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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 G,

    Thank you for your comment!

    I am not an expert on the Triton board, but it’s one of the more rigid backer boards out there so it basically gets the “thumbs-up” from me. Any rigid backer board is fine as long as you also install a good (low perm) waterproofing membrane over it.

    I would suggest you check out their website if you want to find out more about this particular board.


  2. This was really helpful! I’m a little confused on one point. My impression is that the Triton board is an MgO board, so although it’s cementitious, it does not contain Portland cement and has somewhat different properties.

  3. Hi Matthew,

    Thank you for your comment!

    As long as this is an interior wall, plywood is a good option to make the wall more rigid. I would also suggest 3/4″ T&G plywood and fasteners at 8″ apart. T&G is necessary if you have any seams (on the long wall for instance) because it locks the sheets together so each side of a seam can’t move independently.

    Good luck with your project!


  4. Hello Steve !

    We are relatively new diy-ers, currently remodelling our home, unfortunately for one of the showers we are remodelling, there are 2×3 studs spaced 16 in apart.

    After going through your article and All of your helpful comments, it would appear that our best option is to attach plywood for additional rigidity , and then use Durock or similar type product as a backerboard followed by Redguard .

    Do you anticipate any problems with the 2×3 stud walls (none are exterior or load bearing) ?

    Thanks for your advice in advance !

  5. Excelent article. Great summary of a complicated subject, well constructed so that one can make a decision as which product to use with corresponding cost guide. I will save this article for future pending use and as a reccomendation to others.

  6. Hi Ralph,

    Thank you for your comment!

    I think I’m understanding what you are saying, and it sounds pretty good to me. The GoBoard over your drywall is a fine choice, but it’s expensive. It might be easier/cheaper to use regular tile backer board (any type) and cover with a good waterproofing membrane.

    As well…I’m assuming that at least one of your shower walls is an outside wall, so it would be best to install a good membrane on this wall like Subliner Dry, Durock Sheet membrane or Kerdi DS. The GoBoard doesn’t have a good enough perm rating for an outside wall in my opinion. The drywall behind your backer board should be protected with either strategy.

    Good luck with your project!


  7. Hello,

    The small bathroom and laundry room had to be remodeled due to water damage in my doublewide. (Stupid Hot Water Heater). The bathroom had 3/8″ Vinyl on Gypsum Sheetrock Walls. And my Bathtub/Shower Walls were just a thin plastic that was held up to the Vinyl on Gypsum with Adhesive and Plastic Rivets. I was suprised to see that the thin plastic shower walls never leaked through to the Sheetrock. I couldn’t get Vinyl on Gypsum Sheetrock to Replace. I just used Regular 3/8″ Sheetrock and painted 2 coats of primer and 2 coats of paint. My plastic 60″ x 30″ bathtub fits inside the Sheetrock Walls. Meaning the tub flange is exposed and not hidden or tucked under the Sheetrock. Meaning the tub flange doesn’t sit right up against the studs. The Sheetrock is between the Studs and tub flange. I want to screw 1/4″ GoBoard 3′ x 5′ over the Sheetrock to the studs so that I will be out past the tub flange. Then seal the seams per manufacturers instructions. Wait 24 to 48 hours for sealant to completely set up. Then install DumaWall Vinyl Waterproof Wall Tiles with Loctite Power Grab Construction Adhesive and Seal each seam in the DumaWall Tiles with a 100% Silicone has I install the DumaWall Tiles which is Manufacturers Instructions on a Completely 100% Waterproof Seal. My Studs behind the Sheetrock are 16″ on Center. But my thinking is that not tearing out the Sheetrock will help keep the Foam Board more structurally sound. Do you think this will be a good structurally sound, waterproof/water tight seal that won’t leak back to my Sheetrock Walls or Studs? Also is the Foam Board overkill or could I get away with just using one of the other backer board types? I was thinking GoBoard because it’s just easier for one person to install and it’s not a big mess to waterproof it. Just seal the seams and that’s it. Problem is, it’s not that easy to get the GoBoard at my local hardware store.

    Thanks, Ralph

  8. Hello Jerry,

    You can install the Kerdi shower pan with Mapei thinset as long as its unmodified. Mapei has a few unmodified mortars including “Kerabond” and their “Floor Tile Mortar”.

    I would always use a sheet membrane over a liquid membrane every time. The regular Schluter Kerdi membrane is fine to use on an interior wall but I would install the Kerdi DS membrane on the outside wall if I was you, especially if it’s a 2×4 wall. Kerdi DS has a much better perm rating.

    Good luck with you shower project!


  9. Hello Steve:
    Thanks for information. it is very useful for me. I am planing my bathroom now. i will use kerdi-shower tray,Durock cement board,Kerdi waterproofingembrace(one exterior wall,one inside wall).My question is should use Schlutet thin-set mortar laydown tray to the subfloor? can i use Mapei mortar?i already have.and about membrace use it too. Another question is red gard and Kerdi membrace which way is better?

  10. Hi Morley,

    Thank you for your comment!

    I fully endorse your decision to use plywood as your first stud wall covering. I have often attached plywood to the stud face to create extra rigidity. I’ve also evened out a non-planar stud wall by attaching ply to a selectively shimmed stud wall to create a perfectly planar surface prior to backer board attachment. This helps reduce tile lippage.

    I also think it’s perfectly OK to install the Kerdi board over the plywood. You can see in this Schluter video that they are actually OK with attaching Kerdi board over existing drywall so it would be unreasonable for them to suggest that you can’t install it over plywood.

    Plywood is one of the most awesome building materials ever invented so I would never discourage its use to stiffen up a wall that will have unusual stresses applied to it (like a wall hung toilet). Most wall hung toilets do not technically require this, but I certainly don’t think it’s overkill. In other words… go for it!

    Good luck with the project!


  11. Hello Steve
    Are there any issues with installing Kerdi Board over 3/4″ plywood sheathed stud wall in a shower.

    I am completely renovating a 6’0″x 10′-0″ bathroom which will have a 3′-0″ x 6′-0″ walk in curbless shower at one end.
    I would like to install 3/4″ plywood over the 2 x 4 studs (16″ oc) on the 10′-0″ plumbing wall which also includes one of the end shower walls. This is desired for a couple of reasons:
    1) mostly for backing for a wall hung toilet (Geberit carrier) & less critical for a wall hung vanity & LED mirror
    2) the plywood thickness will also reduce the the length of the 6′-0″ by 3/4″ which is reqd to fit the tile module
    3) the plywood at the end of the shower will allow for various locns of the sliding handheld shower head track.

    Within the shower I will attach 1/2″ Kerdi Board over the plywood but only attach thru the plywood where the studs are located. I will be installing 6″ x 18″ Carrera marble over the Kerdi Board within the shower.

    I have emailed Schluter but there only comment was to install over open studs not over plywood, – no other explanation even though I have sent further emails requesting Schluter’s reasoning?

    Not sure why the plywood would not be allowed under Kerdi Board that is only attached at stud locations.
    Is it that unstable of a material even though its behind a waterproofing membrane on an interior wall?

    I have purchased all of the materials a while back or I would switch to another product line.

    Can you shed any light on this?

    Further: Is it worthwhile to use plywood over a wall hung toilet carrier or is this overkill?

  12. Hi John,

    Thank you for your comment!

    I would always suggest using a good tile backer board for any shower wall. Either cementitious or Wedi foam board. If you use regular TBB, I would always suggest a good sheet waterproofing membrane be used as well. This is especially important on any outside walls, even if you decide to install foam board. If you are asking about the finished wall surface, I would always suggest a porcelain tile. Much lower vapor transmission through these compared to ceramic tile. Also a good water resistant grout like Mapei Flexcolor CQ.

    A curbless shower is always a challenging project for any DIY’er but I would always recommend it because it’s a very satisfying project, and a super cool look! However, you should never do this project without following the rule book (manufacturers install instructions, in other words). There is no room for error with a curbless shower. If you are leaning towards a foam shower pan, I would suggest a Wedi pan. The cement and fiberglass mesh coating makes it so much more resistant to damage during install and during its service life.

    Good luck!


  13. Hi Steve , you have packed a ton of info right there!! I wish you were located in Mytown Canada. We are planning a remake of a small bathroom, maybe with curbless entry, maybe not. A Kerri product appears likely. We are considering a Tile redi pan.
    One end wall is exterior. At 35 years old construction, we had drywall fail at around year 5-8. For a finished surface on the shower/tub wall, we used vinyl flooring, and it works well! Tons of patterns and colors. My wife painted it, and it still looks like a new install, to me. What products, material would you use for the interior exterior wall rebuilds, and is curbless somewhat ok ? John m . Thankyou for your hard work here! Thank you.

  14. Hi Richard,

    The GoBoard install instructions suggest that you can use both unmodified and modified mortar to bond tiles to its surface so presumably they are suggesting that both will stick well to the bare board as well as the sealant covered sections.

    Personally I would only use a modified thinset mortar because it should adhere better to the sealant covered areas than unmodified. I don’t really have a favorite modified mortar. You just have to make sure the one you choose is rated ANSI A118.4 (most are). That’s what Johns Manville recommends.

    Good luck with your project!


  15. In your opinion, which mortar has the best adhesion to cured GoBoard Pro Sealant? The product is very sticky when applied, and seals seams and screwheads beautifully. It’s totally waterproof, which is a huge positive. However, I’m a little concerned in areas around windows and doors, where much of the GoBoard is covered in cured sealant, that the tile mortar may not bond well to it. Should I be concerned, and, if so, can anything be done to enhance the adhesion of the cured GoBoard Pro Sealant?

  16. Hello again Gene,

    If I am understanding you correctly, you tub will be on a framed “box” that sits on the concrete subfloor. If so, It must be super strong to take the weight of a cast iron tub filled with water. I would go big on the support. I am not sure what you meant by “framed 26 centers” but if that means 26″ on center, that is not nearly enough. Make it strong…at least under the tub legs. 8″-12″ on center with a 3/4″ plywood deck would be great. Permabase screwed over top with a thinset base would be great as well (apply with 1/4″ square notch trowel). The Permabase would be a much better substrate for tile bonding and it would add rigidity. Claw foot tubs legs deserve extra support because the weight is bearing down on only four small points.

    And it also wouldn’t hurt to install a waterproofing on this box to keep any standing water from penetrating.

    Good Luck!


  17. Hi Steve, thanks so much for your quick response! I have one other question for you on the same project. The cast iron tub will be on a raised 2×6 platform framed 26 centers to accommodate a trap and drain as this is located in a basement over radiant slab. It has a 3/4” subfloor and I will add 1/2” backer (permabase). I’m thinking it would be more rigid to bond the backer to plywood with thin set and screw. Would you recommend that and if so what size notch trowel? BTW, I decided to go with waterproofing for the peace of mind. Thanks,

  18. Hi Gene,

    Thank you for your comment!

    I agree with you. You should not need a waterproofing membrane on that wall if it’s an inside wall, and it’s not going to be exposed to any water. If there is any possibility of splashing against this wall, it would be a different story. Either way, you should consider using a water resistant grout like Mapei Flexcolor CQ.

    Good luck with your project!


  19. Hi Steve, Thank you for your very informative post. I will be installing a tiled wainscoting about 4′ high in an alcove on an interior wall that abuts the furnace room. And was planning on using Durock or permasheild as backer, the tub in this case will be a free standing cast iron clawfoot that will have a curtain track on the ceiling when using shower. Would you still apply a waterproof membrane in this case or would mesh tape and thinset be adequate over seams and fasteners? I understand why it would be necessary in a tub surround or fully tiled shower, but it seems very unlikely that the tile would ever see much exposure to water.
    thanks so much for sharing your expertise!

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