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-   -   Plywood Subfloor Seams (https://www.johnbridge.com/vbulletin/showthread.php?t=94786)

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.


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