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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
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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
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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
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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
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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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Unread 12-22-2019, 10:47 PM   #8
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Deflection test? Situation too funky to calculate on JP calculator

Working on a kitchen floor which had 12x12 natural stone tiles on top of 1/2" hardibacker which was over 3/4" x 6" diagonally ran plank subfloor which was on top of 1 1/2 x 5 1/4 floor joists from the 1950s. 3 tiles in the whole kitchen were cracked and that was directly over a hardibacker joint and in a weak spot in the floor (could feel the "squish" when bouncing lightly).

Tore it all out down to the 1 1/2" x 6" x 12' long floor joists. YIKES. Talk about undersized floor joists! The 2x6's run 12 feet (unsupported) and are 12" on center. I sistered 1 3/4 x 5 1/2 Microlam 2.0 LVLs to each joist and then added 2x6 blocking every 3 feet. I then covered with 3/4" (height restrictions to old floor) Advantech subfloor.

The floor is about 32234234 times more stiff than it was before based on my scientific fat man walking/bouncing tests. The wonderful John Bridge Calculator wont work in my situation. Is there a test I can do to see if natural or ceramic tile will indeed be ok? I have laser levels, a lot of fat friends, and many tape measures if that helps any....
Thanks in advance, love this place.
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Unread 12-22-2019, 11:06 PM   #9
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Are we still working on this project, Bob? If so, it might be helpful to merge the threads so the readers get the full picture.

I can't tell you how much your microlams have helped your situation. I'm sure they did, but how much is something only the manufacturer can tell us, or maybe an engineer.

I can tell you that you probably would have been better off to add a mid-span support, which would have greatly increased your deflection rating, more than likely enough for ceramic tile, and maybe enough for stone.
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Unread 12-22-2019, 11:31 PM   #10
Dun Wright
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Oops! Yes, same project. Sorry about that, can you merge if that would follow the rules? Thanks

I decided to try all of this first in hopes of avoiding a mid span support, as that would protrude into the basement ceiling space. If I can find a solid number/percentage/something that indicates I still have too much deflection, I'll resort to adding a mid-span brace if absolutely needed.
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Unread 12-22-2019, 11:37 PM   #11
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Yeah, let's do get those project threads combined, Bob.

If your existing joists were a full 6" tall and 12" on center you could have achieved a design deflection exceeding L/360 by adding another joist that size to each one. Given that, I would feel confident that adding a 5 1/2" microlam would be even stiffer, but I cannot find a span table for those microlams you added. You're not, however, gonna get L/720 with that addition.

And the blocking does nothing at all to help your joist design deflection.

My opinion; worth price charged.
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Unread 12-28-2019, 02:14 PM   #12
Dun Wright
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Thanks for the reply.

I'm not saying you're wrong and I've heard similar claims from experienced framers, but can you explain how adding blocking DOES NOT help with deflection?

Both in theory and in the field, I have to say I disagree. Blocking seems to spread the load to nearby joists, and more of them, than only the subfloor, in my opinion. Is there any article/science/theory that says otherwise? Honestly asking and here to learn, not saying I know you're wrong by any means.
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Unread 12-28-2019, 04:00 PM   #13
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The between-joist blocking (X bracing is actually the better method) serves to keep the joists from twisting under load when the unsupported span is long. That does help in allowing the vertical joists to display their full deflection potential, and may actually transfer some point loads to adjacent joists, but it does not add anything to the inherent stiffness of the joist itself. The overall design deflection of the joists remains exactly the same as it would without the blocking. The application of a ceiling to the bottom of the joists provides a similar result in keeping the joists upright, but doesn't increase the design characteristics of the structure.

The rigidity of the sawn wood joist is determined by the species, grade, and condition of the wood itself. The design deflection of the structure will be the result of the size, shape, spacing, and unsupported span of the individual joists. It is beneficial to keep the joists in position and vertical under load to maintain that design deflection, but any framing units that merely keep the joists in place do nothing to increase their capabilities.

The same is true of engineered wood joists except that the characteristics are determined more from the design than the inherent characteristics of the individual components. You will frequently see between-joist blocking specified as a requirement for a particular floor design, especially in long spans, and that is precisely because part of the design is that the joists remain vertical under load and some of those joists, by their design, are prone to twisting under load. The design is for the entire structure, often including a glued and mechanically fastened subfloor of a specific type and size. The design deflection is for the entire package and that deflection is not expected to be exhibited unless all the components are included and properly installed. If you were to install additional between-joist blocking to one of those floors, you could expect to achieve nothing at all beyond the original design specifications.

Perhaps someone else will find you an authoritative article or source for your answer, but I'm afraid I don't know one offhand. And then perhaps you'll be able to say with authority that I was wrong. It will not be my first time.

My opinion; worth price charged.
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