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 confused. Everything I read about backer-board products says they are water resistant, not water proof; hence the membrane between the backer board and studs…So why is there no barrier required between the edge of the backer board and the sheet rock at the edge of the shower wall? Without some type of barrier, won’t the moisture in the backer board seep into the sheet rock?

Thank you



Hi JB,

Not normally. The transition should be outside the wet area, and water doesn’t run sideways, it always runs down.



Hi Roger,
Thank you much glad you’re out their helping



Hi Roger,

I hang Hardie backer board in the shower walls, cement board seam and drywall seam meet right outside of the entire tub. The two dont meet flush and i plan to tile 3″ past the outside of the tub and down the wall because the shower tile is the same as floor tile. So my question is how do I fix this without removing the cement board and shimming it. Do I just lay a thicker layer of thinset when laying tiles so the tiles will be flush or do I try to make the walls flush with thinset and let dry and then tile. I have taped thinset seams as u have said and waterproof all the shower even beyond where tile will be placed. But since I didn’t see the seams weren’t flush til after hung I figured I can correct this but now secwguessing myself.
Thank you



Hi Greg,

You can do either of those – they will both work just fine.



The tub was prepared with cement board an inch below where I would be tiling in order to hide the seam behind the tile. I used modified Versa thinset to mud the seam between the sheetrock and the cement board. I applied spackle in some areas only on the sheetrock. The sheetrock will be painted
Since it appears that I will have a lot of tiles left over, I plan to lay subway tile up to 9 inches higher than I initially planned. With this new plan I will be laying the subway tile above the cement board onto the sheetrock and there are a few spackled areas.
I read you can apply Redgard and in another site they recommended a good primer over the area. I have the primer and the area is not as likely to get that much water.
Question 1
What do you recommend?

Question 2
I read online that many use 4 mil plastic behind the cement board and other places that is not mentioned. What’s the deal on that?



Hi Carl,

You can install the tile right onto the sheet rock. Above the shower head it doesn’t create an issue at all.

To waterproof a shower you need either a moisture barrier behind the backer (4 mil plastic) or a topical membrane over the face of it – never both.



Where my bullnose tile meets the drywall, there is about a 1/8 to 3/16 stand off from the buildup of thin set to compensate for irregularities in the wall. Should this area be grouted, silicone caulk that is color matched, or acrylic color matched caulk since paint will need to go on? With the later, should I paint first and then caulk?

Thanks in advance!!!
Me and my beer(s)



Hi Mark,

Ideally siliconed, but against the drywall on the back of bullnose it can usually be grouted without any issues.


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