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Unread 06-10-2022, 01:35 AM   #1
shonuff66
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substrate approach for plank subfloor, porcelain tile

Looking for advice on (1) substrate approach for my subfloor situation, (2) recommended grout width.

Porcelain tile (8x48 and 6x36) is going in my kitchen/dining etc. See attached picture of plank subfloor (house is from 1966). Contractor is saying he either wants to:
1) put hardieboard down (screwed or nailed), thinset, tile
2) put 1/4 plywood down (screwed or nailed), thinset, ditra, thinset, tile

He doesn't want to put thinset right on the plank subfloor. I assume he's concerned about ruining the subfloor (for future remodels it will make it difficult to remove thinset from subfloor, perhaps ruining it). I assume he's also concerned about unevenness, but he didn't mention this.

We live in the bay area (earthquakes happen).

Tile is not rectified. I'm asking for 1/16th grout gap. Based on what I read, I suppose 1/8th is the right approach?

What's your advice on the substrate approach and grout gap?
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Unread 06-10-2022, 05:14 AM   #2
PC7060
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Hi and welcome to the site!

First thing to do is verify the floor is suitable for ceramic tile. Please use the Deflecto tool in the blue bar to enter the unsupported length, size and spacing for your joist. You need a minimum of L/360 rating for ceramic or porcelain, higher is better for earthquake zone.

Second: Regarding the floor preparation, I recommend a minimum of 5/8 structural T&G plywood screw over the existing plank (screwed/no staples); 3/4 would be better. Best case would be to remove the existing planking and attached directly to the joists. If you applied over the existing planking you will still need to screw directly to the joists, as the existing flooring has no more value than semi-inert spacers.

Third: Flatness - The floor does not have to be level but it does have to be very flat. Per the TCNA…

Quote:
For tiles with at least one edge 15” in length or longer, maximum allowable variation is 1/8” in 10’ from the required plane, with no more than 1/16” variation in 24” when measured from the high points in the surface.
Let’s work our way through these topics and then we’ll discuss whether it’s best to use concrete backer board or another product such as Schluter Ditra.

Good luck and feel free to ask any questions that may come up!

Last edited by PC7060; 06-10-2022 at 06:23 AM.
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Unread 06-10-2022, 07:07 AM   #3
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PC shonuff has ya covered, but you didn't mention how thick the planks are, nor if they are tongue and groove.

Given the age of the house those planks are nailed to the joists, likely without glue. Thee very first thing I would do is screw the planks down using construction screws. 2 screws per plank at each joist.
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Unread 06-10-2022, 09:53 AM   #4
shonuff66
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Thank you for the fantastic advice! What a great forum!
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Unread 06-10-2022, 01:40 PM   #5
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Quote:
Originally Posted by Dan
PC shonuff has ya covered
Ouch, that’s bad!
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Unread 06-10-2022, 02:23 PM   #6
jadnashua
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Guidelines for minimum grout line width is 3x the min/max of the tile involved. Stand a bunch of them up on edge, find the tallest, and the shortest at any point along that edge, calculated the difference and multiply by 3. I seriously double 1/16" will work! And, that makes setting the tile without lippage exponentially harder.
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