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Unread 01-17-2020, 11:22 PM   #1
pope
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Purpose of TCNA Handbook's corner taping?

I'm a tiling dilettante implementing B412-19 with waterproof membrane (Redgard) from the TCNA Handbook. My detail (and many others from the handbook) prescribes for backer board installation:

Quote:
Corners—leave space between backer units. Tape joints
using skim coat of latex-portland cement mortar and 2"
alkali-resistant glass fiber mesh tape, but do not fill.
My intuition says that the corners are intended as expansion joints, hence the "do not fill." This leaves me scratching my head, however; why mortar the corners at all? My best guess is that butt joints benefit from the perpendicular corner's reinforced mortar at both ends. Anybody have anything better than a best guess?
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Last edited by pope; 01-17-2020 at 11:23 PM. Reason: clarity
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Unread 01-17-2020, 11:39 PM   #2
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Quote:
Originally Posted by Tim
Anybody have anything better than a best guess?
Dang it, I was all set to start typing a really smart-sounding answer until that last sentence.

My best guess is that it allows for expansion when you don't fill the gap. But you need the skim coat to hold the mesh tape in place.

So you have a gap in the board, and the tile. And that gap is filled with a flexible sealant, such as silicone or latex caulk, depending on the application. Every point in that assembly allows for some movement.

But there are several blokes hereabouts that are more knowledgeable than I, so they might have a better answer, better than my "guess". You may have to wait until morning to get that answer though.
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Unread 01-18-2020, 01:49 PM   #3
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Quote:
But you need the skim coat to hold the mesh tape in place.
Do you mean a corner's mesh tape or some butt joint's mesh tape that ends at a corner? If you mean the corner's mesh tape, then why not just delete it along with its mortar?

I intend to Redgard a 6" wide fiberglass tissue along my corner joints, so I'm having trouble rationalizing the corner's mesh tape and mortar. Upon reflection I've got another theory: Maybe the corner's mesh tape patches up damage inflicted by the corner's backer-board-to-wood screws?
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Unread 01-18-2020, 02:10 PM   #4
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The way it's supposed to be done is mesh tape in the corner, thinset in place, but the corner not filled. I think that's the same thing as what Kevin said.

It will still add a little strength to the corner but allow for movement. Also, you'll be able to redgard the whole thing that way.
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Unread 01-20-2020, 09:01 AM   #5
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If you're asking the purpose of the tape, it helps to keep cracks from telegraphing up from the backer board seams through the mortar and tile. In almost every case that cracks appear over the backer board and follow the seams, tape wasn't used. The mortar is used to hold the tape in place. All of the spacing between the sheets is to accommodate movement as others have said. In every layer of a tile assembly, butting things together causes issues when temperatures change or structures shift.
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Unread 01-20-2020, 12:26 PM   #6
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Quote:
If you're asking the purpose of the tape, it helps to keep cracks from telegraphing up from the backer board seams through the mortar and tile. In almost every case that cracks appear over the backer board and follow the seams, tape wasn't used.
Right, but according to the TCNA Handbook's Movement Joints section (EJ171 I think), in the inside corners of indoor construction (and also the outside corners of outdoor construction), the tape sits above a movement joint and the tile is caulked instead of grouted. Bridging that gap with tape plus mortar seems contrary to the movement joint's purpose, while the joint detail seems to serve no crack suppression purpose.

The joint detail reminds me of a seismic retrofit detail that I once saw for some concrete panels at Harborview Medical Center in Seattle. We wrapped a bunch of epoxied fiberglass strands around the panels to reinforce them in place, but under lab testing the strands would debond at their ends. The panels would fail when these strands debonded up to some major crack in the concrete. We got a bunch more strength out of the panels by running strands perpendicular at the ends to restrain the debonding. This inspired my initial speculation for the purpose of the (inside) corner's tape plus mortar ("butt joints benefit from the perpendicular corner's reinforced mortar at both ends").

Every time I've screwed sheetrock along an edge where I can see the edge's gypsum, I've noticed cracks in the gypsum caused by each screw. Assuming that my Durock behaves similarly, I suspect that each of my (inside) corner's screws has a bunch of fractured backer board in its neighborhood. In my experience with steel and concrete cracking, a crack takes a spike in stress to begin, but then relatively lower stress to continue spreading. This inspired my second speculation for the purpose of the (inside) corner's tape plus mortar ("the corner's mesh tape patches up damage inflicted by the corner's backer-board-to-wood screws"), where the tape plus mortar reinforces the fractured backer board in the neighborhood of screws to stop my initial cracks from spreading. The lack of tape plus mortar over all screws seems to contradict this explanation, but removing half of the backer board for corner screws makes them more likely initiation points for fracturing. The lack of tape plus mortar at all free edges seems like good evidence against this explanation, however.
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Last edited by pope; 01-20-2020 at 12:29 PM. Reason: clarity
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Unread 01-25-2020, 07:32 PM   #7
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Quote:
Originally Posted by Tim
Last edited by pope; 01-20-2020 at 12:29 PM. Reason: clarity
Didn't work, Tim.

Well, at least the part about seismic retrofit of the concrete panels. 'Fraid I couldn't follow that analogy very well at all. But I'm gettin' old, eh?

On your B412 application, though, I do understand your question/confusion about the corner treatment of the CBU walls. But I've found lots of things confusing in that Handbook over the years. Your example is a good one.

B412 requires under Preparation by Backer Board Installers, as you pointed out:
Quote:
Corners—leave space between backer units. Tape joints
using skim coat of latex-portland cement mortar and 2"
alkali-resistant glass fiber mesh tape, but do not fill.
But note that under Installation Specifications in B412 it says in one bullet:

Cement backer board - ANSI A108.11

And ANSI A108.11 says at 5.5, under Joint treatment: Floors, walls, and ceilings: "Fill all joints of CBU's [sic] with tile setting material and tape according to manufacturer's requirements."

And to further reinforce that A108.11 version of how to treat the corners, two of the most common such boards, Hardiebacker in the Fiber/Cement category and Durock in the C1325 category, each requires, in his most currrent online instructions, that you fill the joints with mortar and tape with mesh tape, James Hardie even saying all joints. Now, USG even says you can use organic adhesive for that application, but that's a whole different discussion, eh?

Why does B412 contain such apparently contradictory information? I dunno. I wasn't invited to vote. But I think we can all agree that product manufacturer's installation instructions trump all other material on the matter.

Just thought I'd try to shed a little more darkness on the matter and perhaps someone will come by and clear it all up for both of us. 'Till then I think the best approach is simply to mud all your shower walls and not worry about how to treat no steenkin' CBU joints.
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Unread 01-26-2020, 02:25 AM   #8
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Quote:
Originally Posted by CX
And ANSI A108.11 says at 5.5, under Joint treatment: Floors, walls, and ceilings: "Fill all joints of CBU's [sic] with tile setting material and tape according to manufacturer's requirements."
Ahhhhhhhhhhhhh! And just when I finally convinced myself to take the handbook's instruction. I'm going to stick with the handbook's instruction, as the USG people don't even mention movement joints. If I was tiling my imagination's 100' by 100' wall without movement joints, then the USG instructions would blow up in my beautiful face.

I could only find A108 from 1999, but it contains movement joint provisions that disqualify the USG instructions at all corners:

Quote:
A-3.4.1 Movement joints are required over all
construction, control, and expansion joints in the
backing and where backing materials change or
change direction including terminations of tilework
where it abuts restraining or dissimilar surfaces.

...

A-3.4.3 Movement joints shall be kept free and
clear of all setting and grouting materials.
Note that this also contradicts the TCNA Handbook's EJ171 which doesn't require movement joints at the outside corners of indoor construction.

Going back and reading the "Using the TCNA Handbook for Specification Writing" section from the handbook, I understand that the handbook accepts ANSI 108 in its entirety. In their details they're pointing to the ANSI standards to be helpful, not to limit scope. If you've got the newest version of A108, then could you check 4.4 from A108.02 (movement joint general requirements)? I suspect that 4.4 disqualifies the USG instruction at corner joints just like the 1999 A108 did. I'm curious, however, if the EJ171's exclusion of outside corner joints of indoor construction contradicts the most current A108.

I also intend to ignore USG's

Quote:
Place a 1/4 in. (6 mm) spacer strip or shim around lip of bathtub or shower pan to hold bottom edge of panel off the fixture.
Maybe it's ambiguous enough to not contradict B412, but I'm pretty sure that they're detailing backer board 1/4" above the tub's lip. I like B412's bottom edge detail better. If I were to take the USG detail, then I'd probably obsess over getting some fancy Sika sealant for that gap.
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Last edited by pope; 01-27-2020 at 02:31 AM. Reason: quote origin
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Unread 01-26-2020, 07:42 AM   #9
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In my experience, trying to rationalize TCNA handbook rules is up there with understanding the why behind the electrical code book. Best to accept most such things as “because it says so”!

Seriously though, only way I’ve been able to get an in depth understand of how the rules evolved is to read the applicable committee minutes and their response to industry comments on proposed changes.
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Unread 01-26-2020, 11:40 AM   #10
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The handbook is not intended to be an instruction book.
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Unread 01-26-2020, 12:51 PM   #11
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True enough, Dave, but if the Handbook is to represent an industry consensus on installation methods, I think every effort should be made to ensure it is not in conflict with the actual industry standards and/or manufacturers' instructions.
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Unread 01-26-2020, 12:54 PM   #12
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Quote:
Originally Posted by Dave Gobis
The handbook is not intended to be an instruction book.
I recognize the Steel Construction Manual (AISC) and the Building Code Requirements for Structural Concrete (ACI) as descriptive codes, where you could read the codes and their commentaries to arrive at a strong understanding of steel and reinforced concrete mechanics. I guess the IBC has a commentary, but I've never bothered with it. Maybe it's a descriptive code also. By comparison, I see the TCNA Handbook as a prescriptive code. You won't find any why in it. According to its how-to-use-me section, it's a guide for writing construction documents.
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Unread 01-26-2020, 01:04 PM   #13
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Does anybody have a recent A108? Can you look up 4.4 from A108.02? I suspect that the manufacturer's corner-fill instructions may get explicitly overruled in this section, but I'm not eager to spend another 40 bucks on a single use document.
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Unread 01-26-2020, 01:25 PM   #14
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I think you're mixing instruction categories here, Tim. The movement accommodation joints for ceramic tile installations are for the ceramic tile installation, not for the tiling substrate or backing material. A108.02.4.4-19 pertains to movement joints in the ceramic tile installation and refers you to A108.01 for Subsurfaces and Preparations by Other Trades.

What is it you're looking for in A108.02.4.4?
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Unread 01-26-2020, 01:27 PM   #15
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Tim,
The mfg is not over ruled in 4.4 which states:

«*It is not the intent of these specifications to make movement joint recommendations for specific projects. Specifier shall specify and detail movement joints and show location.*»*
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