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Old 11-19-2008, 09:02 PM   #1
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Correct Sub Floor choice


Hello;

I ripped out my kitchen floor to the joists. The kitchen buts up to the dining room, which is two 1/2" sheets of ply wood with Konecto on top of it, so I figure this to be about 1 and 3/16" in total height off the joists.

I have the wafer board sturdy floor 3/4" in my kitchen right now. We talked about doing tile, but I really do not want the height to be much different then the Konecto.

I read a little bit about Ditra, and how it can be used when you don't have much height to work with. I also read it is very expensive.

I know I need to research the whole tile thing more. What I need to decide very soon, is if the 3/4" wafer strudy floor is a ok choice to put down or if there is a better material for a sub floor?

Thanks very much
Jamie

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Old 11-19-2008, 09:19 PM   #2
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Correct Sub Floor choice


Jamie,

What is wafer board sturdy floor 3/4"? Do you have a brand name for us? It's possible a 3/4" subfloor with Ditra over it may be good enough.

How about the joists system? What type and size? Spacing, and do you know the species and grade? What is the unsupported span of the joists?

Ditra. It's about double the cost of cement boards, but I wouldn't say it's "very expensive". They're two different types of products, hard to compare. I recommend Ditra in almost all floor installations.

Jaz

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Old 11-19-2008, 10:06 PM   #3
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Correct Sub Floor choice


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Jamie,

What is wafer board sturdy floor 3/4"? Do you have a brand name for us? It's possible a 3/4" subfloor with Ditra over it may be good enough.

How about the joists system? What type and size? Spacing, and do you know the species and grade? What is the unsupported span of the joists?

Ditra. It's about double the cost of cement boards, but I wouldn't say it's "very expensive". They're two different types of products, hard to compare. I recommend Ditra in almost all floor installations.

Jaz
Hello
Thanks for responding.

I call OSB wafer board, but maybe they are not the same thing. It is Georgia Pacfic OSB Sturd-I floor 3/4" T&G.

Spruce 16" o.c. 2x10", Longest span between bearing walls is 14'.

I am not positve on the grade of the floor joists. I will have to look again and see if I can find a grade stamp.

Thanks.
Jamie
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Old 11-20-2008, 08:40 PM   #4
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Correct Sub Floor choice


Wafer board is not OSB, OK, we've got that cleared up. That subfloor is a good standard base. If you add Ditra you should be fine for tile as far as the between the joists deflection goes, but barely. You wanna be real safe, add more plywood. minimum 3/8".

The joists themselves are a bit on the so-so side. I'm using Spruce-Pine-Fir, #2 as the lumber. The standard minimum code says these joists will meet L360 deflection up to a clear span of 15' 5". But that is at the standard 40/10 live load dead load usually used for residential. When tile is used as the flooring I recommend a more realistic 50/20 criteria. Now the max span is only 13' 0".

It will probably work but maybe you can do a few things to improve your chances. Like, build a supporting wall under the floor? Sister the joists, if not all the way, sister as much as possible. Another thing is to use a certain thinset that unlike all other brands which suggest a max deflection of L360, is guaranteed to work well even at L 240.

Jaz
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Old 11-21-2008, 10:20 AM   #5
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Correct Sub Floor choice


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Originally Posted by JazMan View Post
Wafer board is not OSB, OK, we've got that cleared up. That subfloor is a good standard base. If you add Ditra you should be fine for tile as far as the between the joists deflection goes, but barely. You wanna be real safe, add more plywood. minimum 3/8".

The joists themselves are a bit on the so-so side. I'm using Spruce-Pine-Fir, #2 as the lumber. The standard minimum code says these joists will meet L360 deflection up to a clear span of 15' 5". But that is at the standard 40/10 live load dead load usually used for residential. When tile is used as the flooring I recommend a more realistic 50/20 criteria. Now the max span is only 13' 0".

It will probably work but maybe you can do a few things to improve your chances. Like, build a supporting wall under the floor? Sister the joists, if not all the way, sister as much as possible. Another thing is to use a certain thinset that unlike all other brands which suggest a max deflection of L360, is guaranteed to work well even at L 240.

Jaz
Thank you for the information. There is a closet about 3 feet into the longest streach of the room, but I am not sure that it is very supportive, only a couple 2x4 headers over a very long opening in the basement below that part of the kitchen. There are really no other areas under there that I could build a wall, it is my laundry room that I just refinished in the basement.

When you say to sister the joists, do you just mean to add another joist next to the current joist and nail it in? So I would have 2 or 3 joists together?

Is the problem we run into that there ends up being deflection and the tiles can / ground can crack due to the deflection?

Thanks
Jamie
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Old 11-21-2008, 10:48 PM   #6
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Correct Sub Floor choice


Jamie,

That's exactly what sistering is, you preferably bolt a new joist to every joists in place. Sometimes you can't install the new one for the entire length. If so, try for the mid 2/3 of the span. You can also us a smaller joist if things get in the way. If you do sister the joists, you will feel a big improvement. I like to jack-up the center of the joists just a hair until the new one is in place.

Too much deflection will cause failure guaranteed. The first sign is usually hairline cracks in the grout. The good thing is that minimum code of L360 is usually OK with ceramic tile, but when the builder spans to the max, there is no room to allow for the extra weight.

Jaz

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