Sub-floor: Do I Have Enough (for Slate). - Tiling, ceramics, marble - DIY Chatroom Home Improvement Forum

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Old 03-29-2011, 07:33 PM   #1
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Sub-floor: Do I have enough (for slate).

After a recent leak we're re-doing some flooring. Engineered wood was taken out and we were hoping to re-do it with slate, but I think our sub floor may be lacking.

We have 2x9 beams (for lack of a better term), 16 inches on center with about 11 1/2 feet un-supported span. However, the 2x9 are where its iffy. They're made up of three pieces, a 2x1.5 inch piece on the top and the bottom and a 1x6 piece of something (vertical) in between, no idea what it is.

I have a feeling that even with additional plywood etc we may not have a sturdy enough sub-floor for slate.



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Old 03-29-2011, 08:33 PM   #2
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It sounds like you have an I joist, which is an engineered joist. In order to determine if you can use slate, you need to know the stiffness of the I joist, which is essentially the L/D ratio (ratio of the deflection at the center point of the joist to the length of the joist under a specified load). If you can determine the manufacturer and joist model, you can either look up the stiffness on their website, or call them. If you cannot determine the manufacturer, it is possible to determine the stiffness of an I joist using some assumptions about the materials, but this is a bit tricky, and it is far better to get the value from the manufacturer.

Natural stone typically requires an L/D ratio of 720 or higher, or the stone is prone to cracking. Typical floor load is 40 pounds per square foot, plus dead load.


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Old 03-29-2011, 08:36 PM   #3
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Hi Kiwi,

Google I joists, then images, and see if anything looks familiar.

I sure that's what your got. Who's gonna do the installation?

You should contact the manufacturer of the joists, (trusses), to see what the deflection rating is in your case. I would like them rated for 50/20 or higher Live/dead load. Typically those trusses are designed to meet L480 at 40/10, not good enough for slate. You need to be at or above L720 @50/20 if at all possible. Otherwise with a nice porcelain tile that looks like slate if that's the look you want.

In the case of natural stone you'll also need a double layer plywood too.

TILE GUY - retired- TROY, MI - Method & Product suitability consulting.

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Old 03-30-2011, 09:39 AM   #4
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Thanks for the replies. I'll have to crawl back under the house again tonight and see if I can find a manufacturer. Even if the joists are suitable I'm not sure if we have the volume available to add another layer of ply, then backer then slate, so we may go and investigate some porcelain tile this weekend. Just wish the closest store wasn't an hour and half away!

Edit - I'm planning on doing the instal.
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