Deflection help [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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10-11-2006, 09:29 AM
Brand new construction. 2 inch by 14 inch I-joists ,16 inch on center. 3/4 inch Advan-Tec plywood . The joists clear span 23 feet. The home owner is wanting to put crema marfil mable(12x12) on diagonal.I usually put 1/4 inch pro-tec cbu ,thinsette and nailed down,but I need to know if I am safe with that. I thought about 1/2 inch pro-tec cbu,but am not sure.?I tried the deflecto-meter but it doesn't work for i-joists. HELP! What thickness of substrate is needed?Or should marble not be installed on this floor?

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10-11-2006, 09:40 AM
Well, Corby, I'm sure you know you must at least install a second layer of plywood over that subfloor for any natural stone installation before installing the substrate of choice.

I doubt very, very seriously that joist structure will meet the necessary L/720 deflection standards. But the only way to find out is to get the manufacturer's name and contact them. There's frequently a phone number on the joists or you can surely find it on their web site. Lotsa the websites are listed in our (Liberry).

My opinion; worth price charged.

John Bridge
10-11-2006, 05:01 PM
Hey Corby, :)

Crema marfil cracks when you only look at it. Don't be contracting to put it on a floor . . . any floor. Get them switched over to botticino or one of the breccias.

Steven Hauser
10-12-2006, 03:54 PM
Hi there,
Though the deflection for an I joist will be above the l/720 closer to l/1100 the deflection along the joists aren't what would hurt the installation.

The seams in the plywood would create the main issues for you. So add the appropriate second layer of plywood according to manufacturer specifications, then use an isolation matting like the Schluter Ditra or Noble CIS membrane.

John is correct about he Crema Marfil, but you do have to use what clients like though.

Good luck


John Bridge
10-12-2006, 04:06 PM
Steven, I don't have to do anything. I'm retard. :D

10-12-2006, 04:08 PM
Thanks for all your input. Still trying to grasp the deflection ratings with nature stone instalations. This floor is rated at over L/1100 ,just found out today with the bulding supply company.
Thanx again Chris Corbin

10-12-2006, 05:25 PM
Mmmm. 14" I joist with a 2" flange, 16" oc, spanning 23 feet and you're getting a deflection rating better than L/1100? Mmmm.

You still have the between-joist deflection to deal with, Corby, and you'll not get it with a single layer of 3/4" plywood even at 16" spacing. You'll also have the problem over the joists where there is a joint in that subflooring layer, as Steven is pointing out. You must have a double layer of plywood for any natural stone installation and it should always be installed following these guidelines. ( Some substrate manufacturers will allow a 3/8" second layer with your joist spacing, but I wouldn't install less than half-inch.

Read all of that article and perhaps it will clear up some of the doubt you expressed in your earlier PM. It explains the reasoning pretty well, I think.

You also get into another area of consideration with that long span you have where, even though you meet the deflection standards, the span is so long that the curvature of the floor with the allowable deflection begins to be a factor.

You're gonna need a really good subfloor installation and a real good decoupling membrane over it to have a good chance at a successful installation there.

Do tell the name and model number of those joists, please.

My opinion; worth price charged.

10-12-2006, 06:15 PM
CX-Boise 14"BCI(r)5000-1.8 SP. I was wrong on it being 23 feet, it is 20 feet 10 inch,sorry about the clear span measurement being wrong.I do not have a specific number stamped from the I -joist,that is the info from the top of the report. I could not get the" these guideline" to pull up on my computer that you are refering to in your last post. It will help me if I could access that information.

10-12-2006, 07:10 PM
Mmmm, dunno why the link won't work. Copy and paste this, , and see maybe won't that work.