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Old 08-24-2007, 11:38 AM   #1
spta97
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Newbie with a lots of subfloor questions

So glad I found this forum - I am hoping you can help me out with my kitchen tile job.

The eat in area of my kitchen is going to maintain the current installed hard wood floor, the area where the appliances / cabinets will be I plan to tile.

The weird thing about my situation is the joist support is not in the center of the room so they layout is like this:

Joists = Unknown wood in good condition, 7.25" high, 1.25" wide, 16" on center
Span #1 = 5.5 feet which yielded a deflection rating of L / 1804 (good stuff)
Span #2 = 12 feet which yeilded a deflection rating of L / 302 (not good)

The tile will not go on the whole 2nd span, but rather about 5.5 feet of it so the "cooking" area will be about 11" of tile in total with 5.5 feet of tile ending in the middle of the 2nd span.

What does this mean for my job? Should I sister / block the joists on the Span #2?

I currently have 3/4" 3.5" wood planks as the subfloor that I plan to replace (in the cooking area) with 3/4" plywood. On top of that I plan to use 1/4" Hardibacker then lay the tile.

Another question, I didn't get the T&G plywood because I feared the wax coating would cause problems with the morter bed for the backer board so I got this stuff. Is this ok?:

Georgia Pacific
Rated Sheething
48/24 23/32 INCH
Exposure 1
404
PRP-108-HUD-UM-40



Thanks!
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Old 08-24-2007, 04:05 PM   #2
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When using cbu, the thinset isn't really holding the cbu down, it is the nails or screws, so it would have been better. The hassle is, along the long sides of the ply, you don't get any benefit of them supporting each other so a big load in that area can cause it to deflect. You should be okay if you put in blocking so the edges are supported.

So, you're saying only about 11" is over that longer span is going to be tiled? Might risk it. You'd have more margin if you used Ditra verses the cbu. See what the pros have to say...
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Old 08-24-2007, 04:15 PM   #3
Brian in San Diego
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I read it differently than Jim. I read it that 5.5' of span 2 will be tiled. I think there might have been a typo and 11" was posted and it should have been 11'. Correct? If that is the case, I would sister the joists in that 5-6' area...or install a beam to shorten the span.

I don't think you want any plywood that has "sheathing" in the description. You need CC plugged or better for subfloor. Most of the big boxes sell BC plywood. You definitely want the exposure 1 (exterior glue)

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Old 08-24-2007, 05:13 PM   #4
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jadnashua - The tile will be 5.5 feet into the second 12 foot span. What is Ditra and CBU?

Thanks..
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Old 08-24-2007, 05:17 PM   #5
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Brian - you read it correctly. The layout is basically like this:


Wall------------Beam 1---------------------------Wall
||----------------||------------------------------||

Tile tile tile tile tile tile tile tile tile tile
Tile tile tile tile tile tile tile tile tile tile
Tile tile tile tile tile tile tile tile tile tile
Tile tile tile tile tile tile tile tile tile tile
Tile tile tile tile tile tile tile tile tile tile

My plywood says "Exposure 1" How can I tell if it is a CC rating? Should I go to a lumber yard?
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Old 08-24-2007, 05:32 PM   #6
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Hey, all this talk and no first name we can call you...spta97 doesn't roll of tongue if you know what I mean.

I know the big boxes will state the plywood is BC, CC or whatever, but you won't get that on sheathing. There are voids in sheathing making suitable for walls and roofs, but not for subfloors where tile is to be installed. I just checked Home Depot online and they carry BC plywood just make sure it's exterior glue. It should say that it is 23/32" (3/4" nominal) BCX. X denotes exterior glue.

I would sister the joists in the area of span 2 or could you install a beam under those existing joists, possibly mid-span?

CBU is Cementious Backer Unit...common brand names Durock, Fiberock, Wonderboard and Hardiebacker.
Ditra is a membrane made by Schluter Systems.

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Old 08-24-2007, 05:42 PM   #7
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Hey 97, how about a name?

We should always go to a real lumberyard, but those big box stores are so convenient, I go there too.

Certainly sistering and blocking the joists if possible is the right thing to do. How are you going to remove some of the hardwood subfloor without disturbing the part you wanna keep? How about under the base cabinet? Are you getting new cabinets? sometimes removing the subfloor makes matters worse,

The deflectorator is a bit conservitive so you might have a little slack? Another thing you can do is try to find someone that sell Hydroment products where you are. Some of their thinsets are guaranteed to work with ceramic even at L240. We always recommend L360 or better but perhaps some added insurance.....Hydroment is a good company, been make stickum stuff for a long time.

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Old 08-24-2007, 06:05 PM   #8
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Hey guys, the name is Tom - nice to meet you

Brian - Did you know what the numbers / letters indicated for the plywood I bought? I can return it if that is the right thing to do but getting to lumber yards is a pita because I work in NYC and leave and get home when they are closed.

If I were to sister the joists does it have to go the whole 12 feet from the beam to the wall or just over the area I want to tile? I would really like to stay away from putting an additional beam in the crawl space - it is not fun down there.

Jaz - I gutted the kitchen with the exception of the eating area hardwood floor so the subfloor that I want to replace is all exposed now.

I did the "jump test" on the existing floor (192 lbs.) and it is solid as a rock. I hired a contractor knock down the wall as part of this remodel and he said that I don't even have to replace the subfloor. He said in he did similar houses and he just used the laytex fortified thinset.

I want to do the right thing but don't want to create more work than I need to. So far it has been 6 months without my kitchen
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Old 08-24-2007, 06:42 PM   #9
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Welcome, Tom. Please go to the UserCP above, find Edit Signature and add that first name there so it will appear with each post for us and we won't hafta search for it.

That "sheathing" is made for roof and wall use and is almost certainly a CD grade. Not what you want for your floor. You want something rated BC or AC that indicates it has exterior glue or says Exposure 1 somewhere in the stamp.

For sistering, it's imperative that you sister at least the center two-thirds of the span, regardless where the tile installation is located.

I think sistering the longer spans will get you where you need to go.

My opinion; worth price charged.
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Old 08-24-2007, 07:14 PM   #10
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CX - I edited my sig, we'll see if it works.

In my first post I indicated that the plywood says "Exposure 1". Do the other numbers / letters negate it for a subfloor?

Regarding the sistering:

1) Which way should the crown face?
2) Do I need to allow the wood to completely dry out?
3) Should I use lag bolts with pre drilled holes?

Sorry for all the questions, sistering is new to me.
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Old 08-24-2007, 07:28 PM   #11
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Tom,

I can't answer all the sistering questions but I can tell you what negates the plywood you bought...the word sheathing. Here's a page from Georgia Pacific's FAQs. http://www.gp.com/build/product.aspx?pid=1400 You'll see they talk about sheathing for walls and roofs...not floors. They also talk about plywood sanded project panels...that's more in line with what you want. You want exterior glue and that will be standard in exposure 1 panels. Or if the sheets are designated ACX or BCX then you'll have exterior glue. My guess, like CX said earlier, is that you picked up some CDX sheathing..it is NO BUENO for a subfloor.

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Old 08-24-2007, 07:33 PM   #12
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Tom,

# 1. Crown? What crown? You'll buy straight ones.

# 2. Joists grade lumber is kiln dried, but store inside.

# 3. Yes lag bolts and washers. You can use 2 nails to temporarily hold them in place 'till you drill holes and bolt.

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Old 08-24-2007, 07:39 PM   #13
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What Brian said. Except I've never personally seen a sheet of plywood that was labled ACX or BCX or CDX. That's how we refer to them much of the time, but you ain't gonna see it onna grade stamp unless things have changed.

1. Always up.

2. Should be "dry" when you get it. Don't buy anything that doesn't indicate KD somewhere on it. That allegedly assures you it was shipped at less than 19% moisture content. Drier is better. If you've got time to buy it and keep it in the house for months, do so. Or use it when you get it home like most folks hafta.

3. You can. Some folks do. I do all my sistering and similar work with construction adhesive and decking screws. Yes, I pre-drill all the holes in one board, usually the sister, but not always.

Jaz is quick this evening.

My opinion; worth price charged.
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Old 08-24-2007, 07:42 PM   #14
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Brian - Thanks for the link. I don't get why it is so hard for them to label "Use this for tile subfloor"
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Old 08-24-2007, 07:46 PM   #15
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Thanks Jaz and CX. I have been searching fror a how to thread on sistering - does one exist?

I am trying to find specifics like what size bolts to use, how many, etc. Also, what type of wood to get.
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