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Old 07-15-2009, 11:54 AM   #1
Ron
 
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replacing subfloor for tile upgrade


Hello all....

I'm new here, and glad to find folks who enjoy and are willing to discuss home ownership issues. I'm trying to be nearing retirement (if that exists anymore in the USA), and have basic tool skills, if given the proper guidance and supervision...

I'm beginning a major kitchen remodel with new counter tops and floor changes. The floor now has linolium squares which was installed over Luan plywood, which is over 3/4" fiberboard, which was layed over a thin (1/2 or 3/8" maybe) ply subfloor over Southern Yellow Pine joists. The Luan was put down because the contractor didn't realize that linoliun won't stick to fiberboard. Why he used fiberboard in a kitchen to begin with is anybody's guess.

We want to install a tile floor (porcelin probably), and I want to replace the fiberboard with plywood.

In removing the old subfloor, I'm planning to use a finesaw to make a vertical cut through the fiberboard at the baseboards, under cabinet toespaces and around an island in the center of the room.

Several questions come to mind AFTER the de-struct is done:

1. Do I need to somehow connect the new plywood to the under-plate/cabinet/island stub of fiberboard?

2. If I use Ditra to save weight, compared to USB, can I get away with laying 5/8" ply instead of 3/4" ply subfloor. I like the vapor barrior part of Ditra, and the reduced height of the finished floor, and, the de-coupling action that it attempts to provide from subfloor movement. If the old subfloor does have a 5/8" base (under the fiberboard), another 5/8" ply would give new subfloor of 1 1/4" to support the new tile. Is that thick enough?

3. I'm thinking that I should attach the new plywood with the old base layer with glue and screws, but am not sure what spacing I should use between the screws, and if they should be deck screws or drywall screws.

Am I on the right track here?

Any words of wisdom or heads up on potential gotchas will be welcome!

THX

Ron

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Old 07-15-2009, 01:07 PM   #2
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replacing subfloor for tile upgrade


There is no need to "marry" the two underlayments.

You should remove everything back down to the original subfloor and start over at that point. DITRA can be used over a minimum of 5/8" plywood subfloor.

You should also remove all the baseboard, it would be silly to do what you propose and isn't the way to do it. To say nothing of the extra work it would take.

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Old 07-15-2009, 01:34 PM   #3
Ron
 
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replacing subfloor for tile upgrade


thanks Bud.... it does make sense to pull the baseboards. I guess that will get me back to the wall plate framing.

The original subfloor had some "give" in it, as I recall, when you walked on it, so I'll plan on covering it with another 5/8" layer, and screw those together to give more support for the tile.

I appreciate the response.
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Old 07-15-2009, 02:35 PM   #4
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replacing subfloor for tile upgrade


The tile requires a 1/4" gap around the perimeter of the room for expansion. The baseboard is what covers that gap and keeps things looking correct.

Deflection ("give") in a floor structure can be a serious issue for a tile installation. There are minimum requirements the structure must meet before it can qualify for a tile installation. The addition of another layer of plywood may not correct the problem or make the structure acceptable.

What size are the floor joists?
What is their spacing?
What is the unsupported span of the floor joists?

With this information the approximate deflection value can be determined.
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Old 07-29-2009, 09:16 AM   #5
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replacing subfloor for tile upgrade


hi Bud...

The joists are southern Yellow Pine 2X10's on 16 inch centers. I'm not certain of the span in that part of the house, but I think it's about 12 feet. I'll need to have some crawl space fun to confirm the span.

I've had four estimates from installers. Two have said that I should pull the pressboard and go back with 5/8 or 3/4 plywood. Two have said that's not necessary and that backer board over the 3/4 pressboard will be ok.

One of the two who said not to remove the pressboard, said that I should use 1/2" backerboard to add strength. My understanding that backer board does not add strength.

any thoughts on that?

Ron
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Old 07-29-2009, 04:35 PM   #6
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replacing subfloor for tile upgrade


Quote:
The joists are southern Yellow Pine 2X10's on 16 inch centers. I'm not certain of the span in that part of the house, but I think it's about 12 feet. I'll need to have some crawl space fun to confirm the span.
OK, without knowing the unsupported span anything is a roll of the dice but I can tell you that at twelve feet of span with this criteria you should be within the deflection requirement but you may be right on the allowable edge.

Quote:
I've had four estimates from installers. Two have said that I should pull the pressboard and go back with 5/8 or 3/4 plywood. Two have said that's not necessary and that backer board over the 3/4 pressboard will be ok.
This depends on what "press-board" is in this case. There are basically two types of product that one may call "press-board".

One type is made of small wood particles and saw dust known as partical board. Partical board is bad news and is never to be anywhere in a tile installation.

The second type is made of small wood shavings and chips. This is OSB (oriented stand board) and is acceptable as long as you don't try to apply tile directly to the OSB. It requires a tilebacker of some description.

Quote:
One of the two who said not to remove the press-board, said that I should use 1/2" backerboard to add strength. My understanding that backer board does not add strength.
If the underlayment turns out to be partical board then those two guys should be sent down the road and never approached again. Anyone that does tile professionally and goes by the book knows partical board is forbidden.

Now the next thing is: Anyone that does tile professionally and goes by the book also knows that no cement board offers any structural strength what so ever, none of it. So that guy right there should be handed his walking-papers for sure.

Quote:
any thoughts on that?
Yup! And you now have them.

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