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Old 02-12-2005, 11:25 AM   #1
PH
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First time laying slate-need assistance

I have installed ceramic tile before, but this is the first time I will be installing slate so I need support from you guys to know I am going to do this correctly. I currently have vinyl over 1/4" ply which is on top of 3/4" OSB and supported by 9" I-joists, 19.2" oc. The area where slate is going has 9' of unsupported spans. I could not find any information on calculating deflection for I-joists (I think I need L/720 for slate-correct)? Can anyone please confirm my joists will support slate? I plan on tearing up the 1/4" ply and vinyl and installing 1/2" B-C Exterior grade ply over the OSB, screwing down 6" oc. I plan on installing Ditra over the 1/2" ply for isolation, support and waterproofing. From what I understand, I need to use modified thinset between the Ditra and the plywood and unmodified thinset between the Ditra and the slate. Does anyone have recommendations for brand of thinset to use? Is unmodified thinset strong enough to install slate to the Ditra? What is the purpose of the latex in modified thinset? Hopefully everything looks okay, thanks in advance for any advice.
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Old 02-12-2005, 12:36 PM   #2
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The first step will be to contact the manufacturer of the joists. Contact info for many of the can be found in the liberry. You want the max span for L/720 under a 20 lb load, not 10lb. Wait for BB on the second layer, but at 19.2, expect probably a 3/4 bcx. If you can stay off the floor for a while, slate is porous enough that you could get away wtih a modified thinset, since the slate will vary in thickness, you'll want a medium set mortar so you can vary the thickness more. The unmodifed is suggested when a very dense porcelain is used because it won't allow the mortar to dry out. Modifieds dry out (latex content) and cure (portland content) The latex adds to the bonding strength.
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Old 02-12-2005, 01:31 PM   #3
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Just to add more information about my joists, I have Boise Cascade BCI 40S.
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Old 02-12-2005, 01:45 PM   #4
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Well they're not in the liberry, but you can contact them by emailing ewpinfo@bc.com
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Old 02-12-2005, 02:18 PM   #5
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Thanks for the information.
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Old 02-13-2005, 08:33 AM   #6
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Hi Paul, Glad you made it.

Our chief engineer, Bob Campbell, is notorious for bugging out on weekends. Claims he has a family and a life off the boards.

I'll make sure he sees this thread.
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Old 02-13-2005, 11:34 PM   #7
PH
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Thanks John. One bit of miscalculation to add, I have one area in my slate project where three joists have an unsupported length of 11' 6". The rest of the area is 9' unsupported. I am hoping this won't throw the idea of slate out the window.
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Old 02-14-2005, 06:46 AM   #8
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Paul, your plans for installing 1/2" plywood and Ditra over your 9-1/2" I-joists is fine for slate. (Your maximum span of 11-6" on 19.2" centers is just shy of L/960.)
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Old 02-14-2005, 08:53 AM   #9
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Thanks Bob for the quick response. What are your thoughts on the modified vs. unmodified between the Ditra and the slate?
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Old 02-14-2005, 10:35 AM   #10
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Un-modified thinset or medium set mortars should be used, according to Schluter. I don't argue with the manufacturers about their products.
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Old 03-01-2005, 02:36 PM   #11
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I want to screw down the 3/4" OSB to the joists and then screw down 1/2" BCX plywood to the OSB. Can I use regular 1 5/8" Zinc plated general screws for the OSB to the joists or should I go up to 2"? I was planning on using 1 1/4" screws for the plywood to the OSB. Do they have to be galvanized deck screws or should they be backerboard screws? I would assume with the moisture from the thinset they should not be zinc plated.
Paul
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Old 03-01-2005, 03:00 PM   #12
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Personally Paul, I would use an exterior rated #8 deck screw (1 5/8"), OSB to the joists. Just a plain 'ol electro plated zinc don't offer much protection. Don't forget to use some pokey on the joists as well, I like a polyurethane construction adhesive, PL makes some good stuff. A 1 1/4" length for the ply to OSB is again what I would use.

You may get some different opinions on this....we have a little discussion going on, another thread, about screw length. Seems like some of us have different opinions, imagine that.
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Old 03-01-2005, 03:18 PM   #13
PH
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Thanks Mike. The OSB is already glued and nailed down to the joists, I just want to add a little extra security. The 1 1/4" for the ply to the OSB should be the same exterior rated #8 screws as well?

Paul
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Old 03-01-2005, 03:36 PM   #14
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Yep. I believe many of the builders around here would want to use #8's. Personally, I like the smaller #6's for underlayment because I get less deformation of the ply using a smaller head.

Then some guys will wanna glue the ply underlayment down, others don't. I don't even want to pewt a stick into that hornets nest, to glue or not-to-glue . If the OSB is laid rough side up like it should be, then I'd prolly glue. If smooth side up, well.....I really don't know how effective glue is sticking to that slick surface.

Buzzzzzz, that outta brind some of the carpenters out of the hive.
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