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Unread 01-28-2015, 10:41 AM   #1
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15/32 Subfloor - Okay?

Hi - My bathroom has 15/32 plywood with 16" joist centers and I am going to build a shower and wanted to install tile.

For the floor I was thinking was level the floor and install a concrete board over the subfloor then tile.

In the shower area I was going to double the subfloor then build the shower up from there.

Do you guys agree with this method? If not what do you suggest? My biggest concern is with the thickness of the plywood...

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Unread 01-28-2015, 10:56 AM   #2
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Welcome, DSH. If you actually go by those initials, pick a couple of your favorites for us to use, eh? Or a first name.

You are correct to be concerned about your subflooring, which is inadequate for either of the areas in your bathroom. Or any other room in your house as far as I'm concerned.

Not only is the plywood too thin even to meet most building codes, it does not have tongue & groove edges and is almost certainly not blocked from below at the unsupported joints. Kinda like starting with no subfloor at all, but you don't actually fall through the joist bays.

It's possible to make a usable floor of it if it's in near pristine condition and you can glue-laminate another layer of nominal half-inch plywood to the existing using a full spread of wood glue and a great gaggle of screws. Otherwise you're options are to remove what you have and start over, or to install a nominal 3/4" plywood subfloor over what you have as though the first layer doesn't exist.

My opinion; worth price charged.

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Unread 01-28-2015, 11:13 AM   #3
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Thanks CX - This house is full of surprises and border line code concerns.... If had the money I would just tear down and rebuild on the lot. That said...

I am blocking the unsupported joints. I have access from below - so I lucked out there. That said you are suggesting I put another 15/32 or sheet over top of it?

I had considered tearing up the subfloor and installing a new floor in the room my concern is support the outer edges. The plywood would over hang the joists by ~4"

So 2 choices

A) Tear it up

B) Install over the existing - Maybe easier?

If that is route to go do I still need a CMU over the ply?

I need to keep forward momentum on this project or I'll go insane I just want to do it as correctly as I can....

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Unread 01-28-2015, 06:59 PM   #4
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Ultimately, it is up to you how to move forward.

Most would recommend tearing up the 1/2" and putting down proper 3/4".

I'm sure you could find a way to support the 4" overhang if you have access from below.

If you elect not to tear up, gluing down another layer of 1/2" is an alternative. I'd recommend the pricier Titebond-III (TB-II is also excellent and while it will be cheaper, it does have slightly less open time and is not waterproof), roll it on with a low nap roller (paint it on) or the flat side of a trowel or a wide taping/drywall knife. I'd further recommend coating both sheets of ply with the glue and then use these Grip-Rite coated deck screws with the star drive at 1 1/4". You won't need a pilot hole and they are threaded 2/3s of the shank to help mitigate screw jacking. You'll pay more for these but they drive like a dream. Get a partner to help with all this and it'll go much smoother. You'll have to move it when laying out the glue to get the sheets married before the glue skins over.

Once you have that completed, it is up to you as to your choice of tile underlayment: Hardibacker, Ditra, Greenskin, etc.

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Unread 01-28-2015, 11:39 PM   #5
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If you elect to add a layer of 1/2" plywood, in addition to the glue, I would recommend you use the guidelines in this article to install it.

The top ten reasons to procrastinate:

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