Subfloor for Encaustic Tile

[Magum Opus]

I have a 1920 Craftsman home on 2.5' piers (so the bottom of the subfloor and joists are exposed to Florida weather). I want to put down Encaustic tiles in my kitchen which is 9' x 14' with floor joists 24" on center. Currently, I have oak hardwood with furring strips running at an angle and spaced rather far apart between the joists and the hardwood. I know that I cannot tile over the hardwood and that it needs to come up along with the furring strips so that a subfloor can be installed over the joists. I'm doing this myself so I need all the advice I can get. I have replaced a subfloor in the past, but that was easy compared to what I'm about to embark on.

Here are my basic questions:

Is 3/4" plywood or OBS the proper thickness for my tongue and groove subfloor material?

Do I need to sister the joists to strengthen them?

Do I need to add joists between the original joists? (is that even a thing?)

We have had a few piers added for support.

I'm starting this nightmare on 1/9/2021

[cx]

Welcome, Douglas.

A geographic location in your User Profile will be very helpful going forward.

1. That would be suitable for the first layer of subflooing.

2. No idea without knowing what you're starting with. If you'll look in the dark blue bar near the top of the page you'll see our Deflectometer listed there. Enter the information requested to get an initial go/no-go reading on your existing joists.

3. See #2.

4. Some photos or drawings or both may be very helpful.

My opinion worth price charged.

[Magum Opus]

Thank for the tip on the Deflectolator. I tried to figure it out from other sites, but had no success. I am not sure of the type of wood. It is dark, almost reddish. Anyway they are true 2" x 8" (those are the real measurements). I have supports spaced 7' apart under them. And I have had new piers added under the kitchen a few years ago. The Deflectolator said L/720. Some good news!

So I take it I do not need to add joists nor sister them. I do have to add 2"x8" cross beams so the outside edges of the new subfloor aren't floating.

Because the floor is not level in one corner, due to 100 years of piers settling. Do I shim under the subfloor? Or add more mortar under the tile in that area?

I'm not looking forward to this and once I start there is no going back. I'm sure I will ask why I got myself into this, as I do every time I lay tile ("I'm never going to do this again".

[cx]

Again, Douglas, some photos or drawings might be very helpful here.

With full dimension 2x8s of any species spaced on 24" centers and spanning only 7 feet you should be getting design deflection numbers a lot higher than that. If, in fact, that's what you've actually got.

And I don't understand the unsupported subfloor part of your comments. Again, that's why the the photos would be helpful.

Your subfloor does not need to be level, it needs only be flat.

My opinion; worth price charged.

[Magum Opus]

You can see from the pic below the new piers are holding a beam. It is located 7' from the edge of the house. The kitchen wall goes 2.25' beyond that. The next beam is about 4'. Don't I need to have an edge to the room for the outsides edges of the subfloor. A beam that goes in between the joists? Otherwise where will the furring strips supporting the hardwood floors connect? They will just hang in the air. You can see they run at an angle.

Is there anything wrong with me using 23/32 tongue and groove plywood for the subfloor and 1/4 cementboard? The tile is 5/8" thick.

The old electric wires are still there.

[cx]

Yes, I see the ceramic "knobs" for your previous wiring. House definitely has some years on it, but all appears in good condition.

What I do not see is the furring to which you refer. The diagonal subfloor boards we see in the photo are the finished flooring above?

And yes, for whatever subflooring changes you might make you'd need blocking between joists where you've cut out old subflooring to install new. But first I'd like to know why we're removing the subflooring we see in the photo?

But no, you cannot install a single layer of nominal 3/4" plywood over 24" joist centers and install 1/4" CBU over that for a ceramic tile installation. The CBU manufacturer's are gonna require joist spacing no wider than 16" on center.

My opinion; worth price charged.

[Magum Opus]

Here you can see the boards with gaps between them that are running at an angle to the joists, those are the furring strips. The hardwood floor is on top of that.

So what should I use on top of the plywood subfloor before I tile?

[cx]

Yes, now I see it and I gotta admit I've never seen it before. Nor have I even heard of hardwood flooring done that way. We live, we learn.

Customarily we would recommend you remove the hardwood and install a layer of nominal 1/2" plywood where you planned your tile installation, but I don't think that's gonna work here. But customarily we'd see a sawn board subfloor where you have only about 2/3rds of a sawn board subfloor, which you're more correctly referring to as furring strips.

If you remove the hardwood flooring, you're correct that you'll need to provide support for your existing furring strips and hardwood unless the cut is made over a joist top. But with your access from below I think it would be easy enough to provide the necessary support wherever you need it.

Does your new tile installation directly meet the old hardwood installation at some point?

Your new minimum nominal 3/4" plywood subfloor would also, of course, be supported at all its edges.

One company, Schluter Systems, makes a tile substrate called Ditra XL, which they indicate for ceramic tile installations over single layer subflooring over joists with 24" spacing. I wouldn't do it, but they say it works. And your Encaustic tiles are not of the same known structure as are ceramic tiles and that might not be in your favor. But that installation style would keep your new tile only a little higher than your existing hardwood. I think.

I would customarily recommend a second layer of at least 1/2" plywood subflooring, then your tiling substrate and tile. With your thick tiles, that would give you a more pronounced transition to your existing flooring, though.

Hang on and maybe we'll get some more enlightened opinions on this.

My opinion; worth price charged.

[Magum Opus]

Nothing is ever easy. I just cleared out my kitchen, so now I can remove the floor. I have yet to determine from above where my joists sit. I'm sure there is one at the long ends (one is the end of the house, the other is a wall that is most likely structural). This only pertains to WALL 1 (see diagram) - I have to cut the furring strips at the joists on the kitchen side. The strips run continuously from one corner of the house to the other. The walls sit on top of them. At WALL 1, where I cut the furring strips even with the joists I will have to add something for the plywood to sit on and this is a problem because on the side of the joist opposite my kitchen there is a partial sister (because the joist is cracked and on the side of my kitchen, where I would add a joist there is a termite eaten beam that runs at an angle against the joist. What should I use here and how should I attach it with that angled piece of wood in my way. I will have to add a piece between each joist at WALL 2 (see diagram) so the plywood sits on an edge. No problem. Unfortunately, I have to deal with WALL 1 first.

The rest should be straight forward. see pics in next post

[Magum Opus]

The ruler in the pic is coming from my kitchen against the wall. No wonder my refrigerator sank in that corner. There is no support.