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Unread 10-11-2019, 09:52 AM   #1
Dun Wright
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Subfloor- old planks vs new ply? What type of ply?

Hi guys,

This house is from the 1950s and has plank subfloor with about 3/16-1/4" between the planks...ie: not T&G. Looks to be about 3/4" thick by 6-8" widths, run diagonally.

Is modern plywood stronger than the old planks or is it the other way around? I'm wondering if I should add 1/2" plywood over the planks, or remove the planks and add a 3/4" and then 1/2" layer of plywood?

If I do use plywood, what type should I use? Something I can get at a big box?

My ultimate plan is support the undersized floor joists from below with a mid-span beam in the basement so the 2x6's only have to span 5'. Then subfloor as mentioned above, followed by Schluter Ditra and 8x8 or 12x12 ceramic tiles. Thanks!
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Unread 10-11-2019, 10:47 AM   #2
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Welcome, Bob.

The tile industry standards require that you install a minimum of nominal 1/2" exterior glue plywood with no face of grade lower than C on top of a sawn board subfloor of a minimum of nominal 1" (3/4" actual) thickness over joists spaced at a maximum of 16" on center before a ceramic tile installation substrate and tile.

But that presumes your boards to have T&G edges and be laid perpendicular to the joist structure, neither of which you have. I would recommend you use at least nominal 5/8ths" plywood over your boards, presuming that your joists are on 16" centers. Is that actually equivalent to the standard requirement? I dunno, but I'd do it.

Or, of course, you could remove what you have and start with a first layer of nominal 3/4" T&G plywood. That might meet minimum requirements depending upon your joist spacing.

My opinion; worth price charged.
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Unread 10-11-2019, 12:45 PM   #3
Dun Wright
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WOW. Fantastic, knowledge filled reply. More than I expected or could ask for, THANK YOU KIND SIR!

If I'm understanding correctly, it sounds like adding 5/8" over the diagonal, non T&G "might" (most likely will) be ok but is not to written standards. Cutting the old planks out and adding 3/4" ply WILL meet written standards and be a slightly safer option.... Given my height issues with the section I am working on, it sounds like 3/4" ply over the 2x6 joists, 12" on center, with a span no longer than 5 feet (I'm running a support beam mid-10 foot span for support) will be a "sure thing" and will also solve my height issues. Did I get all that correct?

Next.... Any preference of ply maker or rating? I did read your minimum requirements above but wondering if there is a "gold standard" or preferred ply to put down or just grab anything from Home Depot that matches those standards? Do I run the long side of the ply across the joists or with the joists?

Can I place the Schluter Ditra directly on this 3/4" ply layer or would it be better to add another 1/2" ply on top? Will likely be using 12x12 ceramic of some sort. Natural stone will be unlikely in this kitchen. Thanks again!
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Unread 10-11-2019, 12:55 PM   #4
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1. Yes. With your 12" joist spacing the single layer 3/4" plywood will exceed any tile substrate manufacturer's requirements.

2. Any AC, BC or CC plywood you find with T&G edges and an exposure rating of EXT or Exposure 1 will suffice. You must orient the plywood with the strength axis perpendicular to the joist structure.

3. With your joist spacing I would be comfortable with a single layer of nominal 3/4" plywood under Ditra. More is always better, and if you're actually considering a natural stone tile the second layer is mandatory.

My opinion; worth price charged.
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Unread 10-11-2019, 01:34 PM   #5
Dun Wright
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Awesome! Thanks to you, this project is starting!

Is the "strength axis" of plywood usually the long direction? IE: The long edge of plywood would run perpendicular to the floor joists?

Any thoughts on Advantech subfloor? Hype or worthy?
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Unread 10-11-2019, 04:33 PM   #6
jadnashua
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Run the long edge across the joists.

Not all OSB panels are created equal, but I've had good luck with Advantec. The only issue is, some thinsets don't like to stick to some of them, but I've not found that to be an issue with Advantec. Considering the fleece on the Ditra only requires a minimum of 50psi, and often achieves 75+, and most any A118.11 thinset should provide at least that, it works. One thing with Advantec is that it is very heavy and you need to be careful about what screws you choose as it is also very dense, and some just don't like to go through it, self-drilling or not. FWIW, Advantec is about 10% stiffer than your typical ply subflooring.

While you have the planks off, double-check all of the joists for being in plane. Fixing that prior to adding the new subflooring is going to be well worth the time and effort as your tile installation becomes much easier when you have a really flat surface.

You will probably need to add some blocking for any walls and the board ends so they aren't just flapping in the breeze. If you have access from below, best to do that prior to cutting out the boards.
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Unread 10-12-2019, 04:32 PM   #7
Dun Wright
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SO MUCH good info, thanks guys!

Great suggestions on blocking before cutting and to level before re-surfacing. Way easier to do at this point!


Advantech flooring and flexbond it is!
Followed by Ditra and Mapei's Kerabond, then tile.


Thanks again guys! If you think of anything else, I'm all ears.
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