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Waterproof Backerboard for Shower

Is there a such thing as waterproof backerboard that does not require a separate vapor shield?

I just found out that my contractor did not install a vapor shield in my newly tiled shower. He said it is not necessary because he used cementboard. I thought some sort of vapor shield was supposed to be installed behind the cementboard, but he says I'm wrong.

There are wood studs, then cementboard with caulk, then mortar and marble tile on top. Does this sound like an adequately waterproofed shower? Or am I looking at leakage/mold problems in the future?

Thanks to anyone who can answer my question.

Re: Waterproof Backerboard for Shower

everything he did sounds correct except the "caulk". the only place that should have been caulked are the inside corners of the shower and the corners where the shower pan meets the wall tile.

Re: Waterproof Backerboard for Shower

well, he should have vapor barrier in behind no matter what, its a issue of the building envelope which is required on all outside walls and the top floor ceiling

as for water proofing the cement board will do but requires deitre mat on the board to waterproof it. denseshield is being used more often now than concrete board.. the difference being concrete board is rot proof but not water proof. denseshild is like drywall but has fibreglass for its skin as opposed to paper so it wont grow mould and its water proof, it simply needs the joints sealed

Re: Waterproof Backerboard for Shower

There are several non waterproof cement boards (CBU); permabase, hardie board, wonderboard and any of the ones that look like cement made into a sheet.

The waterproof boards are Wedi, Densheild, and kerdi board. The first two look like they have some sort of coating sprayed over them, giving them an even, but somewhat bumpy appearance. The Kerdi board is a smooth bright orange. All of these boards require the nail / screw holes to be waterproofed as well as the interior corners. Normally they are sealed to the tub flange as well to provide a 100% waterproof surface.

If he wanted to use regular old CBU, then either -

A- sheet plastic should have been applied to the wall studs then overlapped onto the tub flange, the sheet gets adhered to the tub flange. The CBU does not overlap the tub flange.

B- the CBU is screwed to the studs (not overlapping the flange), then a surface applied membrane such as Kerdi, Hydroban, Hydrobarrier, Redguard or similar is applied to the surface. The waterproofing membrane is adhered to the tub flange.

If your contractor used plain old cement board, your shower is NOT waterproof. It's water resistant, but not water proof. It may or may not give you problems. Caulking the seams of the cbu does little to keep the shower waterproof, but kinda helps. caulking the seams of CBU is not in the current TCNA guidelines.

If your contract called for a waterproof tub, you get your money back.

Re: Waterproof Backerboard for Shower


I tiled all 21 of the bathrooms in my building much the same way as your contractor did yours, and in 21 bathrooms over 20 years, I've not had a single problem, and there doesn't seem to be anything on the horizon either. I don't believe it's because my tenants over the years have been much more careful taking care of my property than you will of yours.

The only difference that I can see is that I never caulked between the tile backer board and the tub. I simply installed the tile backer board so that the bottom edge was 1/8 inch above the tub lip, and installed my bottom row of tiles so that they hung down in front of the tub lip.

Also, my bathrooms are all done with glazed wall tile, not marble.

I'm no geologist, but I expect that marble will be as impermeable to water penetration into your walls as glazed wall tiles.

If I were you I would:

1. Allow your grout to dry as long as possible. Ideally, if you can have baths instead of showers for 2 months, that will allow most of the grout to dry completely. Maybe apply some 1 1/2 inch wide painter's masking tape over the bottom few inches of the vertical grout lines, and then tape over the painter's masking tape with 2 inch wide duct tape to keep everything under that duct tape dry.

2. Use an ACRYLIC film forming grout sealer to seal your grout lines. Really, a penetrating grout sealer should last longer if it works properly, but my experience with penetrating grout sealers is that they don't always work as expected. I am reluctant to use a penetrating grout sealer because I can't tell how well the grout is sealed by inspecting it, and in my business I have to be confident that the grout is sealed well enough to keep it mildew free regardless of whether or not the next tenant stays 1 year or 10 years. With a film forming sealer I can use a bright light to check the sealer on my grout and be confident that it's still well sealed.

3. If you can draw a straight line, you can seal grout. Just hold an inspection light (with a CF bulb in it to avoid the heat from an incandescent bulb) along with a photographic film container half full of grout sealer in one hand and the artist's paint brush in the other. Buy a "Script" paint brush which will have longer bristles for less dipping and less dripping. You want one that when wet with water paints a line about the same width as your grout lines. Then, simply paint the grout sealer onto the grout lines. The reflection of the inspection light off the rough surface of grout and the smooth surface of the liquid sealer will tell you wheher or not you've covered the entire grout line width or not.

Now, it's been my experience that three coats of a good acrylic film forming grout sealer will last 7 to 10 years approximately. I have no difficulty keeping up with 21 bathrooms in my building, and so you shouldn't have any problem at all maintaining the grout sealer on a single bathroom shower.

In my experience, one of the best grout sealers you can buy is marketed by the Glaze 'N Seal company and is called simply "Grout Sealer". Glaze 'N Seal's phone number is 949-250-9104. Maybe phone them and see if anyone in your area sells their products or can order their Grout Sealer for you. If you can't get that one, then either "Gloss Sealer & Finish" or "Matte Sealer & Finish" made by TileLab and sold at Home Depot are both decent acrylic film forming grout sealers. DON'T use any grout sealer with the word "siloxane" on the fine print on the container. Siloxane is a suffix that means "silicone based plastic". Silicone based plastics seal water out of the grout as well as any acrylic plastic, but the difference is (and this is true for all silicon based plastics so far as I know) that nothing sticks well to them. So, when it comes time after 10 years to reseal your grout, the new siloxane grout sealer won't stick well to the old siloxane grout sealer. You don't have that problem with an acrylic based grout sealer.

Penetrating grout sealers SHOULD last longer than film forming sealers because they're protected from the erosive effects of the shower spray by the grout itself. I am reluctant to use penetrating grout sealers on my bathrooms both because I haven't had confidence that they worked properly when I have used them and I have no way of telling how well sealed the grout is. The advantage of a film forming grout sealer is that you can see (using a bright inspection light) the clear plastic film over the grout to confirm the grout is sealed, and so you don't have to go on trust. In my case, I want to be able to check that I've got a thick coat of sealer on my grout so that I know the tiling will remain mildew free regardless of whether the next tenant stays 1 year or 10 years.

You're likely to eventually have mildew growing on the silicone caulked joints around your shower. It's a popular misconception that mildew can't be cleaned off of silicone caulk. It can, and I do it all the time. Post again if you want to know how to do that.

In a nutshell, your shower seems to have been constructed better than mine. If you maintain your grout and silicone caulk, there's absolutely no reason that your shower won't last 20 years and counting as mine have.

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