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Unread 12-02-2004, 12:13 PM   #1
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Join Date: Dec 2004
Location: Ottawa, ON
Posts: 34
Kerdi Mortar Question

Hi Everyone,

I am new, and really cannot believe the wealth of info here. Good on ya!

I installed a Kerdi shower tray over a plywood base the other day. I used Masterblend mortar with an acrylic additive so that it makes a good bond with the plywood. I used the same mortar to set the Kerdi drain.

I mixed up the mortar good and thick, nice consistency, not runny, but workable. I then put the mortar down and laid the drain into it. Unfortunately I used the Kerdi tray as a template to cut the drain hole, so the hole is about 3/8" bigger than the drain all the way around. I contacted the Schluter support desk after I realized that I should have used the drain template - they said it was no problem, the drain would support the weight if someone stepped on it.

When I set the drain into the mortar, I put some weight on it and let it set. When I came back the next day I found that the mortar had a single crack in it almost all the way around the drain. The crack is about .5 - 1 mm wide. The drain does not move, but I am concerned about why it cracked. When I install the membrain I will skim coat this to fill in the void.

I think this could be because some of the mortar leaked out the hole in the floor around the drain, maybe pulling the rest of the mortar down.

So to sum up my 3 questions;
1. Was it okay to use Masterblend with acrylic mix to install the Kerdi tray and drain?
2. Should I cut a hole in the ceiling below my drain to try and re-enforce the hole around the drain?
3. Why did my mortar crack???

Thanks very much in advance.
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Unread 12-02-2004, 06:52 PM   #2
John Bridge
Join Date: Mar 2001
Location: Rosanky, Texas
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Hi Rob,

I think you're okay. Thin set cracks when you put a lot of it on, and it shrinks quite a bit when you have a lot of liquid in it. As long as everything is tight, you're fine.
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