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HomeDepotAssociate 01-09-2015 04:55 PM


And I can't imagine what they hoped to accomplish by painting RedGard over the joints in the floor panels and the fastener heads.
Lurking in here, ordinarily when you are painting redguard are you skipping over the joints or would that be done before painting redguard? (the tape and mortar over the joints). Not to derail this discussion, my condolences to the abandoned project :(

cx 01-09-2015 05:05 PM

Chate' if you'll visit the FAQ you'll find a brief tutorial on how to properly attribute quotes you post here on the site. Very simple once you see it and people won't hafta go searching about to determine what or whom you might be quoting, eh? :)

Nothing ordinary about this particular use of RedGard. No idea what might have been the contractor's intent.

HomeDepotAssociate 01-09-2015 05:29 PM

OOPS! lol. I got half the attribution. Sorry, that irks me too (quotes without the names of the person who said it). Or maybe it doesn't irk you. . In any case, wouldn't you redguard over the joints after you've done the taping/thinsetting? (some of the comments here made me realize I didn't quite understand the handling joints with redguard during an installation)

SWB04 01-10-2015 11:57 PM

7 Attachment(s)
Well, folks, this has been quite a bit of fun. I hired a contractor because between my business and other ongoing home pre-sale renovations, I just don't have the time to deal with it. So then he left, and **whoosh**, I guess I now have the time. :uhh:

I've spent several days reading and planning, so here's a status update, my plan of action, and a bunch of questions. Appreciate the advice, in advance.


My son (Construction Management major) did the research and almost pled with me to go after this guy's license, and seek reimbursement. The Contractor offered me $1000 back, I took it, I figure I'm about $700 out on the deal, but no lawyers and no delays to getting this moving, so I'm good. He's out; that episode is done.

So, I'm my own Contractor (again - how many times has his happened to me; I check references, I interview, but I always seem to have to step in to ensure the job is done correctly). I'm hiring a couple of recommended tile setters to help out, but am doing the subfloor preparation myself.

Where to go from here?:
(a.k.a. help! questions!)


The joists where originally (1991) made by "True Joist Corporation", now absorbed into Weyerhaeuser. They are marketed as the "Trus Joist Silent Floor" system. Specs are here:


Joists are TJI-110 Joists, 11 7/8"H x 1 3/4". They are NOT spaced 12" on center, as I previously posted. That situation is in the garage, which includes a 20'+ span with a second story house above it. The bathroom subfloor is supported by joists 19.5" on center, which is in spec for a 16' span (we're at 14' I believe, so it's actually overspec).

My (Construction Management college major) son took on the task of calculating deflection from LP's documentation, and he arrived at 0.143. The "Deflecto" calculation was 0.210 for a load deflection of L/798 based on Doug Fir. So, based on the superior strength of the composite I-beams, we should be covered in any case.

The subfloor panels are unfortunately, garbage LP waferboard, which was standard in 1991 (I guess OSB is standard today). So here's the subfloor plan:
  • There's no way I can save the glue'd in existing CBU floor layer. So, remove and destroy the 1/2" hardibacker. With the Redgard filled screws and construction adhesive, this could be fun.
  • Add plywood reinforcing to the waferboard subfloor. I've literally read this web site and others for hours, and I haven't seen any uniform subfloor standard for stone. (It seems to be slightly different for every installation.)

    --> Are two sheets of plywood required, or one?
    --> 1/2" plywood followed by 3/4" sheet?
    --> One sheet of plywood of 5/8"?
    --> Should the waferboard be replaced with plywood?
    --> Should I use OSB or another composites, in place of plywood?
    --> What grade(s) of plywood are within spec?
    --> Should I be using something like type 1 mastic to glue down the plywood?
    --> Or, should I use something like liquid nails, and use a 1/8" notched trowel to create an even glue layer?

  • Add a moisture barrier over unmodified thinset. Must this be 1/4" CBU, or would thinner layers, such as Ditra, or RedGard be sufficient??
  • I'm concerned about the transition height to bedroom and closets, and don't want to create a 3" or 4" "step" between the rooms (nor have to add 1.5" of flooring to the other rooms, either). What options minimize the height of the finished floor? (Again, remove the waferboard? ditra or redgard rather than 1/4" CBO?) I expect that I could build a 8" long, 1" high wooden ramp under the carpet at the bath entrance, but more than that will be too noticeable. I might have to add a layer of subfloor to the adjacent walk-in closet to balance that height as well. Dreading that.

Shower and Bath Surround issues:

  • The contractor used a Hardibacker 1/2" CBU to replace and construct the shower walls, after the original installation was demo'd (gypsum board, chickenwire, 1" floated mortar, and 4x4" white tiles). He screwed the CBU to the studs, and as far as I understand, that's an adequate installation. However, he used Redgard over fiberglass mesh for the joints, whereas Hardie requires unmodified thinset over mesh for the joints.

    Question 1: At this point, could I simply coat the entire shower and tube surround with Redgard? Or, do I need to scrap/sand to remove the Redgard at the joints, and install the thinset over the joints? I'm not sure how successful that would be.

    Question 2: The shower tub was a quality build by a local hotmop specialty company. The contractor extended the CBU to overlap the shower tub. I believe code calls for a minimum 2" overlap. The Contractor placed a ledger board as a starting point for tiling up the wall, but no smooth joint exists between the CBU and shower tub. How is this normally handled? Caulk? (In fact, on the bath tub side, there's a considerable gap between the hardie and shower tub - see last picture. Not sure how the contractor intended to deal with that. I expect that I'll have to rework the CBU to overlap further and seal that void.)

While I know that Hardie backer CBU properly installed, is supposed to form a waterproof barrier. However, for the cost of a gallon or two of Redgard, I'm tempted to coat the entire installation (assuming I use 1/4" CBU for the floor) in two coats of Redgard.

Intend to use Versabond for all travertine. I know I should pre-seal the travertine before setting it to prevent stains; I'll do it if I have time.


Kman 01-11-2015 03:52 AM

My understanding is that waferboard is just a cheaper version of OSB, and not something that I would choose as a subfloor. If you were installing ceramic tile, I would probably tell you to add a layer of 1/2" plywood and start tiling. However, you're planning on installing natural stone, and a very soft one at that.

Since two layers of plywood/OSB are required for stone, my inclination would be to remove what you have there and go back over the joists with 3/4" plywood followed by a layer of 1/2" plywood. Both layers should have a face grade of A, B, or C and have an exposure 1 rating. The 3/4" ply should be glued to the joists with construction adhesive, and secured with screws or ring shank nails every 8". The layer of 1/2" ply should be installed with screws or ring shank nails only, no glue, using the recommendations in this article to install it. That will give you the best chance at a successful installation, assuming your calculations on the joist deflection are correct.

SWB04 01-11-2015 04:50 AM


Originally Posted by KMAN
"Since two layers of plywood/OSB are required for stone, my inclination would be to remove what you have there and go back over the joists with 3/4" plywood followed by a layer of 1/2" plywood. "

Thanks Kevin.

Can anyone provide me a reference that indicates that 3/4" OSB (or in this case, LP waferboard) with one layer of either 1/2" or 3/4" Plywood would not be sufficient for a travertine install? Or is the double plywood requirement based on personal experience and "gut feelings"? Must the two layers both be plywood? And, I presume that I could reduce floor height by using either two layers of Redgard or Ditra, versus the CBU for the floor isolation membrane.

I have to pull the CBU, so I suppose I can just cut between the joists with a "saws all" and pull up both the CBU and waferboard at one time. I'm a bit concerned about nicking the joists. I assume this would be R&R wall-to-wall, closet to vanity, tub and shower over to the bathroom, reaching joists as are exposed. I'm not going to touch the already prepared shower or bath tub; too much already invested there.

However, I'm preparing this home for resale (i.e. kids are gone, nearing retirement, etc.). I'm $1000's over budget in this already. I really don't want to tear up the entire bathroom subfloor, but if that's where I have to go, it's coming out today.

Thanks again.

cx 01-11-2015 08:50 AM

Scott, the Marble Institute of America (MIA) governs that installation and requires a double layer of plywood or OSB for any natural stone tile installation, regardless the joist spacing. Each layer must be properly, and differently, installed. Then you would install the tiling substrate of your choice and tile.

There are various combinations of subfloor that would satisfy the industry or/and specific substrate manufacturers, but starting with nominal 3/4" T&G material and adding nominal 1/2" exterior glue plywood with no face of grade lower than C would be my choice.

You'd then add the substrate of choice per that manufacturer's instructions.

But it's your house and your tile and your dinero and you can install over anything you feel comfortable with. With your joist spacing I would not install even ceramic tile without the second layer of plywood.

My opinion; worth price charged.

Pirate 01-11-2015 11:27 AM


Originally posted by Chate'
OOPS! lol. I got half the attribution. Sorry, that irks me too (quotes without the names of the person who said it). Or maybe it doesn't irk you. . In any case, wouldn't you redguard over the joints after you've done the taping/thinsetting? (some of the comments here made me realize I didn't quite understand the handling joints with redguard during an installation)
To answer your question about the Redguard. Redguard is a waterproofing membrane. Applying it only to the joints doesn't make the whole shower waterproof. To properly use it, he whole area needs to be covered with Redguard.

SWB04 01-11-2015 12:51 PM

Hi CX,

I spent "even more" time researching, and finally found a copy of the chapters in the MIA "Dimension Stone Design Manual" that applies to plywood subfloors and Travertine installations. I also found a MIA Q&A document which shed light on a few things.



My take is:
  • MIA still discusses two layers of "plywood or OSB" in the Q&A, while the guide seems to only recommend plywood. MIA does make one significant distinction in the Q&A. OSB and similar products aren't an issue for rigidity. The main concern is OSB expands at about twice the rate of plywood when wet. The existing subfloor is 23 years old, and for 23 years has supported a bathroom environment covered with nothing more than carpet and a pad. There has been no moisture penetration above or below the layer (nor in the tub or shower areas). Therefore, my conclusion is that while the existing 3/4" waferboard is not idea, it should adequately support the subfloor if overlaid with 3/4" exterior "C" or "C" plugged grade (per MIA standards). I'm not concerned about moisture or expansion at the base layer. If we have a plumbing leak that floods the second story floor to the point of damaging the waferboard, the we've got bigger fish to fry than worrying about the travertine floor.
  • MIA recommends 1.5" of plywood, e.g. two 3/4" sheets, versus 1.25", so I'll install one 3/4" plywood layer over the existing subfloor.
  • The MIA guide recommends expansion gaps BETWEEN the plywood, even tongue in groove. I knew that the boundaries of the installation required 1/4" gap, but I never dreamed that I'd be installing the plywood essentially unconnected with every other sheet. That's how I read this (see quote), and I really don't want to do this, since this is a structural layer. The guide also lays out in detail how the glue is applied (beads @ 2" intervals, not applied like mastic with a trowel), and everything else required - great stuff!


    Originally Posted by MIA
    "6.1.1 Plywood subfloors, including tongue-and-groove plywood, must be installed with a gap between the sheets to allow for expansion. Stagger all seams. All subfloor seams should occur over framing, with underlayment seams occurring approximately 25% into the span between framing members. Plywood should have the strength axis running perpendicular to the joist."

  • I'm going to install Ditra in unmodified thinset as my separation membrame for the floor, over hardi-backer (which I actually prefer), to reduce the finished floor height.

This still leaves open the questions regarding the shower and tub surround. I know that I can call Hardie for advice, and their customer service rep will simply quote "chapter and verse". There's no industry guide for resolving "substandard installation" other than ripping it out and starting over, and I don't think this is required. The Hardie was properly installed with glue and screws, and the joints are sealed with Regard and tape. As I said previously, since the Hardie is not truly a structural membrame, if I overcoat the shower and tub surround with Redgard my reasoning is that the installation will be watertight (e.g. reasoning that if Redgard over wallboard is supposed to achieve this, Redgard over Hardie should achieve the same). In other words, I'm not going to attempt to remove the Redgard and apply thinset to the joints of the shower; I'll leave them as is, and coat the entire installation.

So, I'm making a few compromises, but I believe that they are reasonable trade-off's to achieve the objective cost-effectively. If there's something I've missed in my logic, I appreciate the advice.

Thanks, all!

Kman 01-11-2015 01:13 PM

Scott, installing two layers of 3/4" ply over 19.5" spaced joists is a good thing. The less deflection you have between the joists, the better off you'll be, considering your choice of finished flooring.

Plywood manufacturers require gaps between the plywood sheets for good reason. Without those expansion gaps, you'll experience peaking at the seams, and in some cases you'll have sections of plywood lifting from the joists. Some products have the spacing built in along the tongue and groove edges, and some don't. Along the shorter edges, where the sheets will meet along a joist, you must leave 1/8" gap there. If you have new subfloor meeting old subfloor where there is no tongue and groove, it's important to join those pieces together by using some sort of material underneath, either a scrap of subfloor or a 2x4, etc.

As I mentioned earlier, a second layer of plywood installed over the subfloor should be installed without glue, unless you are prepared to trowel glue evenly over the entire subfloor. Beads of glue can actually do more harm than good by creating little voids between the plywood sheets where no glue exists. Trust me, installing the second layer with screws every 4-6" makes for a very solid substrate.

Also, there is no benefit to you in installing Hardibacker and Ditra on the same floor. In that application, they serve the same function and are therefore redundant. You would simply be adding more height to your floor. If you want to keep the height to a minimum, use Ditra over a proper subfloor.

cx 01-11-2015 02:35 PM

Scott, you'll find some contradictions between that MIA manual and the stone installation methods in the TCNA Handbook, which began to publish a few stone methods in, I believe, 2012. And you'll find some clerical errors in each, some of which have caused a good deal of confusion in past. Such as their requirement for substrates to be "level" to a specific tolerance when I'm sure they mean flat. And for the joist deflection in the TCNA saying L/360 is required when the MIA still requires L/720. You've gotta overlook some of that with an eye towards the intent and that's not always an easy thing.

You can find in the TCNA Handbook a published method for stone installation over wood framed subfloors using CBU, for example, that requires a first layer of nominal 5/8th" and a second layer of nominal 1/2" plywood. That method requires a maximum of 16" joist spacing, though. I think they would accept 1/2" over 3/4" over 19.2" joist spacing, but I can't prove that and Kevin's recommendation of two layers of 3/4" might be safer.

The method of gluing of the plywood layers with beads of construction adhesive I would take issue with and the ceramic tile industry actually has testing that indicates some small measure of uncoupling the tile installation exists when the second layer of plywood is mechanically fastened only to the first layer and not to the joists and definitely not glued in any manner.

Whole lotta things to consider and we'll always try to lead you down the most correct path. But if you find evidence you like better, you're certainly free to use that instead. :)

If you decide to use two layers of 3/4" plyoood, I'd very strongly recommend you pre-drill the screw holes in the top layer to prevent screw jacking.

My opinion; worth price charged.

by-eye 01-11-2015 04:27 PM

Scott, sorry for your troubles.
Your shower is built wrong.:gerg: It should have the studs furred out (plum), and a moisture barrier with the hardie properly overlapping your hot mop base, with no fasteners lower that 3 inches above the finished curb. :crap:
You might be able to bridge the gap with kerdi, but it wouldn't be proper.:x:
Since your selling, have you considered changing to ceramic tile? Will the toilet flange accept a new floor height? Are you prepared to undercut the doors and trim?

SWB04 01-11-2015 06:16 PM


Originally Posted by by-eye

Since your selling, have you considered changing to ceramic tile?

Actually, I was checking my fire insurance and thinking more of "dynamite". :loaded:

Actually, I already have the materials, and the "standard" for homes in this price range around here is natural stone. We already have 1000 sql ft of travertine in our main rooms down stairs. The new installation matches that installation downstairs.


Originally Posted by by-eye
Your shower is built wrong. It should have the studs furred out (plum), and a moisture barrier with the hardie properly overlapping your hot mop base, with no fasteners lower that 3 inches above the finished curb.

Are you saying that the bottom of the hardie should be plumb with the top of the hot mop basin?

I believe the studs are vertically plum with the bottom of the basin on the shower control and back sides. The Hardie (moisture barrier) extends over the hot mop base (not sure how far, though). The gap on the bathtub side of the shower needs some sort of resolution. I'm out of time this weekend, but I'll pull his ledger boards and get a closer look next week, and see how and if it can be fixed.

Nothing this contractor did was done correctly. I'm glad I stopped him when I did. Funny thing is, none of the 4 LICENSED contractors I interviewed and took bids from mentioned deflection, load, double plywood subfloors, and so forth. None of them were prepared to perform the installation consistent with industry standards.


cx 01-11-2015 06:25 PM


Originally Posted by Scott
Are you saying that the bottom of the hardie should be plumb with the top of the hot mop basin?

The bottom of the Hardibacker should be plumb with the top of the Hardibacker, Scott, and the wall should be flat. That's important and if you don't have that condition, now's the time to fix it.

Originally Posted by Scott
The Hardie (moisture barrier) extends over the hot mop base

One of your major problems is that you have no moisture barrier, and that's another of the things you really wanna correct.

My opinion; worth price charged.

Kman 01-11-2015 06:26 PM


Originally Posted by Scott
The Hardie (moisture barrier) extends over the hot mop base (not sure how far, though).

Is there plastic or felt paper behind the Hardibacker? If there is, there would be no reason to use Redgard on the surface. If not, you should be aware that Hardibacker is not a moisture barrier. When installed without a moisture barrier behind it, then a moisture barrier, such as Redgard or a sheet membrane, should be used on the surface.

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