Doing an upstairs new bathroom, eventually with a (fiberglass) shower base. Currently has the plywood subfloor down and all the plumbing roughed in. I can't get a straight answer between my plumber and the guy who's doing the floor tile about which should go in first. Both have an "you can do whichever you want" attitude, without telling me which is best for creating a water barrier. The bathroom is directly above the first floor master bedroom and I don't want any water seeping through. As far as I can tell, there's three options:

1) Install shower on subfloor. Run cement board and floor tile right up to it and caulk. (I think this is the worst and will definitely leak.)

2) Install the cement board all the way under the shower, then install the base, then run the floor tile right up to the base. (I think this is probably the right way, but I think my tile guy isn't happy about making two trips and waiting for the plumber in between.)

3) Install the cement board and floor tile under the shower base. (I think this is overkill and the tile guy said he's never done it that way, but it sure is better than Option 1.)

What's the right way to go here?

9 Answers 9


Most of the time, installers will just wait until the tub is installed before they lay down the tile backer, and then just cut around the tub for the backer board and tile. In most cases, this is fine, but not ideal.

The wood subfloor is a permeable surface, and water can easily seep between the gaps of the tub, and the edge of the backer board. The only protection you have is grout, and caulk. That will hold up for a while, but that will eventually fail. As long as the water is mopped up immediately, there should be no problems. However, if water sits on the floor for an extended period of time, it could cause issues later down the road.

Tiling under a tub basin is not necessary. Nobody is likely to ever see under it anyway, and with the cost to tile, is not worth it financially. In the old days, this was much more common. Early bathtubs were freestanding, and all the plumbing was exposed. In some ways this was better since leaks were easily detectable. The downside to modern showers is leaks are not found immediately, and only present themselves long after when there is significant damage done.

I would recommend having tile backer underneath, and behind the tub and shower stall. The backer board can handle getting wet, and won't readily allow mold growth to take place. I left a small piece of tile backer outside for over a year and it was perfectly in tact. If the same thing was done with drywall, it would have completely disintegrated by now.


Shower pan on top of the sub floor works fine. I did this recently and plumbed/installed the pan prior to installing backerboard. I recommend using Georgia pacific denshield board as backer rather than cement backer board. Its reasonably priced and doesn't required a water proofing step. Silicone all edges and bring it down to the shower pan nailing fin lip, leave at least 1/16" gap at bottom of board and shower pan. Use plenty of silicone.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Feb 1, 2020 at 13:48

For what it's worth and for anyone who finds this thread later with the same question, here's what I ended up going with:

I had my floor/tile guy put down cement board and then seal it with RedGard to make it waterproof. (It is a small bathroom, so he sealed the whole floor.) Then the plumber came in and installed the shower base. When the floor/tile guy comes back he'll run the tile right up to the shower base.

Doing it this way seemed to be the best of both worlds: the plumber was no longer worried about water working its way between the tile and base to corrode the subfloor, and the floor/tile guy was familiar with working with the RedGard.

I guess I won't know for sure if this was the right way to do it until I remodel that bathroom in 10+ years.

  • 1
    So essentially you chose plan #2 There were several answers that said go with plan #2, you chose one of those as correct, Since We took the time to answer your question.
    – Alaska Man
    Feb 26, 2020 at 19:52
  • i wouldn't trust a liquid applied membrane on a floor. vertical is ok. lots of red guard failure videos on youtube. Issac Ostrom has great water proofing videos for tile and floor systems. Then again red guard on the floor is likely the same as caulking the joint from the showerbase to the tile and unless you are flooding the bathroom probably all that is needed for a bathroom to last more than 20 years. Feb 26, 2020 at 20:27

Not sure if it's the "right" way, but I would go with 2). It'll be a litte more work putting the cement backer board in around the shower drain. Have to make sure you seal all the seams in the backer board with the right material, thinset+tape?

I assume you're putting in a fiberglass shower base?

  • yes, fiberglass shower base. I'll edit original question.
    – Aeryk
    Jan 29, 2020 at 17:59

The shower pan is what contains the water. Tile, grout, cement backer board, plywood, etc. will not stop water from getting out. A properly installed and drained pan will prevent any water from escaping and causing damage to the subfloor, joists, or downstairs ceiling.

So the important thing is to get the shower water-tight to begin with using the appropriate materials. All paths for water need to lead to the drain!

  • True, except for the front lip where the base meets the floor. That's the gap I'm most worried about.
    – Aeryk
    Jan 29, 2020 at 18:23
  • OP says he's using a fiberglass shower base.
    – SteveSh
    Jan 29, 2020 at 18:32

Plan 2, gives your the extra strength under a fiberglass pan. tile to the pan, and caulk where it meets the floor, just like a tub

Plan 3, It is just wasted money and if you have to ever pull the shower pan out to address problems with plumbing it will be much more work to pull up the tile.

Tell your tile guy you can find someone who is happy to accommodate the home owners decisions. ( but i do not understand why two trips are needed.)

  • "Tile guy" is a friend of the family. Two trips: one to lay the cement board before the shower base, one to do tile after shower base is installed.
    – Aeryk
    Jan 29, 2020 at 19:55
  • pull the shower pan? These are typically concreted to the subfloor, do that and you have to buy a new shower unit. Jan 29, 2020 at 20:52

Option 1.

If you want to go crazy on water proofing then you'd install a liner on your subfloor maybe keri, use keri band to go up the walls and treat it like a shower stall, ardex 8/9 the joins then install the shower and then the tile. Unless the tile guy is saying he needs a layer of concrete board in which case you would want to install that under the shower base too before the water proofing.

This would add a few thousand to the project. Putting tile or concrete board under the shower base will do nothing to enhance the water proofing without a waterproofing layer.

The way I always do this is shower base straight to plywood subfloor. Caulk the joint. You could go with an epoxy grout for the tile to enhance the waterproofing without a membrane. Some caulking is rated for 30 years. If you expect the bathroom floor to be treated like a shower with standing pools of water then spend a few thousand more for the membrane option.


The tub gets installed first. A Fiberglass tub/shower is your water barrier. Putting anything else down does not help. The water would just be trapped between the tub and the other "water barrier" creating an environment for mold and other issues.

The fact is if a tub/shower pan is installed correctly there is almost a zero chance it leaks unless it gets clogged - even then it is highly unlikely. But if it does leak you want to know right away not have the water find itself a path 10 feet away to escape.

Now there is a few things that you can do to help a possible leak:

  1. Blocking on inside of tub/shower. If you can provide as much support as possible to tub on top sides and bottom (flex) add them in. This isn't always an option and is dependent on the type of unit you get.
  2. Super important is adding a mud mix under the basin. You should not be installing on hollow subfloor. You should be using mud and "squishing" the tub/shower into its place. This will add support on flex, it will help stop movement and it will point water to drain. I am not giving directions on how to do this here - but mentioning it is an important step.
  3. Create a small waterproof lip between tub/shower and tiled area. Tons of materials you can use here but just a slight lip that goes under concrete board and attached to tub/shower. You can trim it after tiling and grout/caulk over it.

There really isn't "another way" to do a bathroom. I have done too many to count and doing them any other way creates innate problems and issues with not only performance but makes the install harder.


ok first off i would like to say you should have just hired someone who could have done all the work themselves , but for any of these options before you can put down the shower pan they have to waterproof underneath it and also use a sand and cement mixture to set the pan to the floor no matter what is underneath and usually you should never install over tile because the tile is almost never perfectly level and the sand cement mix is going to destroy the tile and it can get moldy and you would never know . the correct way is to put it on the sub floor with the correct base beneath it and as long as it have the walls installed correctly as well it should last a very long time good luck .

  • 1
    "someone who could have done all the work themselves" is that the homeowner doing DIY? I've never heard of a plumber that also does tile work or tile guys who sideline in plumbing. Are they common in your neck of the woods?
    – FreeMan
    Feb 4, 2022 at 13:20
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