Prep 3/4 Plywood Subfloor For Tile??

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Old 02-17-08, 07:34 PM
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Prep 3/4 Plywood Subfloor For Tile??

Hi,

I'm putting in slate in my bathroom on the second floor. The previous owners had vinyl, I've removed it to find it the 3/4 plywood that isn't exactly level.

I was going to lay a lathe down and pour self leveler on it, but don't know if that is the best why to approach it?

If I do this will the thin set bond with the self leveler? I asked a guy at Home Depot. And he said to screw sheets of hardi-plank sheets onto the plywood and then lay the tile, BUT, he is also the guy who gave me skim-coat and said it was self leveler (luckily I was able to lay it down with a trowel and was able to spread it around pretty good. Any help would be greatly appreciated.
 
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Old 02-17-08, 08:02 PM
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We first need to determine if your current floor is suitable for slate. Since slate is a natural stone, it requires a much more robust subfloor.

You need to determine your floor joist size, your joist spacing (center to center) and it's unsupported span. When you have this information, you need to run a deflection calculation, which is found here:

http://www.johnbridge.com/vbulletin/deflecto.pl

It will tell you if your floor is suitable or not.

Slate will need at LEAST a 1" (two layers of 1/2")exterior grade plywood with a hardiebacker or membrane like Ditra.

This would be recommended installation.

If your budget cannot permit this, then there are other methods.

You do not add SLC under cement backboard. You will thinset and screw your backerboard onto your existing subfloor, then pour slc then tile.

I will move this thread to the flooring section of the forum.
 
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Old 02-18-08, 08:03 AM
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Slate will need at LEAST a 1" (two layers of 1/2")exterior grade plywood with a hardiebacker or membrane like Ditra.
Lets get something straight here. 1/2" plywood for a subfloor is not acceptable anywhere. Anyplace where you see a 1/2" subfloor, you'll notice that it was done by someone without a clue. Yes natural stone does need 2 layers of plywood, but not 1/2" subfloor, never ever 1/2" subfloor. 5/8" minimum t&g subfloor (more is better) and then another 5/8" underlayment (some will tell you 1/2" is ok for the 2nd layer).

You do need to find out if your joists will support a natural stone installation, most need some beefing up for natural stone.

How unlevel is the floor? It's more important that the floor be flat than level. Tile needs flat but not necessarily level.
 
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Old 02-18-08, 08:52 AM
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Care to explain? As I've been told, 1/4" is obviously unacceptable, but two layers of glued and screwed 1/2" ply is a no no?

I was giving the minimum standards.....
 
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Old 02-18-08, 10:27 AM
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Sure would, do some research. Minimum subfloor requirement for a tile installation is 5/8" minimum t&g this is a tcna requirement. 1/2" is not acceptable. Adding additional on top of 1/2" is like starting over. Adding 1/2" to 1/2" doesnt give you a floor equal in strength to 1", no way, no way, no way.

Additionally, its not recommended that you glue the underlayment to the subfloor. Research by people smarter than me and you has shown that just screwing the second layer to the first, avoiding joists is the best method. If you decide to glue, you have to know what you are doing. You need to use an exterior full spread wood glue and work fast to get it down and screwed before the glue dries. This is sometimes difficult to do and not a good idea for a diyer.

There are 2 deflection concerns with ceramic and natural stone. One is deflection along the length of the joists and the second is deflection between the joists. You need 1 1/4" of plywood for stone, not 1" when joists are placed 16" oc. This is to meet deflection requirements between the joists.
 
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Old 02-18-08, 10:58 AM
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Thanks Johnny! I wasn't questioning what you were saying, just getting edumacated.........
 
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Old 02-18-08, 11:18 AM
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Thanks Johnny! I wasn't questioning what you were saying, just getting edumacated.........
Understood. I just makes me crazy sometimes, cause the diyers that come here sometimes read the answers go ahead and do it and never come back here. Don't want em doin the wrong thing.

We can all learn alot from this board and others.
 
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Old 02-19-08, 05:50 PM
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Hi,

Thanks for all the responses. I just measured the joists and put them into the calculator you provided. (THANK YOU). My joist are
10 ft long,
1.5 inches wide,
and about 14 to 14.5 inches apart.

according to the calculator I am ok to use the slate.

With all the replies I'm starting to get a little confused. The plywood is 3/4. When you say (t/g) i'm assuming you mean tongue and groove, which my floor is not.

From what I'm understanding I should put down and additional sheet of PLYWOOD (not hardi-plank)? And it has to be at least??? 1/2 inch or 3/4?? And I'm assuming i'm going to lay it down opposite way of the current subfloor? Glue and screw? Or just screw?

My other question that is still unclear is do I need to put down something so the thinset can stick to the plywood (which I thought was the purpose of the hardi plank, which I'm assuming is the same as the wallboard?)

My floor is pretty level, but not perfect. Do I need to level it with self leveler? Does that go down before the second sheet of plywood? and if it's not that unlevel do I need to use lathe under the self-leverer? And before I probably piss y'all off if the self lever does go over the second sheet of plywood or hardiplank, will the thinset stick to the self-leveler?

Again thank you for your response and help. To give you a little background it's totally open at the moment. (the tub/toilet/sink have been removed and the sheetrock and vinyl taking up. The downstairs is also the same way. This was a old house and the toilet was leaking. I had my uncle come in and they cut the pipes from underneath and added some additional joists to help support a wall that didn't have proper support under it. The pipes have been reinstalled and I'm trying to get the restroom done now, so then I can check for leaks before the downstairs drywall goes up. Thank you.
 
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Old 02-19-08, 06:08 PM
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Add another layer of 5/8" exterior ply to your 3/4 using screws.

Once that is layed, install your hardiebacker board (1/4") or a Ditra type underlayment.

The SLC (Self leveling) will go over the hardiebacker, then you thinset and tile.
Wallboard and Hardiebacker are NOT the same. Wallboard/drywall is a gypsum product, and hardiebacker is a cementacious backer unit which contains no gypsum.

I forgot to add that you thinset AND screw the hardibacker down.
 
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Old 02-20-08, 07:31 AM
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Bull

You can add 1/2" of plywood over what you have. The plywood gets installed perpendicular to the joists, same as the subfloor. Avoid having a plywood seam on the second layer lining up with the subfloor plywood seams. The second layer of plywood should be bc grade or better exposure 1(exterior glue)plywood. Just screws, no glue is necessary. Make sure your 3/4" plywood subfloor is attached solid to the joists. Add screws/nails if needed. The second layer of plywood should be screwed to the first layer of plywood, not the joists. Use 1 1/2" deck screws for this. The screw pattern for the plywood should be every 6" around the perimeter of the sheet and every 8" in the field. Leave a 1/8" gap between plywood sheets. Use a cheap flexible caulk to fill the gap in the plywood sheets. Idea here is the caulk will allow the plywood to move a little, which it will do.

The plywood gives you the added stiffness between the joists thats needed but is not a good bonding surface for the stone. You need to add a 1/4" cement board or an isolation membrane over the plywood. My personal choice here would be Schluter Ditra. It'll only add about 1/8" of height to the floor, and is much easier to work with than the cement board. Ditra gets bonded to the plywood floor with a modified thinset. Your slate gets set over the ditra with an unmodified thinset. Cement board will work fine also if you prefer to use it. Cement board will just add a little more height to the floor.

As to leveling, the floor needs to be flat but not necessarily level. 1/4" out in 10' is accepable. If you need to do some minor work to get the floor within that tolerance, for ditra you need to do that over the plywood before ditra goes down. For cement board you would do it over the cement board. Use a cement based patching compound for this.
 
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Old 02-20-08, 09:26 PM
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Originally Posted by HeresJohnny View Post
Bull

You can add 1/2" of plywood over what you have. The plywood gets installed perpendicular to the joists, same as the subfloor. Avoid having a plywood seam on the second layer lining up with the subfloor plywood seams. The second layer of plywood should be bc grade or better exposure 1(exterior glue)plywood. Just screws, no glue is necessary. Make sure your 3/4" plywood subfloor is attached solid to the joists. Add screws/nails if needed. The second layer of plywood should be screwed to the first layer of plywood, not the joists. Use 1 1/2" deck screws for this. The screw pattern for the plywood should be every 6" around the perimeter of the sheet and every 8" in the field. Leave a 1/8" gap between plywood sheets. Use a cheap flexible caulk to fill the gap in the plywood sheets. Idea here is the caulk will allow the plywood to move a little, which it will do.

The plywood gives you the added stiffness between the joists thats needed but is not a good bonding surface for the stone. You need to add a 1/4" cement board or an isolation membrane over the plywood. My personal choice here would be Schluter Ditra. It'll only add about 1/8" of height to the floor, and is much easier to work with than the cement board. Ditra gets bonded to the plywood floor with a modified thinset. Your slate gets set over the ditra with an unmodified thinset. Cement board will work fine also if you prefer to use it. Cement board will just add a little more height to the floor.

As to leveling, the floor needs to be flat but not necessarily level. 1/4" out in 10' is accepable. If you need to do some minor work to get the floor within that tolerance, for ditra you need to do that over the plywood before ditra goes down. For cement board you would do it over the cement board. Use a cement based patching compound for this.

Thanks again for y'alls input, I do have one more question you put I need to put a 1/2 inch sheet of plywood, another post put 5/8? Does that mean either or?
 
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Old 02-21-08, 07:07 AM
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Bull - more plywood is always better but 1/2" is sufficient for your situation. In my opinion, 5/8" is not needed. If it gives you more comfort, then go with it.

But I'll say it again. You dont need 5/8".
 
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Old 06-16-08, 03:27 PM
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How to determine whether floor is flat

While searching this website I found the comments from HeresJohnny. Is there an easy way to tell whether the floor is flat?

What I have done so far is take a 1/5' level and lay it across the width of the floor. The floor is not quite level; the bubble is off center slightly. 1/8 - 3/32" spacer under it centers the bubble. As I move across the width of the floor, this seems to be a consistent difference.

So it looks like it is not level, but is flat. There must be a better way of doing this though.

In the other bathroom, it is level and flat for ~3', then seems to drop off by 1/8" in the last 1' of surface area. Is this acceptable for tiling, or do I need to somehow use a self leveling compound for this 1/8"?

Thanks, a newbie to tiling.
 
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Old 06-16-08, 04:19 PM
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I take a 5' level or a true 2x4 and check for flatness.

Tile needs a FLAT floor, but the floor can slope, which sounds like you have.
 
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Old 06-17-08, 11:54 AM
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Tolerance should be no more than 1/4" in 10' and 1/16" in 1'. If you are out of plane by more than that you need to do something to get the floor flat before you set tile.
 
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