Help with pre-slope removal and drain

brian78April 7, 2013

Hello all,

I am in the middle of a tile shower remodel and while I have done considerable research (including reading the Kerdi Shower part deux post about 30 times) am I nonetheless stuck on what to do with the existing pre-slope mortar bed.

I removed the tile, mortar bed and vinyl liner with relative ease and now am left with the pre-slope and traditional drain. I was planning on doing the kerdi shower kit but about 15 min into trying to chip out the pre-slope to create the required flat level surface, I realized that there must be a better way :-)

Here is what the existing pre-slope looks like. You can see the left corner looks wet where I was chipping it out. (the right corner is wet because that's where the leak existed that caused mold damage to our adjacent bedroom carpet - hence, this project)

I had hoped to reuse the pre-slope and simply put the kerdi membrane over top but in my completely inexperienced opinion, the pre-slope doesn't look like it was constructed very certainly doesn't look like the pictures in Mongo's thread.

Question 1. The kerdi pre-formed shower pan has to sit on a flat surface, is there any option that doesn't involve me chipping out this pre-slope mortar bed? From the picture does it appear that the pre-slope is OK to reuse? or can I "repair" it, level it, fill it in...sprinkle magic fairy dust on it to make it instantly flat and perfectly sloped?

Question 2. The drain - as you can see from the pictures, the drain is flush with the pre-slope. I am unclear on what needs to happen here. Do I need to get that drain out of there? or is this where I would use the kerdi drain adapter thingy - meaning I bought the wrong kerdi drain? (note: this is a slab foundation, no access from below)

A bonus question - if you are so inclined - the bench is built into the shower already (see pics) and originally had a vinyl membrane over it down into the pan liner. Can I just kerdi over it as is? Treat it like any other seam? i assume yes but maybe someone in the know can confirm.

I can't begin to express my gratitude for all of the knowledge you share so freely in this forum. Thanks in advance for taking the time to read and (hopefully) help me.


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Brian, you sure have your work cut out for you there!

First, because you had the leak, I'd recommend pulling the preslope so you can better assess (and repair if needed) the subfloor under the preslope.

If you wanted to keep what you now have and build over it, I'd venture that your best bet would be to clean up the preslope, use a bonding agent, then use a mortar-type of mix to essentially turn that preslope into a flat slab. Then you could set your Kerdi Tray over that flat slab. You would have an increased elevation at the curb, so be aware of that.

But that might create another problem with using the Kerdi adaptor. The problem with that is your existing drain flange may end up recessed so far down in the new "slab" that the Kerdi adaptor may not be able to make the height transition. That's mostly a guess since I don't know the elevations you now have, or what you'd end up if you did try to level out that preslope. But even if it would work, I'd recommend yanking out that preslope and starting anew. You;re going to be putting so much effort into this shower, right now trying to incorporate the preslope sounds like a smart move that can save you time and effort. But in hindsight, it might turn out to be a colossal mistake.

If it's too onerous of a task to chip out the preslope, you might be able to rent a demolition hammer, it would make short work of it for sure.

With your preslope out and your 2-part clamping drain, if you can use the Kerdi adaptor, that'd be the perfect application for it.

The bench, completely cover it with plywood so there is no flexing when you sit on it. Make sure the bench seat is pitched into the shower for drainage. Then cover the ply-covered bench and your shower walls with cement board. When using Kerdi, I recomend you not use Hardi products. Use Durock or Wonderboard.

    Bookmark   April 7, 2013 at 9:09PM
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Thanks so much for the quick reply Mongo. I feel like I just wrote a letter to my favorite rock star and got a reply ;-)

I actually just got back from home depot where I purchased the wonderboard that I needed to cover the bench and walls, and some portland cement. I had deduced from watching the youtube video on the adapter installation about 300 times that my best option was to build up the current preslope until it was flat and level and then place the tray on top.

The preslope is highest in the corners where it is about 1.25" high measuring from the floor; in the middle it is about .5", as you can see from this picture it slopes rather steeply from the corners and then there is very little slop in the middle area.

So, I was calculating that if I brought the entire floor up to 1.5" to make it level and flat, then the kerdi tray which is 1.5" at the thickest part, then tile which is 1/4", I would have a total height of 3.25" which would leave me 3/4" of space for thinset while still maintaining the 2" minimum curb height (using the 6" kerdi curb).

Of course, now that I sit here typing that out it seems like an awfully narrow margin of error for a DIY'er like myself...*sigh*

I want to do it right so if Mongo says take up the pre-slope that's what I will do. There is a Home Depot tool center about 45 min away, I'm sure they have something that will get this pre-slope out. Let's assume that with a power tool able to chip away the pre-slope I will most likely not end up with a smooth level surface when I'm done :-)

Should I use a self-leveling morter to fill in the divets or will I need to do something else to prep my slab for the kerdi tray? from watching the video it looks like I will need a 1/4"-1/2" "spacer" between the slab and the kerdi tray anyway...should I just build up a 1/2" morter bed using Portland cement? Or, at that point should I just skip the kerdi tray and do a full morter pan on which to place the kerdi membrane directly?

By the bad is the "tinny" sound of the water hitting on the kerdi tray? That is just the kind of thing my wife will complain about for the next 15 years ;-)

Thanks again for all of your help. If you are ever in Dallas I owe you a beer!


    Bookmark   April 7, 2013 at 10:20PM
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One thing regarding the 2" curb height: for clarification, understand that it's not that the curb has to literally have a 2" vertical differential between the flat tile on the top of the curb and the sloped tile on the shower floor. The code requirement is that you need a 2" differential between the flat tile on the top of the curb and the finished height of the drain.

That means that the vertical height of the curb right at the curb might only have to be 1-1/2" to 1-5/8", with the other 1/2" to 3/8" of the 2" requirement being made up by the rise from the slope of the floor.

Another way to think of it if I didn't explain that well: If you plug your shower drain and flood the shower, the depth of the water over the drain needs to be a minimum of 2" deep before water starts cascading over the top of the curb.

It's not the physical height of the curb that gets measured. It's the difference between the drain height and the top of the curb.

That's why we can have "curbless showers". There is no true vertical curb in a curbless. The 2" of vertical is all contained within the rise component of the sloped floor.

My main reason for recommending that you pop out the preslope is to inspect for and then repair any water damage. You're over a slab so that should be okay. But just give the wall framing, the studs and the sole plates a good looking over.

If you chip it all out and you want to use the Kerdi Tray then as you wrote, you'll have to then make repairs to the slab to get the surface smooth. You can do that with SLC as you wrote, or by simply troweling a portland-sand mix into the divots and skim coating the surface.

SLC might be easiest, but if you use SLC, understand that it will run everywhere. So contain it as needed with dams, etc. If you use a Kerdi Tray, SLC might be a fine choice as it'll give you a dead-flat surface.

If you want to avoid the Kerdi Tray, then you could just do a deck mud slope right over your divot-covered surface, there would be no need to pre-treat the divots.

Set your drain elevation. Set your perimeter elevation. Pack the perimeter with deck mud to your elevation lines. Deck mud is roughly 4 to 5 parts sand to 1 part portland cement, dry mixed well, then wetted just enough so when you take a fistful of it it clumps together in a ball but does not drip water, sort of like sand castle beach sand.

With your perimeter elevation set, and your drain elevation set, you then pack deck mud between the two to create the slope. You can shave mud as needed with the edge of your trowel to fine tune the slope. The big thing is to not let the slope level out and flatten at the walls, especially in the corners.

Do understand that my recommendation to chip out the existing preslope is biased. When making repairs, I prefer to start from anew as much as I can versus making repairs over a possibly compromised surface. You have your eyes on the project, and you've certainly done some research. If you think you can build over what you have, then you probably can.

I was at the Shiner Brewery last month. Not too close, but sort of...

    Bookmark   April 8, 2013 at 10:35AM
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Brian, one other thought...Hydroban versus Kerdi.

If this is not a steam shower, then you might want to consider Hydroban. There's nothing wrong with Kerdi, but with your bench, etc, Hydroban might be easier.

Instead of having to fold Kerdi around the corners and make all the seams around the bench, etc, you can just paint Hydroban all over.

You can use Hydroban with a Kerdi drain, and Laticrete (Hydroban's parent company) will warrant that installation.

Just something to think about...

    Bookmark   April 8, 2013 at 10:46AM
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I have been flipping back and forth between Hydroban and Kerdi from the beginning. I decided on Kerdi primarily because I can't seem to find hydroban ANYWHERE locally.

My original plan was the goof proof shower pitch kit and mortar with a Kerdi drain and hydroban. If I order hydroban online it's probably cheaper to go that route anyway.

You think that's the best route to go? I've already rented the demo hammer and I'm at the tile store right now picking up SLC for the inevitable mess I'm going to make of the sub floor lol

    Bookmark   April 8, 2013 at 12:41PM
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I did have one other question Mongo. When I start chipping up the pre slope, won't that leave the drain area elevated from the sub floor? Or is the drain flush with the top of the slab and the pre slope slopes up and away from the drain? I.e drain represents ground zero and nothing should be chipped away from directly around it.

Sorry to be so needy with your time. You really are a life saver. I'm a nerd by trade so if you ever have computer problems shoot me an email, I would love to return the favor.

    Bookmark   April 8, 2013 at 12:50PM
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Just an update. The demo of the old pre-slope is pretty much complete. I have learned 3 very important things about demoing concrete that I thought I would share for other would-be DIYers.

2. if you are smart, let someone else let the tool do the work ;-)

And lastly...Mongo and I have very different definitions of "quick work" ;-)

Now to see how bad I can mess up self leveling concrete.

    Bookmark   April 8, 2013 at 3:39PM
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Brian, read Staceyneils thread on SLC from several years ago. I will try to find it and link it here....
...found it, but I couldn't find it with a GW search, I had to do the search through google using my key words :(

There is also info on the john bridge forum on applying SLC. But staceyneils is a great read.

Here is a link that might be useful: Self-leving compound & radiant floor heat.

    Bookmark   April 9, 2013 at 6:56PM
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Thanks for taking the time to find that link enduring. I think that I'm leaning towards the mud bed vs the SLC + kerdi pan. The "tinny" sound of the kerdi pan had me worried (my wife is picky about things like that) lol. The article you linked was a good read and it pushes me further toward the mud pan route.

I finally got all of the wonderboard hung today so I will have to make a final decision in the morning when these beers wear off :-)

Unfortunately, I messed up hand cutting the kerdi curb so I will have to make a trip to the store in the morning for the materials to do a traditional 2x4 curb.

Above: the curb lines up fine on the top plane...just like the 2x4 template I made

Below: Unfortunately, i guess I cut the curb at a pretty severe angle...guess this is why I don't do this for a living ;-)

I was going to try to cut a wedge or fill in the gap with the dry-pack or something but since this will essentially be load bearing (shower glass door) I feel like that isn't a good option. I figure I will just grab a couple of the curb perfect seems like the "for dummies" version of shower building so hopefully I cant mess that up...I am so tired of going to the tile store lol.

    Bookmark   April 9, 2013 at 11:51PM
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Brian, what photo size are you using? It makes me wonder if you are using a very high res setting on your camera. It is taking a very long time to load these. I have the thread selected to notify me when someone posts, and my email has pictures that are HUGE. Then this page took a very long time too.

    Bookmark   April 10, 2013 at 7:27AM
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Sorry about that. The pictures are taken with my iPhone so they are probably around 2 MB each. I left them full size to show detail but I can resize them to make th smaller so they will load faster. It's probably partly due to the number of pictures at this point. I'll try to tone it down. Sorry!

    Bookmark   April 10, 2013 at 8:51AM
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Hey Brian, no blame:)
Here is a link to the ever popular "New To Kitchen..." thread that has very good info on a variety of topics, and one is photo posting. Take a look.

Here is a link that might be useful: New To Kitchens? Posting Pics? Read Me! [Help keep on Page 1]

This post was edited by enduring on Wed, Apr 10, 13 at 20:19

    Bookmark   April 10, 2013 at 8:18PM
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Ok, so a little more help please!

Demo is done and I am ready to lay my mud bed. I have a slab floor and purchased the Kerdi drain kit with residential adapter which of course fits every drain on the planet...except for mine. :-)

I read a great post on this forum regarding drilling out the adapter plate to fit my bolt pattern, which I did. In my particular case however, I also had to notch out the vertical edge to allow for the bolt heads (See pictures for more detail). My thinking on this is that because the water tight seal is formed at the gasket, notching this vertical lip will have no impact on its water tightness (yes, I know tightness is not a word but I took some poetic license!).

Since I am a lowly DIYer I was hoping for some positive confirmation from the experts here before I mount this thing and lay mortar over it. I dont expect Kerdi to warranty my "modifications" but I just want some piece of mind that I didnt cause the drain to not function properly.

Thanks in advance for any help. I really appreciate and admire the time you all give so freely in this forum.


    Bookmark   April 11, 2013 at 4:05PM
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