correct size for your floor – the width of your floor plus four inches, by the length of your floor plus four inches. That is your floor size plus two inches up each wall. Got that? Good, let’s move on.
In your Kerdi drain box there are two templates. One is for the hole in your subfloor, so the drain fits down into it correctly (it says cut-out template for floor), and one for the membrane (it says cut-out template for kerdi membrane). Grab the one for the kerdi membrane – I’m assuming you already have the hole cut out in the floor, yes? If not you’re doing it wrong… Remove the inner circle – this is the part you’ll be using.
Set the template (the circle) into the center of your drain. It will fit perfectly into the center depression of the Kerdi drain. Measure your shower floor in each direction to the center of the template, the little cross, plus two inches. For instance: if your shower is 36″ deep x 48″ wide and your drain is centered then distance to the cross should be 20″ from the back (18″ plus 2″) and 26″ to the side (24″ plus 2″). Write those numbers on your template in the appropriate spots – 20″ at the top of the cross (to the back wall) and 26″ on the side of the cross (to the side wall).
You can click on any of these photos to see a full-size version.
Now take your Kerdi membrane and lay it out in the same orientation in which it will be in the shower. Measure 20″ from what will be the back of the shower and make a mark, then measure 26″ from what will be the side and make another mark. You should have a cross on your membrane similar to the one on the template. That is the center of your drain in the shower.
Now take your pen and place the tip right in the center of the cross. Gently push the tip of your pen through the template so it pokes out the back of the template like this:
Once you’ve done that fold the template back so you can see the pen tip and place it directly onto the little cross you made in the membrane. Then hold it firmly in place so it doesn’t move and remove the pen (Do this with the other hand also unless you’re taking pictures too).
Now take your razor knife, scissors, bowie knife, whatever, and cut the circle out of your membrane – take your time! It needs to be exact and there are no second chances if you slip.
Now you can lay it into your shower floor and everything will line up correctly without any problems. I normally roll it up from each end to the center so when I lay it into the shower floor I place the cutout directly over the drain and roll the membrane out from the drain in each direction. That way you ensure it’s in the correct spot.
Although this sounds like an extremely long process because there are a lot of words on this page, it’s actually quick and straight-forward. If you use this method you eliminate the possibility of damaging your membrane and you end up with a hole where you need it and the size you need it.
Oh, and if you actually cut it out with a bowie knife – send a picture. I gotta see that!