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Unread 08-21-2007, 08:32 AM   #1
rowiga
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Question Floor Integrity

My wife and I bought an old farm house built in the 1840's. The kitchen floor was built out of six inch diameter trees covered by tongue and groove planking. When you walked on it you felt as though you were on a trampoline. We took the floor out and replaced it with 3/4" tongue and groove plywood over 2 x 10 treated joists on 16" centers. We decided to use treated joists because the floor is over a dirt crawlspace. We were going to put vinyl on it then changed our mind to a floating laminate floor and now we've finally decided to use ceramic tile. The span we're going over is 14 feet and according to the Deflecto result is inadequate for tile (using the unknown wood setting). We want to beef up the floor from underneath rather than add any more height to the existing floor. I was reading that merely adding 2x4's between the joists (blocking fashion) every 24" will do the trick. I was also wondering if I could use two sistered 2x10's between the joists (blocking fashion) down the center of the span to stiffen up the floor. What do you think?

One other question. The plywood floor is painted with an exterior latex (we couldn't afford flooring at the time). Is that going to have an impact on tiling?
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Unread 08-21-2007, 08:48 AM   #2
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Don't know where you are, Bob, but in my part of the country (S. Texas) the treated wood we get is SYP.

The biggest problem with it is shrinkage and warpage, of which you will get a lot. That's why we don't recommend it for use around tile instalaltions as a rule.

And if your crawl space is adequately drained and ventilated and has the requisite vapor retarder over the ground, you really don't need any kind of treated wood down under there at all. And if those conditions are not met, you'll have problems the treated wood won't address anyway.

But that ain't what you called about, is it?

None of the joist blocking you mentioned will do anything at all for your joist deflection. If you're concerned about that, you'll need to add actual supports under the joists. But if you can get an accurate measurement of the unsupported span of those joists, and it's not too much over 14', you may well find a span table that says they meed L/360 deflection standards. Our deflectometer is pretty conservative, and I like it that way. But you might be technically OK with the joists you have. If not, adding posts and beam under there is your only remedy aside from actual sistering.

Your subflooring is another matter. It also meets specs for some tiling substrate manufacturers, but it's still pretty minimal. And that's the area of deflection that causes the most tile installation problems.

My opinion; worth price charged.
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Unread 08-22-2007, 12:26 PM   #3
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We used the treated joists based on the recommendations of the contractor who was helping us at the time. He said it would be a good idea over a dirt floor. The area under the floor is dry but the adjoining basement area has a few wet spots.

I measured the actual unsupported span this morning and found it to be 13'4" which calculates to an acceptable number using the Deflecto (using SYP). I can hear the floor squeak in two spots but the use of screws in the area seems a bit sparce to me but then again my wife always said that I was screw crazy since I got the new cordless.

Does the fact that the plywood is painted cause any problems with the tiling process?
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Unread 08-22-2007, 06:20 PM   #4
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When you use cbu, you install it with a layer of thinset then screw it down. The thinset is there to fill voids and minor imperfections to hold the cbu up, so you should be okay - the screws actually hold it in place. With the paint, you couldn't use a membrane, since then, the thinset would have to actually stick, and it would not be guaranteed. If you can afford a little extra height, consider using another layer of plywood and then a membrane rather than the cbu before you tile it.
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Unread 08-23-2007, 06:12 AM   #5
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I'm completely new to tile work so I don't understand "CBU" and "membrane". Could you tell me what these are?
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Unread 08-23-2007, 07:15 AM   #6
Brian in San Diego
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Bob,

CBU is cementious backer unit. All of the backer boards are usually lumped into this category, Wonderboard, Durock, Fiberock and Hardiebacker to name a few. Membrane is a flexible covering generally used to "uncouple" the tile installation from the subfloor. The one most often referred to on this site is Ditra by Schluter Systems. Membranes are also made by Custom and Noble to name two.

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Unread 08-23-2007, 10:05 AM   #7
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I appreciate all of the help. I see that hardibacker is sold in 1/4" and 1/2" sheets. It seems that most people use the 1/4" thickness...at least that's what HD stocks the most of Is there a "standard"? I suppose using the 1/2" sheet would be better if you can afford the additional height. We're trying to keep the floor level as low as possible since we hadn't originally planned for tile and have other rooms up to grade already that meet with the room we want to tile.
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Unread 08-23-2007, 10:11 AM   #8
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Bob,

The 1/2" is not any better for a floor. The 1/2" is used for walls because it is stiffer.

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Unread 08-23-2007, 11:03 AM   #9
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Thanks Brian. We want to actually tile two adjoining rooms. The kitchen has a plywood floor and the den has a cement slab. How do I get the finished height of the tile on the cement slab (also painted with exterior latex) to the same height in the kitchen? If you say a mud job, I don't know what that is
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Unread 08-23-2007, 12:26 PM   #10
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The thinnest material you could put down over plywood is a membrane. I think Ditra would come in at between 1/8 and 3/16". Keeping tiled rooms exactly at the same height as adjoining rooms is a tall order. Usually that is handled by a "transition"...a piece of wood or other material to transition between the differing floor heights. Schluter makes some transition pieces as do most of the flooring manufacturer's like Bruce, Pergo, etc.

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Unread 08-23-2007, 12:45 PM   #11
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Right now the two floor levels are exactly the same and we're using the same ceramic tile in both rooms but I'll be adding the hardibacker and such to the plywood floor. I'm assuming you don't use the hardibacker on cement since it has to be screwed down so I'll have to make up the height difference of hardibacker and mortar on the cement floor with something. Also remember that the cement floor is painted.
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Unread 08-23-2007, 04:55 PM   #12
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You may need to remove the paint. The best way to do that is probably to rent a scarcifier - sort of a big grinder, to take the top layer off. If you have any high spots, it can grind them down, too.

If your plywood portion of the area to be tiled is sturdy enough to tile, and is the same exact height as the cement slab, then maybe using a membrane over the whole area would allow you to keep the height the same. My guess is that you would still want to make an expansion joint over where the two floor types meet. This can be a joint you use caulk in instead of grout, or an expansion joint installed while tiling (www.schluter.com makes some as do others). You need to check the subfloor area by using the tool "Deflecto" in the blue bar above.
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