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Steve Gehrmann is a remodeling contractor (SKG Renovations) as well as a partner in Redblock Industries

In this post I’ll explain everything you need to know about shower tile backer boards so you can decide which one is best for your shower remodel project and your budget.

I’ll discuss each type of board and how they compare in price and performance based on my personal experience as a bathroom remodel contractor. I even put it into a handy table!

I’ve also provided a bit of a background to the tile backer board story and some install tips at the end to keep you on track after you’ve decided on your board.

This is Steve from SKG Renovations with another addition to my shower remodel series.

Ok, let’s get into it!

Topics Covered:

Purpose of Shower Tile Backer Board

4 Types of Shower Wall Board

Comparing Shower Tile Backer Boards

Comparing Tile Backer Boards – A Deep Dive

Shower Backer Board Install Tips

Purpose of Shower Tile Backer Board

There is an understandable confusion about the function of tile backer board on the shower wall because all backer boards do not share the same performance characteristics.

In other words, different shower backer boards perform different functions yet they are all sold to the same group of people and they all claim to be the best solution for your shower wall.

What most people can agree on is that their main purpose is to provide a secure bonding surface for a waterproofing membrane and tile. But that’s where the clarity in purpose diverges somewhat.

What is Tile Backer Board ?

Traditional Tile Backer Board is made of aggregated Portland cement with fiberglass reinforcement. It serves as structural sheeting and a secure bonding surface for waterproofing membrane, thinset mortar and tile. Some newer boards are cementitious, while others contain silica sand, wood fiber, gypsum, fiberglass, or foam. Moisture and mold resistance are now common features, but foam boards also offer a complete shower waterproofing solution.

Shower Wall Board Problems

In the early days, gypsum wall board was created to replace the very labor-intensive lath and plaster walls, and became the dominant wall board in the 1950’s. It was used everywhere in the home including the shower.

Drywall was considered to be an adequate shower tile substrate because tile and grout was assumed to be mostly waterproof.

It wasn’t long before it was discovered that a significant amount of moisture passes through tile and grout, especially ceramic tile and unsealed cementitious grout.

Moisture behind the tile assembly caused a few major issues.

With no waterproofing membrane, the wallboard absorbed significant moisture, causing it to swell and became more flexible. This led to a cascade reaction of movement, grout line cracking, and more moisture entering the wallboard.

And with moisture and warmth also came mold growth and still further deterioration.

In other words, these early backer boards didn’t hold up well to the moisture exposure of a typical old-school shower.

Enter the Shower Tile Backer Board

In order to solve these problems, the cement tile backer board was created.

A cement based board dramatically reduced the deterioration of the backer by reducing moisture absorption and most importantly, reducing the expansion, contraction, and mold growth that occurred in the old drywall boards.

Less movement in this new cementitious board also meant less chance of grout line cracking, which meant that moisture had a tougher time seeping through and causing the cascade failure we talked about earlier.

Everything was good!…… Well maybe not so good.

The problem was (and still is) that this “solution” didn’t address the first problem of vapor transmission through the tile assembly, or the second problem of absorption and transmission of moisture through the backer board.

The cement backer board still got moist just like the drywall board did, but there was much less movement and deflection so the tile assembly remained mostly intact.

A huge win!

New Shower Board Confusion

Although the problem of swelling/ deflection in the backer board was largely solved, the negative implications of vapor/ moisture transmission through the shower wall were not fully understood for a long while after cement backer board was introduced.

Eventually they realized that with todays modern “air tight” homes, moisture that entered the structure was not dissipating as it was in older homes. Excessive moisture tended to become trapped in the shower wall causing some serious problems.

Eventually some great waterproofing products and strategies were introduced to address the moisture/ vapor transmission problem. And some tile backer boards also address this problem, while others only pretend to address it.

The swelling/ deflection problem is very different from the moisture transmission problem, and they needed to be addressed in different ways.

This makes it difficult to use only one product to address both, even with the high-tech products available today.

That brings us to the actual shower tile backer board products out there today.

How do these boards differ? What materials are they made from? What problems do they claim to solve and which do they actually solve?

Find out below!

4 Types of Shower Wall Board

  • Cement board
  • Fiber cement board
  • Glass mat tile backer board
  • Foam backer board (cement coated & membrane coated)

Cement Board

Face of Durock cement tile backer board
Profile of Durock Cement tile backer board

The newer versions of Cement tile backer board or Cementitious Backer Units (CBU’s), are very similar to the original cement backer board. They are all made with inorganic materials – lightly aggregated Portland cement with fiberglass reinforcement.

They are designed to be impact resistant, fire resistant as well as a rigid, efficient bonding substrate for tile.

The modern versions of these boards often contain polymers with a finer fiberglass mesh on front and back, and wrapped around the long edges.

These boards are NOT moisture resistant, and in fact most absorb moisture quite readily. However, extra additives in these boards allow them to resist fungal growth and deterioration when exposed to water.

Cement boards are generally produced in only 1/2″ thicknesses because they would be too brittle and structurally unsound if they were made any thinner.

This is the least expensive shower tile backer board at around $10/board (3’x5′).

Examples of these boards include Durock Cement Board by USGWonderboard by Custom Building ProductsPermaBase by National Gypsum and  Triton Backer Board by Triton Watertight Systems.

Fiber Cement Board

Face of Hardibacker tile backer board
Profile of Hardibacker tile backer board

There are a few versions of these boards from different manufacturers, but they all utilize cellulose fibers (wood fibers) to make the board more flexible (less brittle).

Similar to cement backer board, these boards are generally uniform in composition (not layered). Some boards incorporate a mixture of silica sand and cellulose fibers while others use gypsum and cellulose fibers.

They also have a uniform (non-layered) and dense composition, which allows these backer boards to be manufactured in 1/4″ and 1/2″ thicknesses.

All the fiber cement boards claim to resist compression, moisture and mold.

These boards can be very similar in price to cement board, or as much as 1.5x the cost of cement backer board.

Examples of these are Hardibacker by James Hardy and Fiberock by USG and Allura Fiber Cement Backerboard.

Glass Mat Gypsum Board

Face of Densshield shower tile backer board
Profile of Densshield shower tile backer board

Glass Mat Gypsum board is a layered product composed of a gypsum inner core with a fiberglass layer on the front face and back face of the board.

Although these products look much like regular gypsum wall board they have an entirely water resistant, paper free (mostly fiberglass) surface, and a water resistant gypsum inner core.

Glass mat tile backer board only comes in a 1/2″ thickness, because it would not be structurally rigid enough if any thinner.

These boards can vary in price from some being similar to cement board, to as much as 1.5x the cost of cement backer board.

Examples of glass mat backer boards are Diamondback by Certainteed, Denshield by Georgia Pacific, and Durock Glass-Mat Tile Backerboard by USG

Foam Backer Board

Face of Kerdi Board shower tile backer board
Profile of Kerdi Board shower tile backer board

Most foam backer boards are made from extruded polystyrene foam, but a few are composed of high density polyisocyanurate closed-cell foam as well.

They are covered with a layer of reinforcement material (either fabric/ fleece or cement) on each side that’s also designed for bonding to thinset mortar.

These boards are up to 80% lighter than cement boards and are highly moisture resistant. In most cases, no additional waterproofing is required after installing these boards and sealing the seams and fasteners.

Most manufacturers of these boards make them in several thicknesses so they can be used to create structural assemblies like benches and partition walls. The minimum recommended thickness for the shower wall is 1/2″ because like some of the other boards, they become far too flexible at thicknesses below 1/2″.

Foam shower tile backer boards are generally about 4x the cost of cement board.

Examples of these include Kerdi Board by Schluter Systems, GoBoard by Johns Manville, and Wedi Building Board.

Comparing Shower Tile Backer Boards

Backer Board Comparison Chart

NOTE: The section for “Structural Rigidity” of Glass Matt Backer Board contains a check, mark but you should keep in mind that Glass Matt Boards can be quite rigid, but only if the wall is flat. If they fracture from being distorted over an uneven wall, they can lose their structural integrity and water resistance.

Comparing Tile Backer Boards – A Deep Dive

The table above is a “quick and dirty” comparison of these boards but some further explanation may be required for those of you concerned about the details (like me   🙂  )

Keep in mind that this comparison will include the physical and functional differences in shower backer boards but also my opinions, based on my experience installing them and testing them.

Four Main Features

The four main features in modern shower tile backer boards are:

Secure Mortar Bonding
Structural Rigidity
Mold Resistant Tile Backer
Waterproof Tile Backer Board

The confusing part is that the shower backer boards available on the market have a random assortment of these features, but none of the boards include all of these features.

It leaves the average DIY shower installer wondering which of these features is essential for your shower wall (if any), which are a load of BS (if any), and why isn’t there a backer board with all these features?

I wondered these things myself…. and I still do.

Secure Mortar Bonding

The most obvious benefit of any shower tile backer board is to provide a secure bonding surface for your tile.

The original tile backer board (gypsum drywall board) absorbed moisture, which tended to cause delamination between the paper face and the gypsum core. This was the first of many problems with using drywall board as a shower tile backer.

Cement board and all other tile backers tend to have a pre-bonded, textured surface that thinset mortar can stick to very securely.

Structural Rigidity

Cementitious backer boards are still the only boards that guarantee high compression strength and unparalleled stiffness. This is important to limit movement and prevent your tile assembly from moving and cracking.

In my opinion, the glass mat shower backer boards (gypsum core) are stiff enough to support the average shower wall tile assembly, as long as the stud width does not exceed 16″ on center.

I would also like to stress that the rigidity of this type of board is quite minimal and may not withstand an impact from a large man falling against the shower wall without fracturing the grout.

Foam shower wall boards are even more vulnerable in this sense. They have generally very poor compression strength and rigidity.

If your shower wall studs are 16″ on center (max.), your tiles are larger format, you have installed a sheet waterproofing membrane, and there’s no accidental impacts against the shower wall during it’s service life, then your shower wall tile assembly should be ok (cement coated foam board will also help increase rigidity to a degree).

But that’s a lot of “ifs”.

In other words, if the finished tiled shower wall is left relatively untouched, it should all be perfectly fine. But if conditions are less than perfect, these boards can compress, and/or deflect between the studs, causing tile and grout line cracking.

Personally, I find these boards a bit too risky to install unless they are cement faced foam boards. The biggest brand of this type of backer board is Wedi. I have installed their products many times with great results.

Mold Resistant Tile Backer

In my opinion, this is kind of a BS “feature” that’s included in most tile backer boards these days.

A simple fact is….. a tile and wall assembly constructed according to “best practices” would not allow any mold to grow, so “mold resistance” would be unnecessary. No significant moisture would reach the backer board, and the little that does would never condense into water.

However, if the wall and tile assembly were not constructed correctly, or if the tiling contractor didn’t know what he/ she was doing, then a mold resistant board may be beneficial….. for awhile.

Excessive moisture would eventually cause any backer board to deteriorate even without the presence of mold. And if the right temperature / moisture level exists, mold will eventually grow.

I have seen several shower demolitions where the “mold resistant” tile backer board was infested with a blanket of toxic black mold after only a few years of service.

The benefit of foam shower backer boards is that moisture would not easily make it through the board so this would offer some protection against mold growth within the board and the stud cavity. But if moisture was condensing on the board’s face, it would not stop mold from growing under the tile.

Waterproof Tile Backer Board

Most of the new generation tile backer boards are somewhat moisture resistant because of a dense surface coating (like Densheild), or a very dense, uniform consistency (like Hardibacker). But none of the glass matt or cementitious boards can inhibit enough moisture to be considered a true waterproofing membrane.

The foam backer boards on the other hand, can inhibit a significant amount of moisture from moving through the board; enough to be considered a true waterproofing membrane.

These boards are completely different than the other shower backer boards because of their closed cell foam core.

Because the foam backer board and foam shower pan products eliminate the need for an additional waterproofing membrane (in most cases), they have become quite popular with DIY installers and professionals alike.

Shower Backer Board Install Tips

Before you attach your tile backer board to the shower wall studs, the most important step you can take is to make sure that the wall studs are flat and plumb.

If you skip this step (most installers do), and the stud plane is not uniform (most are not), you will be creating tile lippage and tile registration problems, but you may also be compromising the water resistance and structure of your backer board.

Important, right?

Because it’s so important and also quite involved, I will be preparing a separate post outlining how to prepare your shower walls for tile backer board installation, so stay tuned!

If your shower wall studs are flat and plumb, there are a few hints below to help you correctly attach your shower backer board.

Cement Board and Fiber Cement Board

If you’re planning to install cement board or fiber cement board, you should pay particular attention to how they are fastened to your wall studs.

I would definitely recommend that you use only official tile backer board screws when installing any cementitious backer board, including fiber cement board.

One of the biggest problems with these products is that they can be quite brittle and can fracture easily if the wrong screws are used. Ordinary screws (even some coated screws) can also corrode if they are not designed for moisture exposure.

I always use Rock-On screws. These coated screws are corrosion resistant and can withstand the high alkalinity of a cement based shower backer board.

Another benefit of Rock-On screws is their high/ low thread configuration. These screws bite into wood like nobody’s business and the H/L thread also makes them perfect for steel studs.

As I mentioned cement board is quite brittle because it’s made up of primarily cement and aggregate, but it’s also very hard. If you use the Phillips head screws, you’ll have a tough time driving these screws in deep enough to make the screw head flush with the board surface.

And if you install the screws with a star-drive head you’ll definitely get it flush, but you’ll risk fracturing the board, especially at the edges and corners. The solution?

Countersinking of course.

You may be thinking how in the world do I counter sink into a cement board that’s so incredibly hard? I use a tile backer board countersinking bit designed specifically for this purpose that I use routinely when installing cement backer boards and fiber cement boards.

It recesses the screw head so it doesn’t get in the way when you’re installing your waterproofing membrane and tile. It also prevents fracturing the board or driving the screw in too far.

Glass Matt Tile Backer Boards

Because the core of this board is gypsum, the screw installation method used for drywall board is pretty much the same, but you should always use tile backer board screws instead of drywall screws.

Also similar to drywall board installation, you need to be very careful not to drive the screws in too far because if your screw breaks through the board face, it eliminates the holding power of the screw.

It’s best to use a driver bit with a stop collar when installing these screws.

This bit is designed to create a depression in the face of these boards just deep enough to flush the screw head with the board face.

Foam Shower Backer Boards

Installation of foam shower backer boards is quite a bit different again, compared with the other backer boards.

Because these boards have such a flimsy low density structure, you usually need to use only the fasteners recommended by the manufacturer to attach these boards. They usually include large washers to increase the surface area of the screw head.

Another big difference is that these boards function as both a shower backer board and a waterproofing membrane so all the fasteners must be covered with a sealer to maintain waterproofing integrity.

This is done by troweling a sealant over the screw heads (provided by the manufacturer), or applying a membrane “patch” over the screw heads with thinset mortar.

Wrap Up

Hopefully I’ve been successful in showing you the differences between these boards and you’ve been able to choose one that’s right for your project. I’ve also tried to give you a couple of good links and suggestions for installation.

If you’d like to ask me some specific questions about this post or tips on how I could improve it, please let me know in the comment section below.  I can usually answer most questions the same day they’re posted. Or if you need some help on other bathroom remodel or shower remodel topics, please check out my shower remodel post.

Good luck with your project!

This post is for information purposes only and should NOT be interpreted as professional advice. You should always consult a licensed local contractor before undertaking any remodelling work in your home. Click here for my full Personal / Professional Disclosure.

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44 thoughts on “DIY Guide to Choosing and Installing Shower Tile Backer Board”

  1. Hi Neil,

    Thank you for your comment! I’m happy that my post has been useful to you.

    In my opinion, foam boards are generally adequate for inside walls, but I always prefer to install a standard cementitious backer board and a good sheet membrane like Wedi Subliner Dry for any outside walls. The Wedi sheet membrane has a .05 perm rating compared to the foam boards by Schluter and Wedi which are around .5 perms. That means these foam boards are 10 times more permeable to moisture!

    For the reasons mentioned in my post, I believe it’s very important to limit moisture transmission into outside walls as much as possible to avoid condensation in the stud wall.

    I am also not crazy about the flexibility of foam boards when installed on a wall with 16″ stud width, unless bracing is installed in the wall and you’re installing a cement faced board like the Wedi foam board.

    Good luck with your project!

    Steve

  2. Hi Steve, really excellent post, many thanks for taking the time to share your expertise. I’m having my shower re done. Having taken tiles off I’ve noticed the previous installer did absolutely no waterproofing at all, just normal plasterboard. And can see all of the issues that you describe first hand. I’m ripping off the boards and taking it back to studs on one wall, and building a new frame on the other. I’m undecided as to whether to use the foam based tile board or the cement type like hardibacker. I think the foam sounds better in terms of its waterproof nature, but I am concerned over the strength of the wall if these are used. Could I ask, if this were your project in your own house, what board onto the studs would you choose to use?

  3. Hi Alysha,

    Wow! I’m super happy that you got some valuable info from my post. It’s very validating and motivating for me as a relatively new “blogger”.

    Good luck with your project!

    Steve

  4. I just wanted to give a big Thank You for such a detailed and informative post. Me and hubby are getting ready to reno our 1950’s pepto-bismol, original-plastic-pearlescent-tile bathroom. There is no fan in this room and everything is approx 70 years old. The tiles are falling off the walls, and I’ve finally had enough. I fear the amount of mold hiding behind the walls, so we are planning on taking her down to the studs and only keeping the bathtub. This is our first big solo renovation. While my parents are SUPER DIY and have been very helpful with other (weekend) reno’s, they live 3 hours away. This is not a weekend-only project, so we’re prepping to do most of it solo (with a 2 yr old underfoot, to boot).

    This post was super informative and helped me understand the differences in backer boards, and which would be best for my situation (durock). One step, out of about 1 billion that I can check off as “I’m pretty sure I understand/made the right choice”.

    THANK YOU!

  5. Hi Bob,

    Thank you for your comment!

    If you’re simply furring out the wall and installing foam backer board over top, you really don’t need to install tile backer board. Plywood would be cheaper.

    Good luck with your project!

    Steve

  6. Hello Steve,

    I really appreciate your insight, thank you. My house is a post and beam constructed from an old barn. Our main bathroom shower is a tub surrounded on three sides by walls covered with tongue and grove pine boards. the tub is surrounded by shower curtains hanging from an oval rod suspended above the tub. I’m planning to tile down to the tub on three sides and enclose the front with a glass door. The walls are mostly plumb however, there is a beam that runs inside the wall horizontally about 4 feet above the tub and sticks out about 1/4″ past the surface of the tongue and grove pine on the wall.

    I’ll need to shim the wall out to make it plumb. Would you recommend that I use a cement board as a sheeting and then apply my foam backer board on top of that?

  7. Hi DS,

    Thank you for your comment!

    You should probably ask this question to your local building inspector but it’s a pretty safe bet that you need to install a fire rated backer board. Since you are removing the original wall material, you would likely be required to meet the current code standards.

    I would suggest you install a cement board like Durock cement board because it apparently has a 2 hour fire rating. Kerdiboard does not seem to have any fire rating at all. And if Durock is not available in your area, it’s pretty easy to look up fire ratings for any other brand of cement board online.

    Good luck!

    Steve

  8. Hi Steve,
    My tiler is replacing our tub surround in a 1970 condo. One side adjoins the building electrical room and another side adjoins our neighbour’s wall. There is 1/2″ gypsum on the opposite side of our studs, then more studs on the other side of that, before the neighbouring walls. The tiler has brought 1/2″ kerdi board to put directly on the studs, and thinset then porcelain 8×12 tiles are supposed to go on top.

    I am mainly concerned about meeting fire rating which I believe is one hour.

    Any advice?

    Thanks,
    DS

  9. Hi Michael,

    Thank you for your comment!

    I will be writing a shower base post that will include info about shower wall drains, but for now I’ll address your questions directly.

    The installation of wall drains like this one are quite involved, so you should make absolutely sure that the contractor you hire is experienced in their installation.

    Personally, I’m not really comfortable with the requirement to cut a big notch out of the bottom plate of your shower wall (see install instructions for WALLD48). This plate is important to prevent flex in this wall. I would never cut a notch out of it for any reason. If you cannot fur out the wall enough to allow the drain hole cutout to clear the bottom stud plate, I would not install it.

    I would also recommend that you not install this wall drain on an outside wall unless the wall is furred out significantly (as mentioned above) to allow for adequate insulation in the wall adjacent to the drain pipe.

    Good Luck with your project!

    Steve

  10. Hi Steve,
    Can you talk about installing a shower wall drain? The drain we’re going to use in our master bath remodel is the Wall 48 from QuickDrain. Any installation issues I should be concerned with? My primary concern is how reliable it is. Is this particular drain style difficult to install? What, if any, are the risks/negatives of using a wall drain? This drain installation needs to be bulletproof in terms of NOT LEAKING! Can you advise me on how to make sure the contractor we choose does a great job?! Would you be comfortable using an experienced contractor that has never installed a wall drain? Thank you!!

  11. Hi Brian,

    If you are installing a mud pan with a waterproofing system like Redgard you don’t need to worry about water wicking up from the pan into the walls and curb because you are applying your waterproofing membrane over the base and backer board. Any undertile waterproofing membrane like Redgard will essentially prevent moisture from getting into this margin between the pan and walls. I would recommend fiber taping the joint between the floor and walls and apply three coats of Redgard. If you do this, you can install the backer board before you install the pan or afterwards. It won’t cause any problems either way. If you install the backerboard after, you should place the board directly on the mud pan. You do not want any gap between them. Here is a fairly good video showing a mud pan installed with a Schluter Kerdi waterproofing membrane.

    Good luck with your project!

    Steve

  12. i read if you install hardibacker or some other cementious backerboard then build the mud pan burying the backer board. water will wick up from the shower pan into the walls & curb and start destroying the backer board. if i built the shower pan then installed the backerboard leaving about 1/4″ gap between the mud floor and walls. of course i would red gard everything. is this a good practice or should i put the backerboard onto the mud floor.

  13. Hi Ivy,

    Thank you for your comment, and your compliment!

    It’s always best to do as much research as you can about products and “best practices” before you hire a contractor, and ask as many questions as you need to feel comfortable with the work they’ll be performing. Any decent contractor will welcome this kind of discourse because they’re proud of their work, and they’ll be happy that a client can appreciate all the work that nobody sees as well as the finished product. Ask your questions……and if they get uptight or defensive, move onto the next one.

    One of my past clients asked me when we first met “Are you going to install the orange stuff in my shower”. And the answer was yes! (she was referring to Schluter Kerdi waterproofing membrane). She was certainly no expert in the details of shower waterproofing but because of her research, she was able to indicate to me that it was important to her.

    Of course you will not turn a bad contractor into a good one by asking these kinds of questions, but it will indicate that you’re concerned about the “behind the walls” details and you’ve done your research. I think it makes a big difference.

    And if you think anything is going sideways during the project, you can always ask me!

    Good Luck!

    Steve

  14. I love your posts – thank you so much! I’m going to hire a contractor to do bathrooms and kitchen. How can I assess whether he’ll use all the best practices you suggest? Like plombing and waterproofing the niche? Just ask?

  15. Hello again Emir,

    Yes, it’s always important to install a waterproofing membrane. I have not used the Durock sheet or liquid membranes but both seem like fine options for inside shower walls as long as the install directions are followed.

    Good Luck.

    Steve.

  16. Steve, thanks for the response. None of the shower walls are outside. Do I still need a waterproofing membrane over Durock cement board? Is the membrane made by Durock good option?

  17. Hi Emir,

    Thank you for your comment!

    I prefer Wedi Subliner Dry or Kerdi DS sheet membranes. These two (and a few others) have very low permeability so they are important to use if one of your shower walls happens to be an outside wall. I would suggest that you check out my waterproofing membrane post if you want more info.

    Good luck with your project!

    Steve

  18. My installer will be using Durock cement board on the walls. What type of waterproofing membrane do you recommend over the Durock board.

    Thanks
    Emir

  19. Hi Roderick,

    Thank you for your comment.

    Although I haven’t installed Wonderboard Lite, I believe that it’s a decent tile backer board.

    However, your contractor should have installed a waterproofing membrane. I would never build a shower surround without applying a sheet or liquid waterproofing membrane to the tile backer board surface. There is an extremely high chance that moisture will condense in the shower wall if there is no membrane, especially if it’s an outside wall.

    If your contractor built a custom niche, the niche box should have a 2×4 frame around its perimeter with plywood backing. Then the Wonderboard should be attached to all the interior surfaces and a waterproofing membrane applied.

    An even easier strategy would have been to build the 2×4 niche frame only, then attach a premade (prefabricated) foam shower niche to this framing (no plywood backing needed). These foam niches come in various sizes so there is really no need to build a custom sized niche unless you insist on unconventional sizing.

    Sorry for the bad news. I know that it’ll be uncomfortable talking to your contractor about this so I wish you good luck!

    Steve

  20. I have a contractor and he used Wonderlite backer board. He did not put any liquid membrane on it or anything. He just put the tile on top of that. Is that safe? He also built a niche, but behind the niche is just drywall, not the Wonderlite backer board…he cut through that to build the niche. Is that appropriate?

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