Drywall to Backerboard transition in tiled showers

by Roger

drywall, backerboard transitionWhen you tear out and rebuild your shower walls you are left with a transition between the old, existing drywall and the new stuff – cement backerboard or drywall (if you’re using kerdi). Whaddya do with it? And how do you do it? And why am I the one asking questions – that seems backwards.

If at all possible, when you remove the old stuff you want to cut a straight line down the drywall to make for a clean transition. If it isn’t straight or was simply torn out without any regards to actually rebuilding it, then find a spot where you can cut a straight line from top to bottom. You want to have a level line for your transition.

So before you begin you want something similar to that horrible graphic right there I just created with a bottle of scotch and my toes. The left side is looking into the wall cavity with one stud, that big brown looking thing? Yeah, it’s supposed to be a wall stud. You are not allowed to give me crap about my lack of Photoshop skills!

drywall, backerboard transitionWhat we need is a way to shore out the new substrate (backerboard) to be solid and on an even plane with the existing stuff. We have a very, very specialized item for this. Listen carefully, because it’s a deeply guarded secret. Ready?

It’s a  2×4.

Take a 2×4 and cut it to the length of either the entire wall or simply from about six inches from the top to six inches below the bottom. The latter is often the only way to do it – you still need to be able to get it into the wall cavity over the tub and around the other studs. It needs to fit in there.

Just take the 2×4 and get it into the wall. Turn it so that the width (3 1/2″) is split between the open space and the existing drywall. There will be 1 3/4″ behind the existing drywall and 1 3/4″ to screw the backerboard to. Once it’s in there it will look nothing like that second horrible graphic – but it will give you the gist of it.

You can see 1/2 of the 2×4 and the dotted line on the drywall outlines the other half. Just screw right through the drywall into the stud to hold it in place.

drywall, backerboard transitionNow you can take your cement backerboard (or whatever your substrate is going to be) and place it up to the edge – leave about a 1/16″ gap between the backerboard and drywall. Then just screw through the edge of your substrate into the other half of the 2×4.

Make sure you measure whatever product you’re using for your substrate. Your existing drywall is likely 1/2″ thick – your substrate likely is not – it is probably a touch smaller. To get them even and on the same plane you can use regular drywall shims behind it.

1/2″ backerboard is rarely 1/2″! It is often smaller – make sure you measure it and shim it out as necessary. Once you get it installed you still need to tape and mud the seam. Just use the same alkali-resistant mesh tape and thinset that you’re using for the rest of the backerboard seams. Don’t know what I’m talking about? Read Installing backerboards on walls for shower tile.

Once that’s all finished you can install the tile as normal, just like the photo below. The transition is directly under the bullnose tile on the edges of the shower. And yes – you can paint right over the thinset if you need to.

Tiled shower with backerboard transition

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I am redoing my bathroom and tearing out the shower walls. First question is what is the best boards to use to avoid mold. I have heard several options and some from what I was told would be overkill. (1) Only tear out the existing mortar and apply mortar and chicken wire over it–no need for tear out (2)tear out to studs and use greenboard mortar/chicken wire over it (I always thought greenboard was frowned upon because of the humidity and moisture) (3) greenboard covered with redgard and don’t really need mortar/chicken wire because it would be a floating wall (4) using backerboard with mortar and chicken wire. I have heard about denshield boards but they are rather expensive.


Susan Jimemez

We hired someone to do our bathroom and he forgot we wanted the tile off set and he tiled half up one wall of the shower when we noticed it. He then took the tile off and it reaped up some of the green board behind it and he covered it with joint compound which he said would be like cement and he tiled over it and said that there was no need to put new green board. my husband and I are worried about mold. Is this okay to do and if so should we make sure to seal the tile and grout twice after he is done tiling so mold doesn’t occur.


Patrick Kelly

I have a small alcove that I will be placing a utility sink in. There will only be 1/8 of free space on each side of the sink. Do you have any suggestions on making the walls either water resistance or water proof from any water that may get splashed on the wall? There is not enough space for tiles.



Hi, I am tiling shower walls in a basement bathroom. I had already installed green board before I decided to tile. Is it ok to put the cement board over the green board? I sure hope so because I worked on it all day😟. I did put a moisture barrier over the green board before I installed the cement board. Any advise would be much appreciated.



Hi Sissy,

Not really a good method, but it should be fine.



Hi, I’m redoing my shower. Im using mold and resistant drywall. I don’t want to take it off now but, are drywall screws ok, can I apply a thinset over the drywall and use waterproof ing membrane after. I know its alot but drywall is up.



Hi Cheryl,

You can use schluter kerdi over drywall. That is the only approved waterproofing membrane for use over drywall.


Richard A

Is it ok to stack 1/2″ (.42″) and 1/4″ to transition to 5/8″ drywall? Anything need to go between them?



Hi Richard,

Yes, you can, and no, nothing goes between them. But it would be easier to put 1/4″ (actually should be 1/8″) shims over the studs, then install the 1/2″.



Hoping you can help! So we are completely redoing our whole bathroom in our old house. We are planning to tile in the bath all the way up to the ceiling. We are in the process of installing the cement backer board but have just realized we are unsure on how to handle the joint between the backer board and the ceiling. Can you give me some tips on this? We came up with the idea to use drywall in the last 6-8 inches so we could just use drywall tape and mud, but seems there should be an easier way. Also I’m fearful there will be a bit of a gap between the wall and the ceiling(the old tub surround must’ve been thicker).



Hi Jackie,

You can use regular drywall tape and mud from the backer to the drywall. It works on backer as well.


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