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Old 07-31-2011, 04:31 PM   #1
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Subfloor and underlayments for stone floor


I will be installing about 600 sq. feet of Vermont slate. I've done a good amount of this at our previous house (1000 sq. feet!). At the previous house most was on concrete with a smaller amount on 16" floor joists with 5/8" subfloor and 1/2" backer board.

Here the floor is 2 x 10 planks on posts and beams. It had been my plan to put down 5/8" T & G plywood over the planks then 1/4" backer board.

I have recently heard about Ditra and EasyMat underlayment mats. I could use 3/4" T & G plywood and either of these two mats.

The choice being a solid or a de-coupled under-surface.

I would be interested in hearing opinions on these choices.

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Old 07-31-2011, 05:01 PM   #2
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Subfloor and underlayments for stone floor


I forgot to mention that I will be putting 15lb building paper between the planks and the plywood.

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Old 07-31-2011, 05:08 PM   #3
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Subfloor and underlayments for stone floor


Hi,

If the subfloor is 2x10 planks, it's telling me your joists are probably around 48" apart. There is no spec that allows anything like that. However, we know it's been done with ceramic/porcelain on the left coast since many homes are built that way. I'd need more info about the construction of your subfloor system, including species and grades.

The subfloor part is much easier to deal with. For natural slate you need a double subfloor that is a least 1 1/8" thick, preferably a little thicker. You're OK there with 2x10 + 5/8 or 3/4 sub. No t&g required btw.

I highly recommend Ditra instead of concrete backer, good stuff well worth the extra cost. I haven't used EasyMat, and no plans to.

So get back about the framing details or any other questions.

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Old 07-31-2011, 06:14 PM   #4
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Subfloor and underlayments for stone floor


The Tile Institute recommends a minimum stiffness of L720 for natural stone. As Jazz indicated, it is possible to compute the stiffness of a floor given full details about the geometry of the joists, including wood species, spacing, clear span, and floor loading (you get the floor loading from code). In your case, given 2x10 planks, if they are adequately nailed to the joists, you may be able to take credit for additional stiffness beyond that of the joists. You get no credit for Hardibacker, and most engineers ignore plywood so far as stiffness of floor goes, however as Jazz indicated you need a minimum thickness of subfloor for reasons other than floor stiffness.
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Old 07-31-2011, 06:40 PM   #5
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Subfloor and underlayments for stone floor


Construction is D. Fir (left coast). 4 x 8 Joists are 4' o.c ("plank and beam construction"). 2 x 10 planks run perpendicular to joists (not diagonal). I am pulling up carpet (uck!) and particle board and renailing the planks to the joists.

I have done this on part of the floor, using 3/4" hardwood (not lick-n-stick plywood veneer), with the 5/8" T & G plywood over the planks. It makes for a real solid floor.

I really like the 5/8" t & g that I have been getting, real nice stuff. I think that they use a better grade of plywood for the t & g stuff.

If I use the Ditra, should I be concerned about moisture being trapped between the building paper and the Ditra. (I really need the building paper to isolate the crawlspace from the living quarters.)
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Old 07-31-2011, 09:55 PM   #6
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Subfloor and underlayments for stone floor


The paper isn't going to hurt anything in my opinion. Not sure how much it'll help either.

You should be more concerned about the stiffness of the joists and the 4' spacing. That floor system works with ceramic tiles, but natural stone is another matter. As Daniel already mentioned slate needs a much stiffer substrate. I have to idea what your deflection rating would be with your set up. Is it too late to switch to a nice porcelain? Otherwise you may need to stiffen those beams.

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Old 07-31-2011, 11:03 PM   #7
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Subfloor and underlayments for stone floor


The 15lb building paper is part of my attempt to seal this house. Mice and bugs have been a problem that I will not tolerate. The mice die under in the crawl space (and the walls) and the house doesn't smell so nice. Hence the vapor barrier. (You would not believe the huge holes we have already sealed between the living space and the crawl space.)

I really have a strong preference for real stone. Every piece is different. (Obviously, tile is easier to install and cut.) The slate is gauged domestic not the flaky soft stuff at Home Depot and Lowes. It seems to be much stronger. I also like that I order it directly from the quarry and it arrives on a pallet. This consists of squares and rectangles sized from 6 x 6 to 12 x 12. I really have a hard time with the high-heeled, over-dressed sales ladies at the tile places. It is such a closed industry with multiple pricing charts to favor "professionals."

So, before I go out to flirt with the sales babes at the local tile franchise, I'd be interested in any suggestions to stiffen the floor I have.

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