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Unread 07-01-2018, 07:03 AM   #1
Hunzi
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Plank subfloor vs plywood and large format tile

Ok, so MrHunzi is off in a couple of hours to help LilMiss with a tile entryway - not a huge area about 40sqft - but it sits between to rooms with wood flooring.

She purchased 12x18 porcelain tiles (1/4 thick).

They already ripped up the nasty old linoleum and removed the chipboard underlayment that was falling apart.

Subflooring was planks - it's a 1953 home.

Ditra says don't lay it over planks.

So MrHunzi's plan is to remove the planks, lay down 1 layer of 3/4" plywood on the joists, then Ditra etc.

I woke up this morning with a nagging wonder if the flooring is stiff enough for the tiles! I ran the deflection calculator and I'm going to assume it's 2x8s with 16 centers, so the deflection was 367. Just enough?

I know MrHunzi doesn't want to have the tiles sit proud of the wood floors - so he's going to resist doing more to stiffen them - but I also know he doesn't want to explain to his little girl why her tiles are cracking.

Do a sanity check for me! Is 1 layer of plywood on joists enough? would putting 1/4 ply or concrete backer over the planks be a better option?

Help me stop him if needed before he goes through all the work to rip out those planks!

Thanks!
Hunzi
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Unread 07-01-2018, 07:40 AM   #2
cx
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I'm guessing that by planks you actually mean boards, Joann. Sawn pieces about 3/4" thick?

If so, your options would be to remove and replace as you have described, or to install a minimum of 1/2" exterior glue plywood over the boards, then your tiling substrate of choice. That would be sufficient only if your boards are oriented perpendicular to the joists.
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Originally Posted by Joann
I'm going to assume it's 2x8s with 16 centers, so the deflection was 367
You really don't wanna guess at that, you need to measure the joist height, determine the wood species, grade, condition, and spacing and measure the unsupported span before making a determination of their suitability for a ceramic tile installation.

If your joist spacing is no greater than 16" on center the nominal 3/4" spacing meets the minimum subfloor requirements for Schluter's Ditra and most any other tiling substrate you might elect to use.

My opinion; worth price charged.
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Unread 07-01-2018, 08:02 AM   #3
Hunzi
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I'm guessing that by planks you actually mean boards, Joann. Sawn pieces about 3/4" thick?

Yes.

I will ask MrHunzi about the exact details of the boards and joists size/spacing- I haven't been over in a while, so I can't recall.

Just wanted to be sure we weren't making a fatal mistake on the kiddo's new house. (Of course, it's nice to practice on someone else's place - LOL, before we find ourselves removing our board subfloor (from 1884) and sistering/stiffening joists up at the Shrine of Our Lady of Perpetual Renovation for the future 1925 Bathroom renovation.

Always ;-)
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Unread 07-01-2018, 07:59 PM   #4
jadnashua
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You need something on top of that new plywood subflooring before you can add tile. Numerous options:
- another layer of ply at least 3/8" thick (works, but less room for error)
- 1/4" CBU, properly installed
- a membrane such as Ditra

You cannot tile directly to a single layer of ply and expect it to last. That first layer needs to meet certain requirements, too, along with proper installation.
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