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Old 12-26-2017, 07:27 PM   #1
ksieburg
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Subfloor newbie questions

Hi, I searched the net and browsed some books and I'm having trouble finding the basics of how to do a subfloor.

Background:

I have some tongue and groove planks in a 25 sq. foot bathroom that I am going to put 3/4" exterior-grade plywood over. I'm going to use 1 5/8" exterior screws. Then I'm going to check for dips and use a self-leveling compound. Then I will install Ditra over that and then some 30 x 61 cm rectangular tiles.

Questions:

1) I am going to leave a 1/4" expansion joint between the sheets and the walls. Is that good?

2) Do I need to put some sort of a mortar over the plywood screws and seams?

3) We are getting a new threshold. Do I remove the current one now and put plywood and Ditra under where the new one will go? I haven't purchased the new one yet but I can if needed.

I'm sure there will be more questions to come. Thanks!
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Old 12-26-2017, 10:51 PM   #2
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First you should verify that your floor joists are adequate to support a tile installation. Use the Deflecto in the blue bar above to check that. Also, what direction are the planks oriented to the joists?
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Old 12-27-2017, 12:06 AM   #3
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How do you get all of this information about your joists when you can't see them? Does it matter that we are only talking about 25 sq. feet?
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Old 12-27-2017, 12:58 AM   #4
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Yes, it matters. And if you’ve got tongue & grove planks, it’s an older home. And older homes were built with everything from under-built to over-built. It pays to know what you’re starting with so you can build appropriately.

If you can't directly see the joists, making an access hole from the top in an area that won’t be compromised by doing so is a good idea. A 3” holesaw can make quick work of it.

And like Kevin asked: Are the planks perpendicular or diagonal to the joists?

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Old 12-27-2017, 08:42 AM   #5
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The planks are perpendicular to the joists. The deflecto meter says that I'm good, although I had to assume the length of the joists (not sure how I would be able to know that without ripping up the floor). Honestly the floor feels solid as a rock and there is no sag when I bounce. Plus there was already tile there before.

So can I get an answer to my questions now ?
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Old 12-27-2017, 09:32 AM   #6
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Karl, we're just trying to give you the best information we can. There's not much use in sending you forward on your project if the foundation is questionable. You can certainly tile over what you want since it's your house. No tile police will be coming over to arrest you if the job isn't up to snuff.

1. Use the spacing recommended by the plywood manufacturer. Typically that is 1/4" around the perimeter and 1/8" between the sheets, but check their literature.

2. You'll be doing that at either the self-leveling stage, or the Ditra stage.

3. If the finished floor will be higher than the current threshold, either replace it or set it on top of the new floor. Otherwise you can tile up to it, possibly, depending on the shape of it.

What is the thickness of the planks?
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Old 12-27-2017, 09:41 AM   #7
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The planks are 1" thick.

No problem, I get it. It's just hard for me to tear up a seemingly solid floor to determine the deflection. It seems sturdy as a rock and this is before I've added the plywood. But I get that that isn't very scientific. But I'm also a week-end warrior with 2 young kids and therefore a short window to get this banged out before xmas break is over.
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Old 12-27-2017, 09:50 AM   #8
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Can you tell what the spacing of the joists is? If they're at 16", you could use 1/2" plywood.
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Old 12-27-2017, 10:07 AM   #9
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They seem to be at 16", yes. And thanks. I am using this expensive OSB board which measures in at .71": https://www.homedepot.com/p/Advantec...2405/202084475
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Old 12-29-2017, 07:46 AM   #10
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Expansion gaps in subfloor too big?

I'm attempting to add a layer of 3/4" plywood on top of my existing subfloor. I did not do a great job cutting my first sheet and the cut was not straight but more wavy. In some spots the sheet it is butting up next to is as far as 1/4" away and in other spots it's the prescribed 1/8". It's also butted up next to the tub and that side has a gap that is sometimes as bag as a 1/2".

Should I just go ahead and redo it?

Bonus question:

When I install the Ditra mat, should I avoid getting thinset in these gaps?
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Old 12-29-2017, 08:58 AM   #11
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I do not see a issue with the gaps. I would try to keep the thin-set out of the gaps. Others in the forum might have different advice.
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Old 12-29-2017, 11:17 AM   #12
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If you want to keep the thinset out of the gaps, fill them with caulk. That will allow for the expansion/contraction while not having to worry about keeping thinset out of then which is almost impossible while trying to get 100% coverage while installing your ditra.
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Old 12-29-2017, 06:13 PM   #13
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Thanks guys. I may add some caulk around the bathtub so that there isn't a void under the tiles where the expansion joint is a little bigger than it should be. But otherwise, I'll leave them be. Can't believe I caught a break!
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Old 12-30-2017, 01:11 AM   #14
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I used sill seal to help keep my movement joints and walls free of thinset. Worked great and you dont have to worry about splashing a little. Gives your nice 1/4 gap around the walls. You could caulk or tape over the joints between sheets but as long as your gaps around the perimeter are open it shouldn't matter if the gaps between the sheets are filled witb mortar.

I'm guessing you just used a circular saw to cut your ply, you can cut a guide as wide as your saw spaces from it's guard and use that to clamp a straight edge to your line and get a nice straight cut that way. They also sell clamp on guides for this purpose.
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Old 12-30-2017, 02:41 PM   #15
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Thanks Nick, that's a good idea about the guide for my saw. I first was using a jigsaw and even with a clamped straight edge to guide it, I couldn't get a nice straight line. Switching to a circular saw and a clamped straight edge worked nicely. I got a little impatient with all of the extra time it took to clamp the straight edge so I started free-handing and that worked pretty well too, honestly. But next time I will just make a 5" wide piece that I can use as a guide and that will make it much easier.
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