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cx 07-10-2011 09:48 AM

First time I've ever noticed Custom's requirement to use bond-breaker tape over plywood subflooring joints when installing Wonderboard. We could have a whole thread about that, eh?

Link To Technical Data Sheet for Wonderboard

CSS 07-10-2011 10:08 AM

I've always placed Duct tape over plywood joints so thinset dont get in there. got a pic of it on the thread I posted about SpiderWeb. Thought I was pose to.

cx 07-10-2011 10:14 AM

Thought maybe somebody would take the hint and start this thread, but that didn't work, eh? So to prevent further confusion on the other thread I copied the initial comment here for discussion.

chuck stevenson 07-10-2011 04:54 PM

2 Attachment(s)
We should clarify sheathing and subfloor.

In my world sheathing is T&G installed with adhesive and attached to the joists with the proper nail or screw pattern.

Subfloor is an additional layer required for certain installations and should be installed per Frank Woeste and Peter Nielsen.

I trowel out a water resistant TB II or water proof TB III to secure subfloor to sheathing following proper fastening schedules.

Attachment 102997

The subfloor shall have the proper rating,spacing, face grades and interior veneers. This is AC 5 ply 1/2" Fir from a lumberyard not the 4 ply crap from box stores. Never understood 4 ply with interior plies running in the same direction.

Attachment 102998

I saw something earlier that the subfloor is never glued. Not in my world.
Adhesive beads? No. Full spread glue over a clean/prepped surface? Yes.

As far as taping the subfloor with proper spacing on top of sheathing would be a waste.

Tilehelperdan 07-10-2011 06:01 PM

My understanding is that the first layer over the joists is subfloor, anything over that (and under the finished surface) is underlayment.

Can we get a link to the requirement, Kelly? Interesting. I never saw anyone treat the seams on plywood with anything.

CSS 07-10-2011 06:51 PM

1 Attachment(s)
Both customs and ardex reps have mentioned to me. If you fill with thinset then the edges have no where to expand. You might as well butt the sheets together.

cx 07-10-2011 08:35 PM

Hereza link, Dan. I pewt it in the original post as well.

I've long been aware of lots of variation in regional terminology differences in the building trades. Tough to keep up sometimes, 'specially when you're on a global Internet site discussing methods and materials with people from all over the country and around the world.

The problem is sometimes exacerbated by some participants being from non-English speaking countries, such as Canada and New Zealand. :D

But to be moderately serious, I've never, ever heard subflooring referred to as sheathing, not from any part of the country. Sheathing to me goes on roofs and walls, usually exterior walls. Never on a floor. Not even in California. :)

Subflooring to me is at least the first layer of material that is connected to the tops of the floor joists. Any structural layer over that is also subflooring. Any material over the subflooring for the purpose of preparing for the final floor covering is an underlayment.

I wish I'd taken the opportunity to bring that up the last time Frank Woeste was on here with us. I have discussed with co-author Peter Nielsen the matter of their calling the second layer of subflooring an underlayment in their most excellent subflooring article. What they're installing there is subflooing. A second structural layer of subflooing. Specifically in preparation for the installation of an underlayment for the application of ceramic tile.

That's my take on the subflooring terminology.

As for the seams in the subflooring when installing tile underlayments, as I mentioned in opening, I've never noticed it being addressed by any manufacturer.

Just another good reason for us telling our visitors to always read and follow the manufacturers' instructions. Clearly ain't none of us gonna be able to keep up with'em all, eh? :shades:


For a really good time with construction terminology, get a group of framers from around the country together and axe'em to say the name of the vertical 2x piece closest to a door opening and supporting the end of a header. :)

chuck stevenson 07-10-2011 09:02 PM


Originally Posted by cx
I've long been aware of lots of variation in regional terminology differences in the building trades.

I acknowledge my terminology is skewed.

AFATS I call that framing member a cripple or jack stud.

Levi the Tile Guy 07-10-2011 09:03 PM


Originally Posted by CX
Subflooring to me is at least the first layer of material that is connected to the tops of the floor joists. Any structural layer over that is also subflooring. Any material over the subflooring for the purpose of preparing for the final floor covering is an underlayment.

I agree with you here CX. I have always thought of underlayment as CBU, ditra, etc... and plywood as subfloor. In the TCNA 2011 hand book pg. 229 - the diagram refers to the second layer of plywood as "plywood underlyament."

cx 07-10-2011 09:42 PM


Originally Posted by Levi
In the TCNA 2011 hand book pg. 229 - the diagram refers to the second layer of plywood as "plywood underlyament."

Yeah, Levi, and I just hate it when they do that. But I think you've been around here long enough to see how much attention they pay to what I think, eh? :)

The only methods in there where they should call that structural plywood an underlayment are those where the tiles are bonded directly to it, such as F124, 143, 150, etc.

Let's hold our breath 'till they see the error in their ways. :D

chuck stevenson 07-12-2011 12:37 PM


Originally Posted by cx
But to be moderately serious, I've never, ever heard subflooring referred to as sheathing, not from any part of the country.

I guess I am not the only one that calls the first layer sheathing.


Back to the discussion. I read the pdf from Wonderboard and it makes sense to use bond breaker tape at the plywood seams.

dhagin 07-12-2011 01:09 PM

I'd don't use much cbu and don't do many floors, but under slc, Ditra, or similar, I typically tape or caulk the upper ply layer joints to keep mortar out of there. Never agreed with the 'fill with setting material' language. When gluing down NobleSeal over ply with NobleBond, i let the glue go in there if it wants to. :shrug:

alberta 03-19-2013 08:14 PM

Wall studs, roof trusses or rafters ,and floor joist's, are all covered by the means of sheathing. Anything that goes over that initial layer, can have numerous different attributes or designations.
On floor joists, it can be referred to as sub floor sheathing.

sheath·ing (shng)
1. A layer of boards or of other wood or fiber materials applied to the outer studs, joists, and rafters of a building to strengthen the structure and serve as a base.
I totally agree with Chuck...

cx 03-19-2013 08:47 PM

Welcome, Alberta. Please put a first name in a permanent signature line for us to use. :)

jwmezzanotte 03-19-2013 10:41 PM

Once I used some cheap caulk I had in the trailer to fill the gaps between the sheets. I think I used ditra between that and the tile if I recall correctly.

Other than that I havent bothered. Not because I don't think its worth the effort, but because I don't have the opportunity.
Most of the time I am called just to install the tile. Most of the contractors I have done this for are not botherd to ask me what I would like done. They never leave a gap between sheets, ever. In fact they are pretty proud of "how tight we got those sheets together".
I'm not sure what the thinking is there, they do leave gaps in the rest of their sheathing :shrug:

All the other tile guys here tile right over the second layer of plywood. The sheets are not installed correctly, and rarely if ever do I see anyone use an underlament.
So its not that I don't want gaps in the sheets, its just that I'm tired of beating my head against a wall. And it doesnt help that I don't have a failure to point to and say "see!"
Its just the way its done here because its been done so long that its the way its done here. Makes sense, except that it makes no sense.
I cant make sense out of that.. :uhh

I install the subfloor properly when I get to do it, which is almost never.
So how big of deal is this gap between the sheets thing? Shouldnt we be seeing a lot more failures here? Especially since its been done that way for so long?
My thinking is climate. Other places in the world have seasons. For a few months here its wet and cold, and then it gets cold and wet, then windy wet and cold, and then it repeats.
We don't have much for changes in temp/humidity, could that mean that there is not as much expansion contraction of the subfloor here in the swamp?

I donno, just curious how important the gaps are.

madronatile 03-19-2013 10:49 PM

Going to hop on this bump. So this would also apply to the joints when installing Wonderboard over ship lap? Which I believe Custom does approve of.

Makes for a lot of taping with 1x8s as subflooring.

Hank B. 03-19-2013 11:35 PM

John, could be climate. I've seen MANY hardwood/laminate floors around here buckle up off the floor 1" high when our monsoons roll around. Just thinking of that happening to some plywood with tile on top of it makes me shudder. I always tape them up. I've always kind of wondered if the first and second layers have similar expansion rates then its kind of canceled out, but if the first layer had gaps and the second layer didn't, seems like sooner or later the bottom gaps would close up and the other layer would have to cause problems.

TooManyToys 03-20-2013 06:09 AM

If the second layer underlayment is glued to the primary plywood subfloor, doesn't this new laminate structure (of say 1 1/4") become a monolithic panel with no expansion joints?

Even when screwed down only, doesn't it semi do the same thing? Unless there is some movement or bending of the screws?

For those that apply glue between the panels, what kind of glue coverage to you get? Are you using a paint roller, notched or flat trowel?

If the gap between panels gets filled, doesn't this negate the gap?

I read last night where John Bridge stated not to glue the panels.

Hank B. 03-20-2013 07:28 AM

you don't glue the panels becaue you might end up with voids caused from uneven glue, and leaving you with weak spots you can't do anythin about couse you glued them together. Proper screws and patern, no glue.

TooManyToys 03-20-2013 08:00 AM

Going by the assumption of the glue being troweled or rollered out similar to veneering .....

How would the occasional void of glued and 8" fastening pattern be any different then all the "voids" between an unglued 8" pattern?

Topspin 03-20-2013 08:18 AM

There was some talk about gaps between plywood sheets in this thread, but it didn't go far.

Thought I heard someone notable on this forum say that if panel gaps are filled with thinset the expansion has so much pressure that it would just crush the thin set. I'd rather not fill and feel better. Per thread link above, I knife flat small bead of caulk sealant in the joints.

In fact this morning I have a kitchen floor to do where the plywood panels are all butted together. My plan is to cut relief lines through the underlayment panels with a circular saw on all panel edges. Then caulk.

TooManyToys 03-20-2013 09:05 AM

Yeah, I hear ya. Even if the thinset was crushed, it is still hard mass that will not compress. I could even be a PITA and say that any material in the gap will reach a compression point of being a solid, so the best gap fill would be a closed cell foam.

Maybe it's not so much of an issue because most homes today have full HVAC and the humidity swings are not as extreme in conjunction with the low expansion of plywood compared to dimensional lumber.

Donnie D. 03-20-2013 02:48 PM

i dont think the plywood of most homes today r gonna move that much..esp. if they r screws and glued down..most homes stay between 60 and 80 tops..ive never seen a tile setter that i know of caulk or tape the seams

Lump 03-20-2013 06:36 PM

Don, if you could see us from there you would see all of our joints are caulked:cheers:

Lump 03-20-2013 06:44 PM

Here are the instructions for advantech http://www.advantechperforms.com/upl...2010023259.pdf

They call for 1/8" gap. This gap needs to be protected with caulk or tape.

Perfect examples were this last summer many of the new homes we installed needed the joints sanded due to peaking, we had little no rain, one of the driest summers ever. Still because the sheets were installed too tight it caused seam peaking.

muskymike 03-20-2013 07:19 PM

Uhh, Brad the seams always peak on OSB. :gerg:

PetrH 03-20-2013 10:22 PM

IMO the maximum moisture content of underlayment plywood will be at the time of install, as the plywood draws moisture out of the thinset used to adhere cement board or tiles. If it does not tent at that time, it is not likely to do so later unless the floor is flooded. I leave 1/8" gaps in my halex underlay, and then tile right over that without caulking gaps. I believe it would be technically superior to caulk the gaps, but is not necessary.

Whenever budget allows, I roll on waterproofing on the floors in the bathrooms as well.

BTW Hank, John lives in one of the wettest places on the planet.

T_Hulse 03-20-2013 11:00 PM

Brad, those 1/8" gaps are filled with hard-as-rocks polyurethane construction adhesive to structurally bond the sheets together. No tape protection or caulking is necessary.
You guys who are caulking those joints, it's really a waste of time putting an expansion joint in the middle of the single tile panel, and it's also contrary to national tile standards not to honor that joint all the way through to the tile surface if you really believe it allows for expansion (it doesn't).
Here's a very key point that is central to this whole discussion: seam peaking in subfloor panels is not from compression that results from expansion (the whole sheet compresses equally); rather is is from water-resistant coatings on the faces of the sheet that are not present at the edges. The water runs off the panel, but has free access to the very-absorbent end grain that is usually lacking the same treatment, and swells the edge. That's why the newer, better panels don't peak as much or at all, because they have heavily waterproofed edges. This is relevant to us because it shows that peaking is a one-time occurrence while the house is being built. And yes, like Petr says the subfloor is never again going to see moisture anywhere near being rained on then covered with wet mortar a month later.

John Bridge 03-21-2013 06:53 AM

I agree with Tom that the greatest amount of sheet expansion is likely to occur during the course of construction. Once the building is dried in the sheets will contract as they dry out. This drying process will continue even after the finish floor is laid or set. Unless the subfloor is somehow wetted down the road, sheet expansion should not be a problem. This assumes that all installation and framing steps were properly completed from day one, and that is one heck of an assumption. ;)

Dave Gobis 03-21-2013 08:37 AM

As long as you don't have a crawlspace.

cx 03-21-2013 09:26 AM

Especially one without a vapor retarder over a properly graded surface and good ventilation or partial conditioning.

T_Hulse 03-21-2013 12:54 PM

The most you can get with a crawlspace (standing exposed water & vents closed off) is a moisture percentage in the low 20's for your subfloor, which is still less than the full-blown rain exposure that is common during framing (especially here! :)).
Even if it were somehow higher than framing, it still wouldn't cause edge peaking; that is not from plain horizontal expansion of sheets. Peaking is from a differential in moisture caused by different amounts of access that water has to different areas of the board, not just by "lots" of moisture; and also by the super-absorbent end grain that sucks up liquid water (not just moisture) faster than the surface. Hardwood floors for instance. We've all seen the peaking when they get wet, but no matter how wet they got, they would never peak if they didn't have a waterproof coating on top and had tile on top that didn't allow the top to air dry much faster than the bottom. As the bottom gets wet & expands, the dry top becomes relatively shorter and causes the whole board to curl. So moisture differential & absorbent end grain, it has nothing to do with compression at the joints. Horizontal expansion doesn't cause edge peaking. :)
Edit: to clarify, also "moisture" by itself can't cause edge peaking. It has to be liquid, running water or a differential in moisture which is not present in a finished tile assembly.

dhagin 03-21-2013 01:22 PM

I've built a few homes in the PNW, and don't necessarily disagree with any of the arguments the "no gap needed" crowd makes. This assumes, of course, that the maximum moisture those panels see is when they're initially installed.

Would anyone gap any sheets, subfloor or underlay, if the project was framed and dried-in before the panels got wet? I've been concerned about this one especially, as we'll occasionally have a few relatively warm & very dry months followed by a year of relatively cool & wet ones. Further complicating issues is folks who leave town in the winters here, during the cool & wet months, turning their heat way down below what I suggest. :)

T_Hulse 03-21-2013 01:50 PM

Dana, that's another whole separate reason this gap thing doesn't work. That gap in the subfloor is not for expansion, it's for glue.
A third reason is that when you say gap, you really mean "expansion joint"; and an expansion joint in the construction that is not carried through to the tile surface is a violation and a waste of time. You dont' want to encourage the subfloor panels to have any more independent movement (from each other) than they already have. It would be dangerous and cause tile cracking if it actually worked as an expansion joint.

dhagin 03-21-2013 03:06 PM


Originally Posted by Tom
That gap in the subfloor is not for expansion, it's for glue.

What's your source Tom? :)

These gaps I'm referring to above are not expansion or movement joints as described by EJ171. :)

T_Hulse 03-21-2013 03:39 PM

There's one source posted in this thread yesterday, the Advantech instructions that wants polyurethane adhesive between the tongue and groove. They show a picture explaining the reason you don't use too much glue is that you'll have to scrape it off the top (not that somehow the boards could move independently if you used less glue). When you built those houses, did you have glue in the T & G?
When you say gap, if you are thinking that somehow that gap was for expansion & contraction, well that's just other words for movement. You are describing a construction joint, like an EJ171A wood version, and if construction joints are designed for movement (whether in concrete, wood, or other), then they must be honored all the way through to the tile surface. The reason there is no EJ171A for wood, like you are describing, is that wood floors are not designed to intentionally encourage independent movement between sheets.
An expansion joint that is not carried through to the tile surface is a worthless expansion joint (and potentially harmful if it actually works).

John Bridge 03-21-2013 04:03 PM

I believe the 1/8 in. required gap IS for expansion during the course of construction. ;)

Lump 03-21-2013 05:37 PM

12 percent max moisture content is also a good rule of thumb to go by prior to installing any flooring....even backer board. Take the time and protect the gaps.

dhagin 03-21-2013 06:53 PM

2 Attachment(s)
I guess i don't get it.

TCNA F143-12 Plywood Subfloor Plywood Underlayment Ceramic Tile, for instance, requires gaps (their term) between sheets of both subfloor & underlayment. These gaps are to be offset minimum 2" so naturally don't carry through the assembly.

APA also recommends gaps or spaces between sheets of subfloor & underlayment, . Their stated reason is to allow for expansion. :shrug:

a few APA ref's:
Attachment 140506

Attachment 140507

eurob 03-21-2013 07:04 PM

Dana, From the first APA link , is this gap -- spacing of 1/32 recommended at underlayment butt joints -- or space that you are refering to ?
Page 4 and 5 from the same link -- L335 -- are pretty interesting .

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