Using 1/4” cement board for shower wall

Question

I realize using 1/4” cement board for a tiled shower wall is a general no-no, but I’m trying to find a creative work around here.

I’m trying to build a tile enclosure for an existing shower pan in a basement bathroom. The existing shower pan is set against a block wall. The block wall is plumb in the upper half, but in the lower half it leans outward gradually. My plan was to furr out the lower half of the wall using tapcon screws to make it totally plumb for tile. Right above the shower pan flange will require a 5/8” furring strip. However, when I consider a 5/8” furring strip, 1/2” cement board, 1/8” thinset, and 5/16” thick 3x6” subway tile, my total distance from the block is 1-9/16”. The distance from the block wall to where the shower pan lip ends is also 1-9/16”. So it would be flush to where the pan lip ends, while there would be a 5/8” reveal of the shower pan lip on the other two walls of my alcove shower. So what I’m wondering is- do you think if I placed my furring strips closer together, say 8” apart, I would have enough rigidity to go down to 1/4” cement board on this block wall?

Thanks.

Answer 1

Rather than furr out the block wall, you might consider using "thick set" mud AKA "fat mud" to plumb the wall. Thick set mud is meant for thick applications, in contrast to "thin set" mud which is meant for adhering tile. Old-school showers up until relatively recently were usually "mud set" installations (i.e., thick set mud was used as the base) and are very high quality. For these mud set applications, the entire shower enclosure was plumbed and squared with thick set mud.

Here is a quick video that discusses thick set mud a bit. If you search youtube for more videos on thick set mud, you will see examples of how to use it to plumb a wall.

Answer 2

Your wall will not be strong enough. If 1/4” cement board was attached to drywall, plywood, or plaster that has been properly waterproofed it would be fine. You might want to check your local building codes, I am fairly sure they will not allow that. 1/4” cement board is just not that strong structurally.

Answer 3

Why would use of cement backer board for a wall tile application be a no-no? I had thought it was pretty much the standard now, having largely displaced green-papered drywall for that purpose. That said, I'm not certain there's any need for it over a wall that is already made of concrete (whether poured or block) - unless the condition of the concrete makes it unsuitable for directly applying the tile.

It sounds like the primary goal is to make the block wall more planar. That's more-or-less like a stucco project. It could be achieved by building up layers of thin set applied directly to the concrete. Don't exceed the "maximum thickness per application" guidance on the package because shrinkage and cracking could become a problem if you do. Use a board or straight edge as a screed or at least a guide to avoid building up the surface too much.

If there's as much as 5/8" of build-up needed it make make sense to make a little batch of concrete with pea gravel and sand aggregate. A concrete like this could achieve in one application the same amount of build-up that might require 3 or 4 applications of thin-set.