space between cement board and tub

hondagirlNovember 11, 2011

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.

Thank you for reporting this comment. Undo
cat_mom

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!

    Bookmark   November 11, 2011 at 11:54PM
Thank you for reporting this comment. Undo
bill_vincent

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.

    Bookmark   November 12, 2011 at 5:27PM
Thank you for reporting this comment. Undo
cat_mom

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.

    Bookmark   November 13, 2011 at 8:45AM
Thank you for reporting this comment. Undo
bill_vincent

Gotcha. :-)

    Bookmark   November 13, 2011 at 10:05AM
Thank you for reporting this comment. Undo
hondagirl

So I've got to squish in lots of thinset, let it dry and sand it smooth?

Pic of part of the gap.

    Bookmark   November 13, 2011 at 10:13AM
Thank you for reporting this comment. Undo
enduring

Hondagirl, how did your project turn out for you? Did you get the tile on?

    Bookmark   June 21, 2012 at 7:29AM
Thank you for reporting this comment. Undo
torontotim

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.

    Bookmark   June 23, 2012 at 11:44AM
Thank you for reporting this comment. Undo
torontotim

Oh shoot - didn't realize this thread was so old before I posted. Hope things worked out.

    Bookmark   June 23, 2012 at 11:45AM
Thank you for reporting this comment. Undo
bill_vincent

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.

    Bookmark   June 23, 2012 at 4:10PM
Sign Up to comment
More Discussions
Contractor messed up shower pan. Fixable?
My contractor who I'm firing tomorrow messed up my...
gwrussp
Does anyone have their light switch NOT by the BR door?
Our new bathroom will be 5' deep by 11' long. There...
southofsa
Kohler Elevance Tub - for disabled - gorgeous
Pretty new product from Kohler. anyone who is looking...
elphaba_gw
wall mounted sink with hidden pipes?
Hi all, Would like a small-ish wall mounted sink, but...
randyelipson
Photos of your finished bathrooms?
Is there anywhere on this site where people on this...
bixa2805
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™