still needs waterproof membrane on concrete wall and concrete flloor?

[lmei007]

I am building a shower down the basement. The shower has one concrete wall and the floor is concrete. I am thinking just put waterproof membrane where wood is behind. That means that concrete wall and the concrete floor will not have the waterproof membrane. I will tile on the concrete wall with thinset directly and build the slopped-base directly from the concrete floor. Does this sound right?

thanks,

 

[lmei007]

Thank you, John.

Then what I can do for the concrete wall which is plain concrete without any paint and seals? Kerdi membrane or Redguard waterproof or ...? I have missed the step to build a frame off the concrete wall.

Why we cannot tile directly off the concrete wall? I can make it even and smooth with mortar which will just like a cement board. Then tile it with thinset.

For the shower base, it is not the direct bond with the floor to make base stay and stable, I think. It is the frame make the base nowhere to go, I think. Think about this, we usually put the waterproof liner which will make the top shower base mud nowhere to bond (I assume mud cannot bond with regular plastic membrane, such as Oatey shower pan liner, well).

Could you please explain more to me?

 

[Sponsor]

 

[John Bridge]

Hi,

I think you should be able to smooth the concrete wall and then apply a membrane directly to it. I would make sure you're not getting moisture from the ground outside. You need waterproofing all the way around the shower, and it needs to be connected to a waterproof and sloped shower floor. Concrete by itself is not very waterproof.

 

[lmei007]

Sorry, I forgot to mention that the concrete wall is an interior wall. There is another room other side of the wall.

Ok, I will buy more membrane and apply it to all walls and the base.

Thanks,

 

[Cacher_Chick]

I would be concerned with the joints where the existing wall meets the new walls. Anytime you have a corner where the materials are not the same, you will be likely to have a problem due to the differing expansion and contraction rates of the material.

Cement and/or concrete block is not waterproof. If you do not waterproof those surfaces you will have a problem with mold and mildew due to the inability of the shower to dry out.

 

[Jadnashua]

When in doubt, look at the acceptable backer materials listed in the Kerdi Shower handbook. A concrete wall, unless it had hydrostatic pressure water problems from outside, should be fine - but, it would be colder unless you added some insulation there. As opposed to John Whipple, I see no good reason not to consider Kerdiboard on that wall if you wanted some insulation. If the wall isn't plumb or flat, you can treat the Kerdiboard like a huge tile and use the 'spot' method to set it using a long level to get it perfect. If it is flat and plumb, just comb some thinset and set it like a huge tile.

Generally, you want to do a bonded mortar bed on your concrete slab. At least a couple of ways to bond it: spread a slurry of Portland cement down, then cover with deck mud before it dries out, or put some thinset down, and do the same thing. Either will tie the mudbed to the slab. On a slab, I'd prefer to make the curb out of something like cement pavers or bricks, but over a subfloor, personally, I don't think it matters. If the wood there got wet, you'd have bigger problems.

 

[Jadnashua]

And, John throwing more darts..the OP says the concrete wall is interior, so there is NO requirement to insulate it, but the other side may still not be heated, and insulation may be useful. There are lots of ways to insulate a basement wall, and it could be done from the outside, or the inside. But, he's not talking about an outside wall, so it's just John throwing darts again.

 

[lmei007]

The concrete wall was cast in 1950. The portion is about 6' away from the exterior wall. Another side of the wall is a planned bedroom with heating but not insulated on that portion.

The basement is dry. The water table is low. My sum pump well is dry all year around in normal years.

 

[Jadnashua]

If it doesn't have cracks in it, I'd use it as one wall and put the Kerdi on it directly. If it is cracked, then you'd want to reconsider. If it is not particularly flat or plumb, then you have two choices: build out a studwall (or slats and then a panel), or use something like Kerdiboard to give you a flat surface. The Kerdiboard would be faster. Well, you could try to grind down bumps, but watch the dust - would require buying a diamond cup wheel and a grinder if you don't have them, though.