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Unread 03-25-2013, 09:28 AM   #76
TooManyToys
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So it seems that conscientious from the manufacturers is somewhere between 1/32" and 1/8".

Here's another one that I haven't seen here yet:

http://www.performancepanels.com/sin...TOKEN=72208544


"Dimensional Stability and Flatness

It is a well-known fact that when wood loses or gains water, it shrinks and swells. The moisture content of structural panels is generally 2 to 8 percent when manufactured. When acclimated to ambient conditions or when exposed to elevated humidity or wetting after manufacture, the resultant increase in moisture content can lead to dimensional increase in thickness, length and width, but APA testing shows plywood and OSB exhibit greater dimensional stability than other wood-based building products.

The shrinking of solid wood along the grain with changes in moisture content is about 1/20 to 1/40 of that across the grain. Therefore, for structural panels, the tendency of individual veneers or layers to expand or shrink crosswise is greatly restricted by the relative longitudinal stability of the adjacent plies.

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The average coefficient of hygroscopic thickness swell is about 0.01 inch per inch of original thickness for each 10 percent change in equilibrium relative humidity. In normal conditions of dry use, relative humidity may vary between 40 and 80 percent, with corresponding equilibrium moisture contents ranging from 6 to 14 percent. Total dimensional changes of a 48x96-inch panel exposed to this change in conditions may be expected to average about 0.05 inch across the width and 0.09 inch along the length.

Three conditions of shape distortion constitute warping: cupping – when the axis of deformation is parallel to face grain; bowing – when the axis of deformation is perpendicular to face grain; and twisting – when one corner of a panel is bent out of the flat surface plane. Warping can be prevented by proper panel storage, including cutting the bands on bundles to prevent edge damage if moisture absorption is expected, by storing panels on three or preferably more evenly spaced stringers, and by weighting down the top panel in a stack to help avoid damage from humidity. Proper attachment of the panels to framing will typically pull a warped panel flat. The ultimate performance of the panel is normally not compromised by warping or cupping.

Certain plywood constructions (combinations of veneer thickness) exhibit greater resistance to warping than others. The ideal construction has about 50 percent of the veneers running in each direction and is assembled in a balanced manner about the central plane. As a general rule, the greater the number of plies, the better the stability.

For most applications, a minimum space of 1/8 inch should be provided at panel ends and edges to allow for expansion due to pickup in moisture content. When spacing recommendations are not followed, there is increased possibility of unsightly appearance due to panel buckling.

Coefficient of Linear Thermal Expansion

The thermal expansion of wood is much smaller than swelling due to absorption of moisture. Because of this, thermal expansion can be neglected in cases where wood is subject to considerable swelling and shrinking. It may be of importance only in assemblies with other materials where the moisture content is maintained at a relatively constant level.

Plywood and wood expand upon heating, as do practically all known solids. The thermal expansion of wood, however, is quite small and requires exacting techniques for its measurement. The effect of temperature on plywood dimensions is related to the percentage of panel thickness in plies having grain perpendicular to the direction of expansion or contraction. The average coefficient of linear thermal expansion is about 3.4 x 10-6 inch/inch per degree F for a plywood panel with 60 percent of the plies or less running perpendicular to the face. The coefficient of thermal expansion for panel thickness is approximately 16 x 10-6 inch/inch per degree F.

Wood structural panels are typically uniformly flat, making them a good choice for many applications. Concerns about warping can be alleviated by proper storage, handling, application and selection of panel grade and manufacture.

For complete details of APA testing, consult APA Technical Topic Dimensional Stability, Form TT-028. "
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Unread 03-26-2013, 04:22 PM   #77
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THIS is the most demanding recommendation or requirement -- preping the substrate -- when dealing with plywood , before tile installation
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Unread 03-26-2013, 06:52 PM   #78
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This copied and pasted from my website. Not too far off from CBP, but with atleast one difference I saw-- after gapping the plywood, caulk the joints with a non-hardening caulking to keep thinset out of the joints:

Quote:
For those who still want to tile directly over plywood, though, Here are the specs. Shortcut anyone of the specs I'm about to give you, and you can kiss that floor good-bye:

There needs to be two layers of plywood, the top layer being spruce or fir (preferably fir), exterior grade BCX or better. The layers need to be screwed (spec calls for every 6" along the edges, and 8" in the field-- I go every 6" throughout, ONLY into the bottom layer-- NOT into the joists) BUT NOT GLUED. When laying the second layer of plywood in, make sure the joints of the top layer fall at the 1/4 and 3/4 mark from the layer underneath. You don't want the joints in the two layers to be any closer than necessary. Also, when laying them in, leave about a strong 1/16" between the sheets for expansion, and make sure you're laying it in with the grain going across the joists. Make sure, when screwing down the top layer, that you're going no further than the bottom layer of plywood. DO NOT drive the screws into the joists. This completely negates the effect of double layering the floor by transmitting the movement from the joists right to the top layer of plywood. Once it's all screwed down, take any cheap latex caulk you can find, and caulk the joints between the sheets of plywood. The reason for this is those joints are for expansion, as I said before. Now you're going to go over the plywood with thinset. Sorta kinda defeats the purpose of gapping the plywood if you fill that joint up with thinset. That's ALL the caulking is there for-- to fill the joints with something that will remain pliable and at the same time, keep the thinset out. Last prep spec is that just like with cement board, you need to use a fiberglass mesh tape to bridge those joints . Easiest to use is the self sticking tape, and then just go over it with thinset when you set the tile. Last thing that's different and this is paramount-- the thinset. Just about ANY bag of modified thinset will tell you it can be used to go over plywood. DON'T BELIEVE EM!!!!! The ONLY thinset I'll trust is an UNmodified thinset, mixed with a liquid latex additive, full strength, such as Laticrete's 317 thinset mixed with their 333 additive, or Mapei's Kerabond thinset mixed with their Keralastic additive. The reason for this is it'll give you the highest latex content possible in a thinset, which does a couple of things for you. It's the strongest stuff you can find, and it's also the most pliable, so that it'll take the extra expansion and contraction that plywood goes through, as compared to cement board.
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Unread 03-26-2013, 07:31 PM   #79
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Quote:
Originally Posted by BillV
...the top layer being spruce or fir (preferably fir), exterior grade BCX or better.
Not to be picky, but we are being picky here, are we not? Exterior is not a grade of plywood, it's an exposure rating of plywood. The grade in your example would be BC.
Quote:
Originally Posted by BillV
Last prep spec is that just like with cement board, you need to use a fiberglass mesh tape to bridge those joints
What ever is the purpose of that, Bill? Don't believe I've ever heard that suggested.
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Unread 03-26-2013, 08:18 PM   #80
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For the same reason you mesh tape CBU joints-- to strengthen a weak point. Even before I ever HEARD of the JB forum, this is something that was always required, where I came from.

Quote:
Exterior is not a grade of plywood, it's an exposure rating of plywood.
Granted. But it's still a requirement.
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Unread 03-26-2013, 08:46 PM   #81
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Quote:
Originally Posted by BillV
For the same reason you mesh tape CBU joints-- to strengthen a weak point.
Don't believe you've quite got apples and apples there, Bill.
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Unread 03-26-2013, 08:52 PM   #82
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Let me give you apples and apples. In an industry where many tile contractors will swear that there's no safe way to install over plywood because it "always" fails, I don't have any problems. Never did. If it's a waste of time and money, then let me put it this way-- I sleep nice and easy after an over plywood install. Like the one I did yesterday. Remember-- I'm the guy who cloths all the corners and CBU joints with Hydroban, too.
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Unread 03-26-2013, 09:15 PM   #83
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Mmmm, mesh tape on plywood seams and reinforcing fabric in liquid applied waterproofing membranes is not apples and apples either, Bill.

I was just trying to get a handle on the reasoning for the mesh on the plywood.
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Unread 03-27-2013, 03:52 AM   #84
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Kevin, any word from Gerold?
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Unread 03-27-2013, 05:52 AM   #85
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Quote:
Mmmm, mesh tape on plywood seams and reinforcing fabric in liquid applied waterproofing membranes is not apples and apples either, Bill.
Sure they are. In your mind neither one is required. In my mind, both are cheap insurance.
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Unread 03-27-2013, 08:47 AM   #86
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Quote:
Originally Posted by Brad
Kevin, any word from Gerold?
Nothing yet. Ill give him some time, then call too. Sent him the following email a few days back:

Hi Gerald,

Just a quick question to see if NTCA has a definitive recommendation on whether to gap an underlayment plywood over a T&G plywood subfloor. We’re talking about a small gap between panels – like a 1/16th of an inch or 1/8”. And if so, do we need to fill that gap with a flexible sealant or cover of some sort to prevent thinset from getting between the sheets?

There is a rousing debate going at this thread: http://www.johnbridge.com/vbulletin/...t=94786&page=5

Started in 2011 but its gained steam over the last few days.

Here’s what I see: Some underlayment mfgrs say to gap, some say butt lightly together. APA says to gap but also mentions that some mfgrs say lightly butt, so where does that leave us? Now keep in mind that we are talking about remodel situations mostly, where the subfloor is not wet from a new construction scenario.

Summarized:
The tile guys that say gap – some say don’t fill the gap with thinset as it will form a solid inflexible seam that will eliminate expansion capability. Others who gap say that the thinset is so weak in compression that if the plywood were to expand, it would pulverize the thinset so no consequence.

The tile guys that say no gap – feel that there is no measureable expansion at the underlayment level and it’s not a concern.

So the question to you is . . . does the NTCA have a policy, recommendation, advisory, on whether to gap or not, and if so, should the gap be preserved somehow?

Either let me know and I’ll pass it on to the JBForum crowd for you, or please wade in and let us know what the NTCA recommends.

I just hope that you don’t say . . . “follow the manufacturer directions for the products used” because what we are seeing is confusing. Any help available my friend?

Kevin
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Unread 03-27-2013, 03:39 PM   #87
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If you fill that gap with thinset, You've eliminated the whole benefit of gapping the plywood! You gap it to allow it to expand and contract. If you fill it with thinset, how's that going to happen? it's not. Use a cheap crap caulk that'll stay pliable, and problem solved.
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Unread 03-27-2013, 05:01 PM   #88
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Bill, I think you came in late on this thread and didn't read far enough back. We pretty much decided a ways back that the expansion is probably going to take place during construction/before dry in. Then Gobis mentioned there could be a problem with a floor over a crawl space.

But I don't think there is a problem filling the gaps with thinset. It seems to me that from that point the movement, if any, will be contraction and not expansion. But I've been wrong before.
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Unread 05-17-2014, 06:36 PM   #89
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Gaps in Plywood

The TCNA is pretty specific about leaving gaps at the edges of ply, but they also say to follow the settings material instructions when setting things like cbu, or a membrane, and then tile. They also discuss the requirement to use T&G or block the joints, use construction adhesive and proper fasteners.

Then, if you look at say Laticrete's floor prep http://www.laticrete.com/portals/0/tds/tds152.pdf or most cbu manufacturer's installation instructions (Wonderboard being an exception), none of them talk about filling those gaps with something like silicon or taping them, or some other means of keeping them free.

So, what do most of you do, and if you do not caulk or otherwise fill them, assuming the rest is done properly (gaps, adhesive, fastener type and schedule), have you ever attributed a failure in your floor to not prepping the joints by filling them with some resilient material first?
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Unread 05-17-2014, 07:20 PM   #90
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I would trust the guidelines more then a cbu manufacturers lack of specs.
Me however, I am totally guilty of just thinsetting right over the gaps without a care in the world.

I install all my CBU using un-modified as a leveling bed and roofing nails to hold them. Don't know if that is why, having a cleavage zone to absorb shear stress, but I have never had grout bounce out or any problems with CBU.

However I do moisture testing on the wood floor before i CBU. If it is in the EMC (equalibrium moisture content) of my geographic region for the species of wood, I feel comfortable going over it as it is acclimated.

I have seen problems such as panelization on jobs where guys used modified and screws over floors that were still damp from new construction.
Other then that and of course guys who don't put any thinset under the CBU. I can't say I have seen any problems from un-gapped joints. But then again someone like Dave G has much more experience seeing tile failures than I.

As far as seeing plywood joints too tight together, the question beacons..did they slam it together when it was installed OR did it already swell to fill the gap they had left as per spec. Plywood expands 1/8 in four foot, The T&G is designed to allow for that.
Just seeing a tight joint you have no way to know if it was installed wrong or if it is merrily behaving as planned...unless you do a moisture test.

Now as far as OSB that does not really shrink back after bieng exposed to moisture. It may have at one point expanded and filled the gap but now is dried out and stable. If it got wet again can it keep expanding more..of course, what can you do?

To me I rather jsut make sure the floor meets deflection specs ,do a moisture test,stay with my un-modified/roofing nail system. That has never failed me then to start cutting the tongues out of floors and compromising the integrity of the floor..and as far as adding blocking after cutting the tongues..how do you do that with finished basement cielings?

If the floor is not exposed to high levels of moisture then why would it expand to the point it damages the tile. crawlspace..maybe, flood..maybe, typical well maintained house over full heated basement like we have in my neck of the woods. I don't think it is an issue worth losing sleep over.

But then again I don't always clean my buckets after every thinset batch either so I am probably not a good example to take advice from.
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