newbie tiling kitchen floor, deflection and underlayment [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Tennessee Prospector
04-01-2011, 08:01 AM
I've been searching and and learning lots from this site. Thanks. But I'm swimming in information that leaves me confused.
I have a greatroom, 24'x32' supported by open web trusses 24" high and 32' long set 19.2"oc. There are two additional trusses mounted one third and two thirds of the way across the 24' width to add strength. The subfloor material is Advantech made by Huber. It's an OSB type sheet 23/32" thick, tounge and grooved. I'd asked the builder to provide support for tile or stone floor, whatever I decided on. Now I want to tile the kitchen end of this greatroom, 24'x13' and I'm trying to find deflection information. I have the engineer's drawings and data for the trusses but I can't decifer the information. The data is below. Is the floor system strong enough?

Also, do I need to add plywood? 1/4" CBU underlayment or something like Ditra? I want to mate up to a 3/4" solid wood floor for the remainder of the greatroom.

Thanks for any and all replies.

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04-01-2011, 11:39 PM
Hi Carl, welcome! I not sure bout the trusses but I do know that being on 19" centers most CBU manufacturers won't warrant it. You could use Ditra XL however if you are wanting to match 3/4" hardwood. If you are planning on natural stone you will need to add another layer of 1/2" AC or BC ply . I'll ping our Injuneer. :)

Tennessee Prospector
04-02-2011, 04:42 AM
Thanks Mike. So if I'm understanding this, the problem of 19" centers with one layer of subfloor is the deflection between the trusses? We plan on using a porcelain tile slate lookalike, feeling the real slate might be too much weight for the floor system to support.

04-02-2011, 06:44 AM
I can't see anything in those numbers that says what the deflection is.

Porcelain tile will be a better choice, both for your kitchen and for this floor's ability to carry them. The deflection requirements are 1/2 as stiff as for stone, both in the subfloor and the supporting trusses. I agree with Mike's suggestion for Ditra. As for the trusses, contact the manufacturer to verify they meet L/480 for combined live and dead loads.

Tennessee Prospector
04-03-2011, 07:16 AM
Thanks Bob,

I didn't see anything I'd recognize as deflection numbers from what I've learned on this site and others. I'll try calling the truss company.

If I understand what I've read about Ditra and Ditra XL they're both approved for use in my situation without the addition of a plywood underlayment.

Provided my trusses are built to L/480 standards am I on the right path for success?


04-03-2011, 09:36 AM
Welcome, Carl. :)

Yes, if your joist system is designed to L/480 deflection criterion, as is almost certainly the case with that long span, they'll be fine. But do verify that with the manufacturer.

For the subflooring, yes, you meet Schluter's minimum standards for ceramic tile over Ditra. Some of us would want a second layer of plywood with that joist spacing, but we're not the manufacturer of the substrate product, eh? :)

My opinion; worth price charged.

Tennessee Prospector
04-03-2011, 04:02 PM
Thanks CX,

I think I'm getting it. If all I want is minimum then here it is. But if I want it better than minimum I need more wood underneath. It looks like a 3/8" plywood underlayment, modified thinset, Ditra (the thinner original), unmodified thinset, and tile is a better way, right? And I'm close to the 3/4" target thickness of my wood floor.

Thanks again to everyone!