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So my house has 2x6 car decking/tongue and groove/whatever-you-want-to-call-it subfloor that I am going to tile over, both in the kitchen and the bathroom. The rest of the floors are 3/4" oak as you can see in the pictures.

Underneath the subfloor is post and beam construction--4x8 beams about anywhere from 3-5' apart from one another (3' from the outside wall, then 5' thereafter). So the 2x6s span from 3-5 feet at a time.

There are two spots that I replaced some old rotted sections of the T&G which you'll notice in the pictures below. The older sections of subfloor were a bit springy in spots so I nailed some 2x4s flat from below, putting a nail into each piece of 2x6 along the way. This stiffened them up considerably and there's hardly any give to them anymore.

My question is how should I go about laying tile over this? I have the 3/4" wood floor to tie into so I don't want to go too crazy with additional underlayment.

From some initial research, it seems my best option might be to go with a layer of 1/2" plywood screwed to subfloor, then thinset/screw some 1/4" concrete backer, then thinset/tile. If I start with ply on the subfloor, do I need to use tar paper underneath that or should I use some self-leveler before laying down the ply (or do you do this on top of the plywood?).

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  • Did you end up using a vapor barrier between the plywood and the 2x6's? – bigmike7801 Sep 30 '16 at 15:16
  • I didn't. Tried to prep the 2x6 as well as I could with self leveler in some spots and planing down other spots. Then I used liquid nails and 1/2" ply screwed down to the 2x6, then 1/4" backer board thinset and screwed to the ply. Then taped/thinset the seams of the backer board. Made it solid as hell and mostly flat/level. My taping job was not the best so I had to grind down a few high spots at the seams. – tbox Sep 30 '16 at 15:26
  • Part of the structural code states the equivalence of using ply to a vapor barrier. I'm on my phone so I can't look it up right now, but I think it's due to the glue in the ply. So you can skip the vapor barrier if using ply underlayment in some cases, though its always better to use a vapor barrier anyway. I skipped it because I used the liquid nails and needed a good surface for it to bond to – tbox Sep 30 '16 at 15:28
  • Thanks for the tips. I feel like I'm looking at pictures of my house when looking at yours. I'm in the Pacific Northwest and my house was built in 72. My biggest problem is that I can't get underneath my house to add any floor joists so I'm hoping that the way it currently is will be ok. – bigmike7801 Sep 30 '16 at 15:56
  • No problem. I'm in the PNW too (Oregon). House is mid 50's ranch. Here's a vapor permeability rating chart: buildingscience.com.678elmp02.blackmesh.com/sites/default/files/… Plywood is even less permeable than tar paper so it sounds like you'd be adding an unnecessary step by doing both. If you make a hole in your floor like I did, you can get under there easily :) – tbox Sep 30 '16 at 17:02
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I quite literally did this exact same thing a few years ago.

First ensure that all of your structural posts have support beneath the house to foundation.

Then I got decking screws and screwed the planks down everywhere they were already nailed.

Then I bought about 50 sheets of CCX ply and lots of subfloor glue. I glued and nailed ply over the entire floor using shanked dipped nails.

Now I had a very stout floor about 2" thick....

Ditra over that, tile over that.

  • thanks for the tips. it's a crawlspace underneath but the posts and footers are all good (I've been down there way too much. ugh). So you just glued the ply to the subfloor without any vapor barrier? (I read somewhere ply can act as its own vapor barrier...) No leveling or anything? – tbox Aug 4 '16 at 23:45
  • @tbox I realized I forgot a step, edited! Use decking screws in every board where it was nailed. – Matthew Aug 4 '16 at 23:47
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Yes, you can do it. This is how they did it during Victorian times when plywood didn't exist:

If the tiles were to be laid over an existing wooden floor, the floor boards had to be pulled up, sawn into short lengths and fitted between the joists. Concrete filled in the spaces and made the base flush with the upper face of the joists, and created a level surface finished within 1" of the finished floor line. A layer of cement mortar was then laid on top. This allowed the tiles to fit in the same amount of space as the floorboards they replaced.*Before laying the tiles, skirting boards or shoe moldings were to be removed, and replaced after the tiles were laid. This eliminated having to cut the outer tiles to fit exactly, and resulted in a neater appearance.

Alternatively, you should be able to measure the joists' spacing and use decking screws to screw down the subfloor to the joists (every 6"- 12" down the length). They didn't have deck screws back then or cement board (they poured concrete). Unfortunately, they don't have no photos that I've been able to find on the web of how the victorians did it, only a description. In a bathroom, you will have to put down half an inch to 3/4 of an inch of plywood if there's going to be a tub. What I'm describing is how you would tiled regular flooring in the household.

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