Shower Tile Basics or Shower Tile 101

by Roger

I get a LOT of questions from my readers about basic shower construction. I understand that my readers don’t consider this stuff basic and there’s no problem with that. The problem is that I end up answering the same questions over and over and over… So, to save what very little is left of my sanity (which is a number roughly equivalent to absolute zero) I will cover some basic things here so I can simply reply ‘read this’.

If you’ve been channeled to this page by one of my smart-ass comments please take no offense to it, I’m here to help. Please understand that I currently have over 12,000 comments (questions) on this site (seriously) which I’ve answered – every one of them. I’m just trying to make your life (mine) easier.  I will continue to answer every question I’m asked, I’m just super cool like that. 8) If, after reading through this, you still have questions feel free to ask them in the comments below.

You can also download my shower waterproofing manual which should answer a lot of questions and cover basic techniques and methods you may be confused about. Go ahead, it’s free.  So without further ado (doesn’t even look like a word, does it?) let’s get on with it. (For all my readers who feel the need to correct me: I KNOW it’s actually ‘adieu’ – I was being facetious. Thanks. :D )


First and foremost – tile is not waterproof. Grout is not waterproof. Adding sealer to your tile or stone will not make it waterproof. Your shower should be completely waterproof before a box of tile is even opened! No matter which waterproofing method you choose, proper substrate preparation is the only thing that will make your shower waterproof.

If you have a leak in your shower – stop using it immediately if at all possible. If that is not possible (it’s your only shower) have the shower repaired – immediately. If you see water leaking it is likely not nearly as much water as you don’t see leaking into your wall cavity and structural framing. By the time you ‘see’ most leaks the framing is normally already considerably compromised.

No, there is nothing you can put over your tile to make your shower waterproof if you have a leak – not even sealer. A tile or stone sealer is made to make your tile and grout stain-resistant, not waterproof. It does this by sealing the pores of the tile and stone to slow (NOT STOP) the absorption of liquid and prevent staining. It only means you have more time to clean up the spilled red wine cherry kool-aid before it stains anything.


Drywall is not an acceptable substrate for your shower unless you are using Schluter Kerdi waterproofing membrane – that’s it. Cement backerboards are the standard and there are also other products such as waterproofed, gypsum based boards like Denshield and waterproofed, foam-based sheets like wedi or kerdi board.

Cement backerboards are not waterproof. They are water stable, which simply means that they will not swell or disintegrate when exposed to moisture or water – they won’t change size. But they are just like your driveway, they will soak in water, hold water, and dry out, just like your driveway when it rains. If using backerboards there needs to be a waterproof membrane utilized as well.


If you are using a topical waterproofing membrane such as a liquid like redgard or hydroban, or a sheet like kerdi, do not use a moisture or vapor barrier behind your substrate. If you have a vapor or moisture barrier behind your substrate do not use a topical membrane on the front of it. This combination creates two waterproof barriers with your substrate sandwiched between them. any vapor or moisture trapped between them has absolutely no way to dissipate. This is lovingly referred to as a ‘mold sandwich’. It is not tasty. Use either a moisture or vapor barrier behind your substrate or a topical membrane on the face of it. One or the other – never both.

With that said, if you want to use a topical liquid such as redgard on the seams of your backerboard, after you tape and mud them, you can do so without problems. If your moisture barrier and backerboards are properly installed there is no real reason to do so – but if it will help you sleep at night go ahead and do it.

If you are using a topical membrane and you have an exterior wall with either plastic facing or kraft paper facing you need to cut slits into that facing before installing your substrate. If you do not it will create the aforementioned mold sandwich. Give moisture or vapor somewhere to dissipate.

Shower-tub transitions

There should be a gap between your tub or acrylic shower base and the bottom of your backerboard. If you are using a traditional barrier waterproofing method you do not need to do anything with this gap. Do not fill it with silicone! This will trap moisture running down your barrier and it will have nowhere to go. If you are using a topical method you can fill it with silicone if you want. If you are using liquid you should fill it with silicone. This creates a waterproof plane between your membrane and the tub or base so water or moisture ends up in the drain rather than in your wall.

When you tile you can tile right over that gap. You should not fill this gap with thinset behind the tile – it will crack due to movement. Your wall and tub or base will expand and contract at different rates – it will crack any cement-based product you place between the two. This includes thinset as well as grout. You do not need a solid backing behind your tile over this gap – it should be less than 1″ wide. You shouldn’t normally be walking on that tile in that particular spot. Yes, it can just hang there.


If your grout is cracking it is due to movement 99.9% of the time. Type ‘cracking’ into the search box up there and you’ll find in-depth explanations for your viewing pleasure.

If you have white, or lighter than normal grout when you’re finished grouting it may be efflorescence. This is mainly due to minerals in the water being left on the surface of the grout when the water evaporates. It is usually indicative of either incorrectly mixed grout or using too much water while cleaning the grout – not wringing out your sponge enough. This is normally only on the surface of your grout. Scratch the very top layer of your grout in an inconspicuous spot with your fingernail. If you have the correct, or at least a darker, color beneath the surface that is the likely cause. The easiest, quickest fix, provided it is only the very top layer, is to get some drywall sanding sponges and go over the grout lines very lightly. Just like burnt toast – scrape it to the color you like.

Corners and changes-of-plane

Caulk. :D

Disagree with me?

Like any other website I get my fair share of people who disagree with my methods or techniques – it really doesn’t bother me. I am more than willing to have a civilized, intelligent conversation about anything tile related. If, however, you simply attack me personally and act like an uncivilized ass I will call you on it – and not in a nice way. I write this blog in a particular manner, it does not mean that I take my profession lightly – I most certainly do not. If I wanted to be a pompous bastard I currently have 19 letters I can place after my name – all tile related. But I’m not a pompous bastard – you can just call me Roger. :D

If you are a contractor with a customer who has come here for information and I’ve told them you’re wrong – realize that the extent of my knowledge of that particular project is limited to what I’m told by your customer. It does not mean that I am ‘out to get you’ or anything of the sort. I normally limit my replies to those situations by stating the current TCNA and/or ANSI standards pertaining to what I’m told. If you disagree – please let me know in a civil manner, If you’re correct I’ll back it up – I do this everyday, too! If you’re an asshole about it expect the same in return (It’s the comment by ‘Kanela’ with the bold print before it) – and please have a thesaurus handy to interpret my reply – I’m a very well educated asshole.

That’s it for now. I will likely add to this post on a regular basis. If there is anything I’ve caused confusion with please, for the love of God, let me know. I can change this page since, you know, it’s my website and all. If you have any questions at all please ask them below – I’ll answer, really. :D

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Hello Roger,

My question is for a tub to backboard/tile transition when using a liquid topical membrane, like RedGard. You state that if a liquid membrane is being used, then the gap should be filled with silicone. I need some clarification on this. Are you stating that, prior to setting the tile, the 1/4″ gap between the backboard and tub lip should be filled with silicone, or just the 1/8″ gap between the tile and tub (like illustrated in the attached photo? Or both? Thanks!




I am wanting to pebble around my Jacuzzi tub it is plywood frame how do I go about it I have hand picked the pebbles I want to use just unsure on how to waterproof the plywood before before starting the tileing process and help would be appreciated



Hi Kathy,

You need to put a proper substrate over the plywood. 1/4″ backerboard would be easiest. Then you can use any type of topical waterproofing on it. Or you can google ‘greenskin’, that will actually be your easiest and quickest option.



I have a question about the bottom course of the shower walls. As you know a shower pan is not flat on the edges where the pan & wall meet. Do I need to cut the tiles to the contour of the pan or float them in a level straight line slightly hovering the pan? I know there will be a gap if I float it, but the gap in the middle of the pan edge is larger than the corner edges. When starting the course in the corner, the tiles are level. But towards the middle of the walls, the gap gets larger and tapers back to level to the opposite corner. I have to keep it level to wrap the enclosure, but the bottom course is really got me confused. Some wisdom & insightful knowledge would be a great help.



Hi John,

I set them on the tub in the corners and shim them up as it runs to the center to maintain a level course. Just silicone that open area once it cures.



So I keep a straight line going around the circumference of the shower, correct? What if there is a ¾” gap between the pan tile & wall tile? I know the pan should be level on the outer edge going around. Unfortunately the person who skimmed the mortar (not me) didn’t know proper procedure & towards the middle of the wall there’s a dip. My brother(the one who skimmed it) seems to think that cutting the tile to contour the dip will look better than what you suggested. I think it will look like s__t if I try to cut for the contour, because it will throw everything out of level. Also how big of a grout line is acceptable in the corners?



Hi John,

A level line around the perimeter, yes. If you cut to the contour it WILL look like crap. Can you skim over the deck to straighten it out? I normally have between 1/16″ and 1/8″ in the corners.



Sorry for the late reply. But unfortunately the pan has already been tiled. So adding/skimming mortar ain’t gonna work. I thought the same thing. I’m considering going with my initial thought and what the suggestion you made. So it doesn’t matter if there’s a gap as long as the bottom course is level wrapped around the perimeter? And fill the gap with silicone?



Nice article. But what about REPAIRS? As a handyman I get plenty of calls for loose for re-caulking, grouting, etc. Have a job now with several tiles falling off the wall. Grout failed. Water got in. Paper peeled off backing. Which looked like green board drywall. I would usually fix by removing the tiles. Cleaning. Treating with molded idea. Fans. And drying until moisture meter shows normal. Then reattach through tiles and grout. Plus skim coat grout the whole shower to fill in all the other holess and cracks. Plus re caulk allike joints too. Obviously the shower is jacked. But if someone’s selling or remodeling soon sometimes a few years is all they’re looking to buy. WHAT ABOUT SEALER? Or something on that damaged drywall before I reattach he tiles? There’s no paper left. It’s wet for now. But solid.



Hi Mark,

Did you read the article??? In the VERY FIRST subheading I stated, very clearly, that tile is NOT waterproof – that water WILL get behind it. Grout has NOTHING to do with it. Pinholes in grout, or missing grout, have NOTHING to do with it. So your premise that you ‘skim coat grout’ the entire shower has absolutely nothing to do with preventing water from penetrating the tile assembly.

I do not have any articles explaining repairs of any sort. The reason is simple: if I remove a tile and it has greenboard, or any other type of substrate behind it that has ‘paper’ coming off of it – the entire shower gets replaced or I don’t touch it. See, once I touch that shower in any type of capacity to ‘repair’ it – I OWN that shower, the entire thing. If (when, and it will) it leaks again, or more problems arise in that shower I am completely responsible for it.

To answer your specific questions: Water did not get in because the grout failed – the grout failed because tile was installed over an improperly waterproofed substrate – the grout had absolutely nothing to do with it. Grout failing is simply a symptom of a seriously larger problem. Drying until a moisture meter reads normal will only last until the shower is used – one time. Doing that may fix the aesthetic surface for a ‘few years’, but there are issues behind that tile, in the structural framing, that will be deteriorating over those few years that will not be seen until that jacked shower is torn out.

Sealer DOES NOT waterproof tile in any form at all. The ONLY approved waterproofing for use over drywall is schluter kerdi, and once the paper is gone it’s iffy, at best. And putting a temporary band-aid over a small area of an improperly waterproofed shower will do absolutely nothing at all to repair that shower. All it’s gonna do is make it look pretty for a little while because the underlying issue has been covered up.



I am putting a tile shower in the basement. I have 2×4 walls on the outside wall of the house with fiberglass batt insulation with a vapour barrier. Can I put a kerdi wall right over top of the vapour barrier….or do I need to drywall first or????
Can you please advise me the correct way.
thanks …much appreciated



Hi Richard,

I’m unsure what you mean by a ‘kerdi wall’. You can put kerdi-board directly over it, or you can put cement backerboard or drywall over it with the kerdi membrane on top of it. You can not put kerdi directly over the barrier.


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