space between cement board and tub

12 years ago

For my bath surround/shower, my GC put up cement board without the 6mm poly. I need some sort of waterproofing and was thinking about using HydroBan. However, there is a 1/2 inch gap between the bottom of the cement board and the top of the bathtub flange. What is the best way of dealing (waterproofing) with this space? It is too big a gap to silicone.

Featured Answer

cat_mom

12 years ago

Hydroban is great. We used it on all three tub or shower surrounds.

DH says the cement board should come over the flange, without touching the tub rim/ledge itself (maybe 1/4" space). He suggested filling in the gap with a strip of hardi-backer (easier to work with than cement board)--~3/4 wide. He also suggested "spackling" the joint(s) with thinset, not joint compound.

GW's resident tile guru's and GC's will probably provide further suggestions and advice as well.

Good luck!

Comments (9)

bill_vincent
12 years ago
DH says the cement board should come over the flange, without touching the tub rim/ledge itself (maybe 1/4" space).

DH is wrong. Cement board should come down TO the flange, and then fill the gap with stiff thinset. If the cement board comes down over the flange, it'll end up bowing out.
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cat_mom
12 years ago
To clarify; DH furred-out the studs so the hardi-backer comes down straight over our flanges (our tile guy's recommendation), so our hardi-backer does not bow out. Forgot to mention that we had done that.
bill_vincent
12 years ago
Gotcha. :-)
hondagirl
Original Author
12 years ago
So I've got to squish in lots of thinset, let it dry and sand it smooth?

Pic of part of the gap.
enduring
11 years ago
Hondagirl, how did your project turn out for you? Did you get the tile on?
Tim
11 years ago
That gap is supposed to be there to accommodate for expansion/contraction of the different materials.

Tile is intended to bridge the gap.

Honestly in my opinion you're much better off that your GC didn't install 'waterproofing' behind the cement board. The notion of putting poly behind the cement board by stapling it to the studs and then peppering it with screws through the cement board as a waterproofing technique is ridiculous.

Cement board is porous, grout is porous. Water will go through the grout, through the cement board, through the poly (or worse accumulate between the cement board and poly) and rot the cement board. It is NOT water proof - it will hold up better than drywall when wet, but not forever or for the long term.

You also don't want a 'water sandwich' i.e. poly behind the cement board and then waterproofing on top of the cement board as this just completely traps any moisture in that envelope. That's a basic tenant of all vapor barrier installations.

If it were my bathroom, I'd be installing a waterproofing membrane like Kerdi on top of the cement board. It would extend to the top edge of the tub covering that gap. Tile then gets installed all the way to the edge of the tub.

Grout goes everywhere except along the bottom edge of the tile where it meets the tub - there you put silicone caulking to allow for the movement of the tub without cracking the grout. The tub will move/flex/expand/contract.
Tim
11 years ago
Oh shoot - didn't realize this thread was so old before I posted. Hope things worked out.
bill_vincent
11 years ago
But for the record, it's not meant to be waterproofing, but only a vapor barrier, and as for being ridiculous, it IS spec'ed both by TCNA and ANSI.