Pre slope mortar thickness at drain flange on concrete slab?

[Sam Hillman]

Howdy Folks!
I'm doing a tub shower conversion on a concrete slab. After hours of searching the Internet and getting no definitive answer I thought I would post here and see if someone can help me.

My question is:
How thick should my pre slope mortar be where it meets the drain flange?

I'm trying to determine how high my drain flange needs to be. I've seen some folks say they set it flush with the slab, I've seen others say an inch to an inch and a quarter above the slab. I can't seem to find any definitive information on this. Please advise.

P.S.
For what it's worth I'm using the "Goof Proof Pre Pitch and Quick Pitch" products to achieve my slopes if that makes a difference to anyone.

Thanks!
Sam Hillman
Arizona

 

[Jadnashua]

The differences are probably between doing this on a slab verses a subfloor. The deck mud makers want a minimum of 1-1/4" (some 1-1/2") when used over a wooden subfloor (floating mudbed), but you can typically go much thinner IF you do a bonded mudbed over a slab. To achieve a bonded mudbed, you need to put down either a slurry of pure concrete, then cover it quickly with the deck mud, or spread out some thinset so that the deckmud is bonded well. A bonded mudbed tends to work better if you have at least about 3/4" at the drain, but some people get away with tapering it to zero. The setting bed on top, on the other hand, must be consistently at least 1-1/4 to 1-1/2" thick everywhere (follows the slope of the bottom layer, and thus sloped).

The perfect pitch stuff is not really your friend...it leaves a weak spot in the mudbed. Draw a level line around the shower wall/curb, pack deckmud so it is tight at that height, then use a screed to shape from that point to the drain after filling in the area in between.

Deckmud is not the strongest stuff, but is fine in compression. Just don't try to dig at it. You do need to pack it in place - it's not like concrete. If you cover it with plastic overnight, it will be stronger. If it dries before the chemical reaction, the surface will be softer...cement needs moisture to cure.

IMHO, I'd much prefer to use a sheet membrane verses doing a conventional shower pan construction. In that case, you only need your single sloped layer, then the membrane, and you tile directly to it. That requires a different drain that would accept the membrane being bonded to it verses a clamping drain unit. That also means a lower step in/out of the shower (but it's less than 2" or so unless the shower is very large).

Check out www.johnbridge.com for help with building your shower.

 

[Sponsor]

 

[Sam Hillman]

The differences are probably between doing this on a slab verses a subfloor. The deck mud makers want a minimum of 1-1/4" (some 1-1/2") when used over a wooden subfloor (floating mudbed), but you can typically go much thinner IF you do a bonded mudbed over a slab. To achieve a bonded mudbed, you need to put down either a slurry of pure concrete, then cover it quickly with the deck mud, or spread out some thinset so that the deckmud is bonded well. A bonded mudbed tends to work better if you have at least about 3/4" at the drain, but some people get away with tapering it to zero. The setting bed on top, on the other hand, must be consistently at least 1-1/4 to 1-1/2" thick everywhere (follows the slope of the bottom layer, and thus sloped).

The perfect pitch stuff is not really your friend...it leaves a weak spot in the mudbed. Draw a level line around the shower wall/curb, pack deckmud so it is tight at that height, then use a screed to shape from that point to the drain after filling in the area in between.

Deckmud is not the strongest stuff, but is fine in compression. Just don't try to dig at it. You do need to pack it in place - it's not like concrete. If you cover it with plastic overnight, it will be stronger. If it dries before the chemical reaction, the surface will be softer...cement needs moisture to cure.

IMHO, I'd much prefer to use a sheet membrane verses doing a conventional shower pan construction. In that case, you only need your single sloped layer, then the membrane, and you tile directly to it. That requires a different drain that would accept the membrane being bonded to it verses a clamping drain unit. That also means a lower step in/out of the shower (but it's less than 2" or so unless the shower is very large).

Check out www.johnbridge.com for help with building your shower.

Thanks very much for the reply. Because you brought up the 'single mud bed / membrane style' of shower pan please allow me to ask a question or two.

I'm doing the walls with Kerdi board so I had considered just getting a Kerdi drain and doing the membrane on the mud bed as well BUT then I thought why not just use the membrane on top of the tradition mudbed / vinyl / mudbed pan utilizing the drain I already bought? Is there any reason I shouldn't do that? I didn't want to use the Schluter Kerdi styrofoam pan because I'm doing a pebble floor and they don't recommend tiles smaller than 2" on the Kerdi pan.

So is it your recommendation to just buy a Kerdi drain and do a single layer mud bed? If so should that also be 3/4" at the thinnest (i.e. At the drain)? Also, my plan it to use thinset to bond the mud bed. The thinset I'm using for everything is the 'Schluter All Set'.

Thanks,
Sam

 

[Jadnashua]

A properly installed sheet membrane should not leak, but in general, it's not a good idea to install two waterproofing membranes. Especially in your situation...without using the Kerdi drain, there's no good way to make a seal between the membrane and the clamping drain, so there would always be a path for moisture to get in between the layers.

SO, yes, make your single layer mudbed, slope it a bit steeper than the 1/4"/foot for your pebbles, and use the Kerdi Drain with it. I'd like that minimum of 3/4" at the drain, and the manufacturers call for more with deckmud. Also, when setting your drain, you DO want to mix up some deckmud with a fair amount more cement for use right under the flange of the Kerdi Drain...this will help it to bond. The deckmud you use for the rest of the floor would be much leaner.

Check your riser pipe...it needs to be quite accurately plumb, since with the large diameter drain body, any error will be exaggerated because of the length.

 

[FullySprinklered]

Here's your definitive answer: It's all been done before.

 

[Sam Hillman]

A properly installed sheet membrane should not leak, but in general, it's not a good idea to install two waterproofing membranes. Especially in your situation...without using the Kerdi drain, there's no good way to make a seal between the membrane and the clamping drain, so there would always be a path for moisture to get in between the layers.

SO, yes, make your single layer mudbed, slope it a bit steeper than the 1/4"/foot for your pebbles, and use the Kerdi Drain with it. I'd like that minimum of 3/4" at the drain, and the manufacturers call for more with deckmud. Also, when setting your drain, you DO want to mix up some deckmud with a fair amount more cement for use right under the flange of the Kerdi Drain...this will help it to bond. The deckmud you use for the rest of the floor would be much leaner.

Check your riser pipe...it needs to be quite accurately plumb, since with the large diameter drain body, any error will be exaggerated because of the length.

Thank you very much for taking the time to reply. Kerdi drain it is then! I'm using Kerdi board and Kerdi products for everything else so I might as well go all in I guess. I was just a little weary of the single mud bed and Kerdi membrane concept but doing more research it seems a lot of professionals give it the thumbs up so I'm feeling a little more confident that it's a solid solution. Thanks again for your help.

Sincerely,
Sam