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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  • Allen Siegel

    My old 1/2 ” Drywall is thicker than the 1/2″ cement board. How do I get a smooth transition?

  • Dave Walters

    Hello !

    Great info on this thread, thank you so much. I am converting a tub to a walk in shower, to be tiled to the ceiling. The existing tub surround fiberglass material goes up 5-6’ above the tub, and is screwed to the (metal) studs, no wall board behind it. Painted drywall above that to the ceiling. Am I OK installing concrete backerboard in the empty spaces, leaving the last few feet of drywall? This assumes I line up the transition edge well enough, and waterproof all the way to the ceiling, over the last 2’ of drywall. Since that is so high up I want to assume the normal concerns about tiling over drywall in a shower are mitigated. I would rather not tear out all the drywall if leaving it is no big deal. Thanks.

  • JDavies

    Hi Roger,

    First off, I gotta thank you for your informative and funny blog. I never knew tile could be a hoot!

    The Floor Elf was one of the resources that helped me weather a recent crisis I caused by yanking on a loose tile in my bathroom. Sluuuuuurk! Off came two rows of tile in my hand, attached to what I thought was a wet sponge but turned out to be drywall. Like an idiot I kept pulling.

    I ended up with an 8.5″ x 7′ hole around two sides of my tub. And company coming in a week. In a house with one bathroom! I had no hope of matching the old patterned tile, but I couldn’t afford the total gut-and-rebuild I wanted to do.

    So I did the best half-assed patch job I could, following advice from your site and other places on the internet. A local tile boutique sold me a sheet of slightly cut-up Kerdi-board for $50 and I was off to the races.

    Back to the topic of transitions, my final finish issue has me stumped. It has long annoyed me that the tub cuts into the trim of the bathroom door. The trim consists of a wide pilaster with raised mitered casing on the outside.

    I assume that when the house was built (1926) the tub was freestanding and clear of the door, but when the claw-foot was replaced with a flush-mounted tub, it wouldn’t fit without cutting into the door trim.

    This meant the mitered casing stopped at the top of the tub, and the tub continued about a half inch into the pilaster. Gobs of caulk did not keep the casing and the base of the pilaster from eventually rotting.

    Right now, having pulled off the casing to get at the outer edge of the tiles (which were cut right up to the edge of the casing) I have a 7/8″ gap between the cut tiles and the pilaster.

    I would like to replace the pilaster, which except for the rot is nice never-painted varnished wood, and maybe the outer casing if I can, but how do I keep them from rotting, butted up against the tub as they are?

    I’ve thought about tiling the 7/8″ gap where the casing used to be with lengths of the 4×4 patch tile I have, or some oddball bullnose trim kind of tile, but that still leaves the pilaster to soak up water from the tub enclosure.

    What would you recommend to a DIYer who wants to do things as right as a half-assed budget allows?

    Photos attached. Yeah, the tiles don’t match, but they do look better than a yawning hole! The dark part of the gap is Kerdi-board with a smear of thinset; the light part is old painted drywall.

  • PhLo

    I’m working to fix a leak in tiled wall around a very old tub for my parents. Every illustrated scenario online describes presumably newer tubs with significantly tall flanges or lips around the top edge of the tub. This old tub’s edge barely curves upward, maybe only 1/8″ or slightly more. My concern is, after the cement board is installed and waterproofed, should the crack/gap between the cement board and shallow tub lip be sealed somehow to prevent water from leaking into the wall, since everyone says to assume tiles and thinset won’t prevent water from getting to this area? Because of the shallow slope, it seems it wouldn’t take much water to slip over the back edge into the wall. If i do seal it, should thinset or caulk be used? Over the cement board I’ll reinstall the same old tiles, regrout and caulk the bottom from tile to tub. I included a photo so you can see the very shallow tub lip.

    Thanks for all your articles and comment responses. I’ve read through many and learned a lot for this project.

    • Roger

      Hi PhLo,

      Once you install the backer you need to silicone between the bottom of the backer and the tub lip to seal it. You did not say what particular waterproofing you are using, but it then is installed over the silicone all the way down to the tub. The combination of the two will seal that space and prevent water from getting back in there.

      • PhLo

        Thanks, Roger. That’s a very confident answer, and I’m gonna run with it. Silicone it is. I haven’t selected a waterproofing membrane yet, as I’m going to “shop around” to see if there are any cheaper alternatives to RedGard available locally. I don’t need a whole gallon for this small project, as I only took out a few rows of tiles near the bottom where the water damage occurred. I know it’s not ideal because drywall still exists behind the upper part of the shower. But we don’t have the budget to take out all the vintage tiles that line the entire bathroom and redo everything. Hopefully thinset at the top where the new cement board meets the old drywall will keep water from sneaking between the two. Fortunately the drywall was only damaged and crumbling near the bottom. The lower part will be “properly done” with cement board and waterproof coating. All of this is really just a band-aid, and a larger renovation will probably get done years from now if finances allow. The whole tub needs replacing someday. Its finish is totally shot, now sandy, porous and hard to clean. I thought about the vapor shield option behind too, but since we aren’t going all the way up with replacing (as we should, I know), and there’s not much tub lip to silicone the plastic vapor shield to, the RedGard-ish topical option seemed like the more sensible. I’m glad I have read your comments about waterproofing over the silicone because my first hypothesis is to not do that since silicone is so stinking malleable. I figured the waterproofing coat would just flake off the silicone since the stuff flexes so easily… but I’ll do it as you say since I have no other ideas, and I’m sure you’ve dealt with this junk before.

  • Bill Leiderbrand

    Can I just cut the old drywall following the rim of the tub. And replace with backer board or do I need to take out ALL drywall and how would that be done without taking out tub?

    • Roger

      Hi Bill,

      You can just cut the drywall.

  • SueInVA

    Love your shower niches. What did you use for the shelf? I’m using white subway tile for the shower and have a decorative tile for the back of the niche but having a hard time finding something white for the shelves that’s finished on the top, bottom, and front.

    • Roger

      Hi Sue,

      I use the tile itself. It is not wrapped around anything, it’s simply tiles bonded together to form a shelf.

  • Dave McLachlan

    Does drywall have to be “primed” before installing kerri membrane?

    • Roger

      Hi Dave,


  • Manny

    Question: I am replacing a ceiling in a bathroom and the tile on the wall goes all the way to the ceiling. When I put the greenboard on the ceiling how do I join with the wall tile (where the tape and then mud would go if it was drywall or green board and not tile on the wall)?

    • Roger

      Hi Manny,

      You can’t unless you remove the top row of tile, then it’s just tape and mud as normal. The best you can do without doing that is silicone and hope it doesn’t move much.

  • Rastussir

    I am installing a tile shower in my basement, and will need to transition from existing drywall to hardiboard. When you install the 2×4 this way, is it strong enough to hold when installing screws through the backerboard and into the 2×4? Will it push the screw heads through the sheetrock as there is nothing behind it to support it?

    • Roger

      Hi Rastussir,

      Yes, it’s strong enough, unless you’re trying to push the screws in with undue force. Never had any issues with that.

  • Eric Sadler

    I squared up my walls and leveled up my studs. The I put up the hardiboard. In certain spots where the hardiboard and my sheet rock meet it is unlikely even. The hardiboard comes out fear there than the drywall. Not sure what to do.

    • Roger

      Hi Eric,

      If you don’t want to pull down the backer and shim out the drywall (who wants to do that?) you can always float the drywall up to flush with the backer and repaint.