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Unread 02-07-2020, 05:36 AM   #1
Fox Mulder
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Perpendicular plank subfloor

Good morning,

I’m a long time reader of JB for the excellent advice, and have been a member for a long time, but have rarely (if ever) posted.

I’m a residential builder, so I have more skills than your average DIYer, but I’m just doing some tile in my own home. It’s built in 67, 2x10 joists, about an 11 foot span. Deflecto meter says I’m good to go. Problem being, I have a plank subfloor. 1x10s that run perpendicular to the joists. It’s in good condition, I replaced a plank that had a nasty spike knot near the edge. I’d like to keep my height to an inch or so, so I don’t really want to lay OSB on it.

Long story short, I’m wondering what you guys would think about using 1/2 inch Goboard over the planks? On a wall install it can span 16 inchs, it seems like it would work with my floor. My thought was to screw the planks tight, then screw the Goboard as well.

I’m laying 24” x 12” ceramic on the floor. And it’s my house, so I’m not worried about the warranty so much, but I also don’t want to have issues. So if it’s dumb idea, feel free to say so, but if it’ll work, I’d like to hear that also. I know that I could cut the planks out and sheet the floor to maintain the height, but I have limited time to work on my own house, I do almost all of it alone, and we have a 3 year old daughter that likes to “help”. So the faster I can get this done the better for all of us!

Thanks in advance for the advice, I look forward to hearing from you all, even if it’s potentially more work!
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Unread 02-07-2020, 07:18 AM   #2
ss3964spd
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Welcome, Fox, it's about time you posted something.

I don't know much about GB's properties, is it just a typical tile backer with no structural integrity? If so, then it's a no-go over those planks. And oh, do those planks happen to be T&G planks?

If the GB is NG, then 1/2" B/C exposure 1 ply is your best bet. You can minimize additional height by using a sheet product - something like Stratamat I think, instead of 1/4" CBU.

Those 12X24's are gonna want a very flat floor, so definitely check if yours is.
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Unread 02-07-2020, 09:15 AM   #3
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Hi Fox,

I would install a good grade of 1/2 in. plywood (that's minimum) and then a tiling membrane over the top. Ditra is what I recommend. I wouldn't guaranty that installation. For a guaranty I would have to remove the boards and install two layers of plywood.
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Unread 02-07-2020, 11:46 AM   #4
Fox Mulder
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Thank you guys for the advice!

It’s not tongue and groove planks, so that’s a bummer. Goboard is stiff, but I don’t know about it adding structural integrity.

I’ve never used ditra, so I’m a little intimidated by that, but it sounds like there are obviously some benefits of using it over the various different backer boards. If I tear out my planking and replace it with 3/4 ply could I install ditra over that? Or if I go over it with plywood, are we talking real plywood or would 1/2 osb be sufficient?

Thanks again gentlemen, and an extra thanks to you JB for hosting such an amazing forum!
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Unread 02-07-2020, 12:19 PM   #5
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Could certainly remove the planks, install 3/4" T&G ply, then cover that with 1/4" cement board or, if you need to shave off a little height, Ditra. Or if even a little more, something like Stratamat.

If you leave the planks go over them with real 1/2" plywood, no face lower than C, exposure 1. Screw the planks to the joists, and screw the ply to the planks, but avoiding the joists. If the planks are nice and flat, carry on. If any are cupped or crowned, remove/replace.

If I can install Ditra so can you. I made my life a little easier and bought it in flat sheets instead of a roll.
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Unread 02-07-2020, 01:43 PM   #6
jadnashua
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You should download and read the Ditra installation handbook...it will answer many of your questions.

Note, there are a few versions of Ditra, and one that is nice, you might consider is Ditra-Heat, where you snap in electrical heating wire so you can warm the floor! If you have pets, they will have found a new, favorite place to rest!

Back roll the Ditra to help take the curl out of it (easier in the summer!). Make sure to wipe or damp mop the floor. The floor isn't too wet unless there's liquid water on the surface. Thinset does not stick to Ditra, but needs to flow around the fleece, so you don't want to lose any moisture to the dry subfloor, which is why you want to dampen it...it also helps to remove any dust. Once the mortar cures, it locks things in place.

Thinset mortar really needs to be mixed properly, so read and heed the instructions as the texture and performance can vary radically if you shortchange anything in the timing. If the area is large, some like to use a roller on it, but that isn't necessary, just faster. You do want to ensure you've pressed over the entire surface, though, so a roller is more reliable.
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Unread 02-07-2020, 08:13 PM   #7
Fox Mulder
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Thanks Dan and Jim! I think I will download the Ditra handbook and read it. I know there must be a significant advantage to using it, or all the pros would do something different. I think I will sheet over what’s there, just so I don’t have to rip everything up in my kitchen if we decide to do the same floor in there, as it’s just a few feet from the bathroom. Thanks guys! I’m sure I will be back with more questions soon!
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