slate floor in bathroom


Old 03-23-13, 06:23 PM
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slate floor in bathroom

I am getting ready to install a slate floor in my bathroom which is 13' x 9'. The slate will cover 9' x 9', a shower will be on the outside wall and will be 9' x 4'. I have 3/4" plywood sub floor, 9 1/4" x 1.5" joist that are 16" on center. Please advise on floor prep, floor support and cemente board vs kerdi and any other sugestions. Thank you in advance.
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Old 03-25-13, 06:49 AM
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Welcome to the forums. I moved your question into its own thread, you'll get better visibility this way than tacked on to the bottom of an old thread.
Old 03-25-13, 08:08 AM
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Welcome, boatguy. What is the total unsupported span of the joists transversing the bathroom area? That's the only criterion you left out. So far it looks good. Slate may be rough on the feet, so be ready for that. It, too, can fleck off if it is true slate. You aren't using this as a floor to your shower, are you? May be difficult to make it transition to the drain.

As far as floor prep, use modified thinset as a layer under your concrete backer underlayment. I like 1/4" hardie backer, and their associated cbu screws in 1 1/4" or longer. With the cbu in place and fastened down, you can lay in your slate.
Old 04-01-13, 09:52 AM
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If you are using natural slate, you'll need and additional 3/8" of ply and Ditra. If it is that porcelain slate that Home Depot always has on sale, 3/4" sub floor is sufficient for Dtira or Wonderboard or Hardi. I personally, would go with the porcelain. It can take a lot more flex than natural stone. If using boards, make sure to use those fancy Backer-Board screws. I have used regular flooring screws with good results but I looked close and noticed that the larger heads of the Backer-Board screws pull the board down much better. If using Wonderboard, make sure that your drill battery has enough torque to drive the screws in without stripping the heads. One time, using my 12v Dewalt, I couldn't get the screws to sink and and my plug-in hammer drill just stripped the heads off without sinking the screws. I got a new battery and, problem solved. Also, I find Wonderboard is too heavy and fragile but it comes in 1/4" so your floor transition to the hallway will be less noticeable. Shluter-Ditra is a definite if it is real slate. Not sure if 16" on center is approved by the tile counsel but I have installed crema marfil with 16" centers using Ditra; no problems. For Ditra you'll need both modified and unmodified mortar. Oh yeah, and make sure to put 1 3/4 flooring screws into the 3/4" sub floor and to the joists to make the floor more rigid. You can download the Ditra installation handbook here:

Schluter-DITRA - Schluter-Systems
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