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Unread 06-24-2020, 11:45 AM   #1
Paul_A
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Subfloor Question

I am preparing to put tile down in a 5' x 5' bathroom and adjacent 5' x 7' laundry room. I intend to put 3/4" pressure-treated plywood over the subfloor and tile on top of that. This all sits on 2x10's, 16" on center, 12.5' total span.

-- Looking at some other threads, should I have tightened up the subfloor with screws instead of nails?

-- Is there any special way to fasten the plywood? I was going to screw it, but should I try to screw it down into the joists, or just into the pine subfloor? Or does it matter?

-- I have 3 4' x 8' sheets. Two pieces will do the 5x7 room, with one seam perpendicular to the joists. If I do the same in the bathroom, I'll have to cover a 1' x 5' space with two small pieces of plywood. Does it matter if I join those two pieces where the toilet sits (12" rough-in anyway).

As far as the rigidity of the floor, will any of this make a difference in such a small area?

Thanks,
Paul

Ps, thanks for the Liberry.
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Last edited by Paul_A; 06-24-2020 at 11:45 AM. Reason: Add'l comment
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Unread 06-24-2020, 12:15 PM   #2
cx
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Welcome, Paul.

Don't do that.

While there is a ceramic tile industry accepted method for tiling directly to plywood, your subfloor package is not going to qualify. And even if it did, I'd recommend you not tile directly to the plywood anyway. It can be done, but it's a bit tricky and unforgiving of any missteps. Your use of treated plywood would be your second misstep thus far. If you already have the treated plywood, find another use for it or return it if possible.

The requirement for tiling over a sawn board subfloor is that you install a minimum of 1/2" exterior glue plywood with no face of grade lower than C and that you fasten it only to the existing subfloor and not directly to the joist structure. The rationale is to provide a more stable surface for your tile substrate and tile. And the method presumes that your subfloor boards are perpendicular to the joists and of a T&G edge style, neither of which you have. It is generally accepted that you can do the same over your diagonal, square-edge boards (see my warranty information below), but some of us recommend the use of a little thicker plywood. I honestly don't think it's necessary, but especially with your cut-up subflooring it certainly won't hurt.

As for fastening, I prefer screws (and glue) for any part of a wood framed subfloor, but deformed shank nails are also acceptable.

In the Liberry you could have found a good article on how best to install the second layer of subflooring (which is incorrectly identified as an underlayment). You'll not need the information about where to place the end joints, but you might find the other information useful. I'd recommend screws for fastening the second layer.

On top of all that you'll need a tiling substrate. Either a CBU or similar or a tiling membrane will do. Lots of choices in those categories.

My opinion; worth price charged.
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Unread 06-24-2020, 01:01 PM   #3
jadnashua
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If you want more moisture resistance for your subfloor, you could use one of the products like Ditra on top of the plywood. When you seal the seams with the specified banding material, it becomes waterproof.

In a bathroom, that normally isn't necessary. Same is true in a laundry room, but there, you have the possibility of more of a flood. If there's no place for say a burst hose to drain in the laundry, it won't make much difference, as that water will just flow out into the hallway anyways. So, if you can during your remodel, if that's a concern, you might want to install a floor drain. It would need a trap primer so it doesn't dry out and let sewer gasses in, though, so that's a consideration as well.
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Unread 06-24-2020, 01:18 PM   #4
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I echo what CX said, don't do it. All your hard work and great intentions will be for naught if you do.
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Unread 06-24-2020, 02:55 PM   #5
Paul_A
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Okay. Definitely not what I was hoping to hear. The local tile place suggested just to go straight onto plywood, and I was glad to avoid the expensive uncoupling membrane (I think one roll is short a few square feet), or CBU, with the expensive fasteners.

So, do I understand that my choice is:

1. Leave the subfloor as it is, but add a layer of plywood and then either membrane or CBU? That's likely to make the floor pretty high. Or,

2. Replace the subfloor if I'm determined to go with tile straight onto plywood?

Thanks,
Paul
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Unread 06-24-2020, 04:41 PM   #6
jadnashua
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Industry standards for applying tile directly to plywood is with a TWO layer assembly...IOW, you'd need two layers of plywood if you wanted to tile directly to it. I haven't checked to see if it is one of the recommended methods when the first layer is planks then with the second being plywood. To keep the overall height down, a proper T&G ply at least 5/8" thick and having exposure 1 or EXT glue would be required as the first layer, then either cbu or a suitable membrane.

You also need a high-end thinset rated for that application, which tends to be expensive as well. A reliable bond directly to the plywood requires a fairly complex substance.
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Unread 06-24-2020, 05:19 PM   #7
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Quote:
Originally Posted by Jim
I haven't checked to see if it is one of the recommended methods when the first layer is planks then with the second being plywood.
Jim, Method F150 in the TCNA Handbook calls for the first layer to be a minimum of nominal 5/8ths" T&G plywood and the underlayment layer to be minimum nominal 1/2" plywood for a Residential rating. No provision is given for a sawn board first layer.

Paul, you could remove what you've got and start with a single layer of nominal 3/4" T&G plywood subflooring and use a membrane in lieu of a CBU and have a much lower overall installation than what you initially proposed. You could also install the minimum nominal 1/2" plywood over what you have and use a CBU and still be at the same final elevation as what you initially proposed.

Adding the half-inch plywood over what you've got would be a much better subfloor than the single layer new nominal 3/4" plywood subfloor.

My opinion; worth price charged.
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Unread 07-05-2020, 12:32 PM   #8
Paul_A
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Underlayment Layout

I have (5) sheets of Hardie Backer (3' x 5') to cover two rooms: Laundry room is 5' 4" x 7' 3" (plus a 10 doorway sticking out); the bathroom is 5' 4" x 5' 2". (Please see my sketch.)

Two sheets do the bathroom. Does it make a difference how those two are laid out? Either way I lay it out, I'm short, either 2" or 4". Probably better to fit in a 4" piece than 2"?

Three sheets will do the laundry room. I want to have one piece in the corner as shown, as that's where the washer will sit, and I've put in some extra 2x10 bracing underneath

I'd appreciate suggestions on how to cover the remaining floor.

Laundry room tiles are 12" x 24". Bathroom tiles are 8" x 8".

Thanks,

=====================

Thought this would be a separate thread, as subfloor is taken care of.

So, to update, I gave up on going straight to plywood. Instead, what I wil have is, layer-by-layer:

-- 2x10 joists 16" on center (long direction in the attachment, , ~12.5' span);
-- pine subfloor, as shown, but now screwed & sanded level;
-- 3/4" tongue-and-groove plywood, Exposure 1, not pressure treated, screwed every 8" into the joists with 3" screws (had to stretch what I had on hand) and every 12" in between joists with 1-5/8" screws;
--1/4" Versabond (grey);
-- 0.41" Hardie Backer;
-- 1/2" Mapei Ultraflex 2 (don't feel like exchanging it);
-- Tile

Please let me know if there is anything is wrong with the above.

It seems that I will have to fit a few odd pieces of Hardie Backer. I don't know the best way to go about that. In the bathroom, I can have either a 2" strip or a 4" strip. I presume I can get away with that if it's under a part of the floor that never receives any traffic or load to speak of, eg, under the vanity or behind the toilet. I have a few different ways I could go at fitting the big pieces relative to 1) the edges of the plywood, and 2) the joists (yup, read that paper).

So I have attached a couple of possible layouts and would appreciate any pertinent thoughts. I had two other people jump up and down on the floor with me, and I couldn't notice any flex.

Thanks,

=

Ps, Just noticed a mistake on the second drawing: Those pices of hardie Backer would extend right all the way to the wall.
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Last edited by Paul_A; 07-05-2020 at 06:19 PM. Reason: Mistake in one of the drawings
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