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Unread 12-15-2005, 02:32 PM   #1
Dancy House
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1 1/8 plywood subfloor vs. double layer

I'm building a subfloor to support a natural slate floor in the kitchen and living room area. My research is telling me to use two layers of plywood. However my current plan is to glue and screw 1 1/8" sturdifloor plywood to the joists. Then thinset and screw a cement backer on top with Redgard and megaflex thinset to lay the slate tiles.

Is 1 1/8" plywood a good way to go, or will I gain by using 2 layers of 23/32" plywood? Using the 1 1/8" will save me some money, but I want the best floor possible for having a crack-free slate floor.

Thanks for your help.

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Unread 12-15-2005, 02:42 PM   #2
Dave Taylor
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Hi Parke... welcome aboard the TYW ship.....

Wile yer at it.... give the Pro folks here a fair idee of what will be holding this "subfloor" up. Joist make, type, spacing on center, width, height and... longest unsuported span.

Thanks :--)
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Unread 12-15-2005, 02:49 PM   #3
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Parke, the double wood floor is the recommended (approved) practice. The 2 layers add a bit of "decoupling" that stone floors need.
You don't need the Redgard, unless you want to waterproof the floor.

If you are using an un-gaged stone, then you need a medium set mortar, such as Custom's Marble & Granite Fortified Premium Mortar. This mortar has more body so the stone won't sink into areas where thicker amounts of mortar are required to keep a consistant surface level. You certainly don't need the super-duty mortar you are planning to use.
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Unread 12-15-2005, 03:31 PM   #4
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Thanks guys for your quick responses. The joists in one area are 2x8 SYP with a span of 8' at 16" OC. The other area is 2x10 with a 10' span, also 16" OC. This is on a pier and beam foundation.

If I use 2 layers of 23/32"... the first layer is glued and screwed, and the second layer should only be screwed? Or should it also be glued and screwed? I'm guessing that gluing it would defeat the whole "decoupling" feature.

Going back to the idea of using one sheet of 1 1/8" sturdifloor... would a product like Redgard offer the "decoupling" needed for a stone floor? The reason I ask is because I'm trying to match the new slate to an existing finished floor level, and two sheets of 23/32" is going to be a little too tall, and I have not been able to find any thinner t&g plywood.

Also, I was planning on using products like Redgard and the megaflex thinset to help eliminate any cracking since it is sitting on a pier and beam foundation with a clay soil that is prone to shifting. Maybe overkill, I don't know.

Thanks, and your suggestions/tips are greatly appreciated. (FYI, I'm building the floor, but a pro will be laying the tile.)

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Unread 12-15-2005, 03:55 PM   #5
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1) Go to the Schluter.com site and download their guide. Lot's of good info in that even if you don't use their products.

2) Redguard is not going to do what you wanted and I really don't think waterproofing a floor in a kitchen is a good idea.

3) The first layer of T&G ply is glued and screwed. The second is not T&G and must not be glued or screwed to the joist., the panel is screwed to the field of the first. By not using T&G you should be able to find the right thickness.

4) Heck, while your at Shluter check our Ditra, seams the stuff does a lot of good and helps with the thickness situation.

5) ...gee, tgime for a cold one!
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Unread 12-16-2005, 06:21 AM   #6
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I am no expert.
I am simply trying to learn this stuff.

But, I don't think I'll ever get this stuff right.

Won't the backer board provide all the uncoupling necessary?

Just my opinion, but I would think a 1-1/8" single layer floor would provide more support between the joists than two layer unglued, and the backer board would provide all the uncoupling needed.

Why would he need to provide additional uncoupling?
Is more needed?
Is the recommended procedure now going to be to provide two separate means of uncoupling?

I thought the two-layer unglued was recommended when using Ditra and even then only if the floor needed additional strength. And not glued simply because of the possiblilty of improper glueing procedures.

Please correct me where I am wrong.
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Unread 12-16-2005, 07:02 AM   #7
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Parke, Star, JBowers, this 2 layer not-glued subfloor system is very counter-intuitive, but has passed both laboratory testing and field experience. Stone floors need the extra stiffness and decoupling of a 2 layer subfloor because the stone may have natural flaws that are unseen until the stone is installed and only then when it fails. My advice is to go with what works.

Installing Redgard as an antifracture membrane will not buy you anything if the building isn't stable, and really isn't appropriate for a backerboard installation, unless you want the waterproofing feature. And it is a mess to install and not cheap to buy. If you have extra money to throw around, consider spending it on Ditra and Kerdi. They would replace the backerboard, provide decoupling and waterproofing, save about 1/8" in height, and are much lighter than backerboard.
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Unread 12-16-2005, 09:05 AM   #8
Dancy House
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Thanks again for the excellent information. Based what you guys have said and what I have researched, here is my plan. Let me know where there are flaws.

- first layer is 3/4 Sturdifloor T&G plywood, glued and screwed to joists
- second layer is 1/2" CD grade plywood, not T&G, screwed to subfloor, not joists, with 1/8" gaps.
- third layer is either backerboard or Ditra.
- and finally the damningly beautiful slate that we're working so hard for.

Let me know how this sounds, and thanks for the help.

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Unread 12-16-2005, 10:36 AM   #9
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Don't use CD plywood. The minimum grade is CC(plugged and sanded).
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Unread 06-14-2019, 06:33 PM   #10
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Sorry to bump an old thread but is this still the common knowledge- because as far as I know there is way less deflection with 1 layer of 1-1/8 plywood vs 2 layers of 3/4” plywood

Is the double layer still the preferred method?
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Unread 06-14-2019, 07:41 PM   #11
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Hijacking this old thread, but I’ll answer. The 1 1/8” Advantech is a very rigid subfloor panel which is typically installed as part of a new floor system to meet requirements of tile or stone with widely spaced joists.

Two layer plywood system typically occur as part of a retrofit to increase floor performance for stone.

Same end goal but one was designed in from the beginning and the other added as the flooring requirements change.

Last edited by PC7060; 06-14-2019 at 07:58 PM.
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Unread 06-14-2019, 07:51 PM   #12
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The reason for two layers is to prevent the joints over the joists from bending upwards, potentially producing a stress crack. Adding a second layer, properly offset, prevents that first layer seam from projecting upwards enough to preserve the natural stone with potentially numerous internal imperfections. A manufactured tile is more consistent and can usually handle a single layer..

Think of the ends of the panels as little levers...a weight in the middle between the joists makes the edge of the joist act like a fulcrum, pushing the end up slightly even with the construction adhesive considering it's only attached by typically about 3/4" overlap.

IOW, use two layers to mask that joint's movement.
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Unread 06-14-2019, 07:52 PM   #13
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Actually, the double layer of plywood is required by the MIA (Marble Institute of America, now called something else) for natural stone tile installations regardless the joist spacing.

The thinking was, and still is, that with two layers it's possible to have a subfloor with no joint over a joist top that extends all the way through to the surface as will be the case with your 1 1/8th" subflooring panel.

And there is a good article in our Liberry showing the best way to install the second layer of plywood such that it's joints are not over the joist tops at all, but rather at the point between joists where there is least stress.

Better subfloor and not even to mention how very much easier it is to handle something less than that wretched 1 1/8th" stuff, even if you do hafta do it twice.

My opinion; worth price charged.

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Unread 06-14-2019, 08:15 PM   #14
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Good info Jim, CX.

I’m currently building a additions with a second floor system using 1 1/8” Advantech over Weyerhauser 210 series TJI. Love the wide spacing for plumbing runs but I’m sure glad I’m not the one lugging and installed that subfloor!
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Unread 06-14-2019, 09:27 PM   #15
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That 1 1/8" is good for only one thing, in my opinion. Stair treads. Cause after the sheet is cut up the pieces are only 12" wide, it's much lighter.

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Last edited by Kman; 06-14-2019 at 10:52 PM.
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