Crumbly mud pan - help me mongo-wan-kenobi

brian78April 13, 2013

Hi again,

I promise I used the search function both here and on the JB forums and must have read through 150 posts about "crumbly" mud pans. What I am confused about is that sometimes the advice was "rip it out" and sometimes the advice was "its fine, maybe put a little thinset on it if you are worried". I guess my inexperience doesn't allow me to understand the difference and/or which advice is appropriate for my particular circumstances.

I read the "how to" in the JB liberry on how to mix deck mud. I mixed it using portland cement from home Depot and "all purpose sand" (also from home depot) With a 5:1 ratio. I mixed the dry ingredients, then added water in small doses until the mix was just as described. It clumped, held its shape and left my hand relatively clean. It went in well, packed great and when I finished I was ecstatic and patting myself on the back. I counted the seconds until it had been 24 hours and found that it was very crumbly.

To be more specific, it has the following characteristics:

  • It's been curing for 26 hours

  • I can walk on it, even jump on it and it feels very solid, there is no "give" to it

  • brushing your finger across it yields "crumbles" down to about 1/4" at which point it starts to hurt your finger to scrape at it

  • using a sharp blade such as the edge of a trowel I can scrape down seemingly to the bottom (i only went down about an inch or so)

  • It is less crumbly in the further parts of the pan where I started and more crumbly at the curb edge where I ended up (It took me about 3 hours to lay because I have OCD about things like this)

Obviously I don't want to redo all that back breaking work (I don't see how you mud men do this stuff every day!) but I want it to be right. Do I need to pull it out and start over? Or is this one of the instances where its OK to tile over it? Its a kerdi drain/kerdi membrane shower pan if that impacts the course of action.

Thanks in advance for any advice.


ABOVE - pan immediately after finishing

ABOVE - pan after curing for 26 hours

ABOVE - "crumbling" with my finger

ABOVE - "crumbling" with my finger closest to the exit

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3 hours? On the same mixed batch? That's your problem right there. You should be only hydrating small amounts at a time. If you can't get it applied in 30-45 minutes, then throw it out. It's started to chemically set by then, only in a dry pack mix it doesn't show like it does with something with some soup to it when it starts to harden up.

    Bookmark   April 14, 2013 at 12:46AM
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Thanks for the response. I feared that might be the case based on the fact that it was less crumbly in the starting corner and much more crumbly in the finishing corner.

So I am assuming your response means I need to dig it out and start over? :cry:

Also when mixing I was mixing based on weight. I don't know how to do that in small batches. Can I mix it by volume instead? I.e a scoop of cement and 5 scoops of sand then add water? Repeating in small batches until I'm done?

    Bookmark   April 14, 2013 at 10:11AM
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I am so confused about the shower setup and water proofing. I am going to use a general contractor, but its still good if I know the procedure myself, so I can tell if anyone is cutting corners.

What is the best method for constructing a new shower? What brands of material should be used? Any red flags I should watch out for?


    Bookmark   April 14, 2013 at 6:56PM
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Oh Brian...I feel for you. Frustrating for sure. Didn;t see your post until today, I was out of town for a few days, just got home last night.

As live_wire_oak wrote, time is a factor. Also how much you "packed" the deck mud into place can affect the integrity of the sloped mud.

Packing helps consolidate the mix and helps the particles bind together.

I usually mix a batch and do the perimeter, a few inches out from the wall. That establishes my elevations at the walls. Then I'll set the drain. Then I'll wet and mix another batch or two, depending on the size of the pan, and fill in the sloped middle.

I mix all my dry materials together into one large batch, then portion it out in drywall buckets. Then I'll add water to the drywall bucket amounts as needed.

And yes, portion it out on 5:1 basis by volume, not weight. Shovel fulls. Coffee cans. Magic marker lines in a drywall bucket. Whatever works for you.

The following is optional, but once my mud slope is complete...elevations good, slope good, corners good, all packed down...I go over it real hard with a steel trowel at a low angle. Sort of like when you trowel and try to bring cream to the surface of a regular cement mix...which you won't be able to do since this is a very lean deck mud mix...but to me it helps consolidate the surface just a bit more.

Understand that deck mud can be shaped, so it doesn't have to be precise when you first set it. You can add a little here, shave some off there. So get it mixed, pack it down, then shape it as needed...

It ain't rocket science...I know, because I used to be rocket scientist! lol

Good luck with the repairs. Be patient, but be deliberate.

    Bookmark   April 15, 2013 at 12:29PM
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Thanks Mongo, informative as always. I wonder if at least some of my problem was my mixing by weight. I basically did half a bag of portland cement (~45 lbs) and 3.75 bags of sand (~225 lbs). The bags are roughly the same size though so by volume I'd wager it was probably closer to a 7 or 8 to 1 ratio.

It clumped just like the pictures in the JB liberry though so i reckon the time was probably the biggest problem.

I am about to try again so say a little prayer for me :-)

    Bookmark   April 15, 2013 at 2:41PM
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Ok, I got the new mud pan in tonight. it's already harder after an hour than the other pan was after 2 days so I am feeling pretty good that this one will work. Hopefully I can get to the Kerdi install tomorrow.

Quick question; In the kerdi installation video where they use the tray they say to use a 1/8" x 1/8" square notch trowel. In the installation video on a mud pan they say to use a 1/4" x 3/16" V-notch trowel. Should I be using the v-notch since I am installing over a mud pan? They didn't make the distinction in the videos. One was an old video and one was newer with the kerdi curb etc.

Thanks again for all of the help everyone.


    Bookmark   April 15, 2013 at 10:14PM
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Due to the somewhat rough texture of the mud pan, I recommend you use the V-notched trowel. It'll leave a little more thinset on the bed than the square-notch.

If you'll have a seam in the floor, don't let that seam fall on the drain flange.

After dry fitting your piece(s) of Kerdi, pull the Kerdi, then use the flat side of the trowel to burn a coating of thinset into the surface of the mud bed. The goal is to embed the thinset into the mud, while also leaving a veneer thickness of thinset behind. Then comb it off with the notched side of the trowel, with the trowel at a roughly 45-degree angle, leaving a nice combed surface. You'll get the hang of it within a few swipes of the trowel.

Set your Kerdi over the thinset, then use the flat side of the trowel to embed the fleece into the thinset. Move the trowel parallel to the thinset ridges to minimize entrapped air bubbles under the membrane.

As I embed the sheet, I work up to a somewhat firm pressure on a low angle trowel. You can start light, and move to more pressure. Again, you'll get the hang of it in no time. If you're moving thinset around under the Kerdi, which happens quite often, you can bring the excess thinset out from under the membrane at the edge of the sheet of Kerdi. For this photo I raised the edge of the drywall knife so you can see the thinset being pulled out:

The goal is to get a smooth surface, no humps, no lumps, etc, with the material the same color. You'll see the Kerdi go from the bright orange to a muddied orange-brown as it embeds in the thinset.

As you complete a section, I recommend you then just leave it overnight.

For an order of work, it might be best to:

1) Protect the mud bed with cardboard
2) Kerdi the bench surfaces, wrapping/lapping the Kerdi on the bench surfaces onto the shower walls about 2-1/2 to 3" or so.
3) Then Kerdi the mud bed

Let it sit overnight.

4) Protect the Kerdi on the floor with Cardboard, then
5) Kerdi the walls around the bench, and working your way out of the shower,
6) Kerdi the rest of the walls

You know your shower best; the shape, access, the size of the area you have to work in. So you can shuffle that work order around as you see fit. The goal is to simply work your way from in the shower to out of the shower, and to not walk on any uncured thinset.

If you feel you can do it all in one day, no worries, have at it. Just plan your thinset batches accordingly. You don't want your thinset firing off in a bucket while you are scratching your head trying to hang a sheet or detail an overlap. So it might be best to mix small batches, mixing a fresh batch of thinset for each task.

If you do drop a tool and ding or puncture the Kerdi, just put a Kerdi patch over it. Seams need to overlap a minimum of 2", so...

    Bookmark   April 16, 2013 at 11:42AM
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