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


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08-08-2004, 07:37 PM
What are the options to strengthen my floor for stone??

Deflection: I finally have the information to submit. 2x12 joists, 16" centers,
3/4 plywood overlay is the existing floor (not tongue&groove). Span is 11 feet. Not sure if wood is Douglas Fir, house built in l960s.

While the above is fine for tile it is not strong enough for stone from what I have read. Would increasing the overlay to 1 1/4 inches be enough for travertine stone 18"x18"? Do I also need to sister the joists?? I do have a toilet drain that runs across the joists also. Increasing the overlay to 1 1/4"will make a huge difference in floor height from the existing bedroom with hardwood into bathroom. Best suggestions to minimize that.

What is Sturd-I-Floor plywood and is it something I can buy at the big box
store? Does it come in different thickness sizes??

When you also apply the Red Guard paint to the hardibacker for tub surround how long do you have to wait for it to dry. Do you do this before putting on the 2 inch tape at seams?? Or would you put it on after the seams have been

Thanks for your help donna

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08-08-2004, 07:45 PM
The deflectolator reports that your joists are plenty stiff along their length. The only concern is deflection between the joists. You'll need more subfloor, but I don't know the rules for that so wait for others to chime in. :)

Thank you for using the John Bridge Forums Deflect-O-Lator :-)

For joists that are Unknown wood, but in good condition, 11.25 inches tall, 1.5 inches wide, 16 inches on center, and 11 feet long between supports, the deflection calculated is 0.161 inches.

This translates to a deflection of L / 819.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile or Natural stone, Congratulations!

08-08-2004, 07:51 PM
Hubby just informed me that he is not sure if joists are 2 x 12 or
2 x 10. If they are 2 X 10 then the deflection is too much for
stone. HELP

Won't know the correct size until tomorrow and I think they are
2 X 10 because of a recessed light that we fitted in for basement
shower - -had to use one with smallest height.

08-08-2004, 08:19 PM
You could reduce the span to 9 feet by building a load bearing wall 2 feet or so from one end. You could make a nice closet out of the space too. You're only other choice is sistering the joists. :)

08-08-2004, 09:22 PM
Dave - - I for sure don't want a load bearing wall at 9 foot - - this remodel
is taking over my life!!!!

Oh - - wonderful - -hubby verified 2 X 12 joists.
OK - -now how to I do two levels of plywood to get the 1 1/4" down in depth??
How can I reduce the transition?

Is there some special wood that will give the strength that is not so thick??
Is there anyway that 3/4" plywood and 1/4" hardibacker will work with stone????

08-09-2004, 05:51 AM
Sorry, Donna! There is no substitute for subfloor thickness. You either have to put 1/2" more plywood on top of your current subfloor, or under it. Judging from your comment on the light fixture, I'd say going underneath the subfloor and adding strips of plywood between the joists is not possible.

You can use a decoupling membrane (Ditra) to keep your floor thickness down. For Ditra, the new plywood has to be 3/8" thick (the 1/2" thickness listed above is for backerboard floors). Combined with the Ditra, your subfloor willbe about 1/2" high. Assuming your stone tiles are 3/8" thick, you new floor should be a little less than 1" higher than the hardwood floors. A tapered transistion strip, made of the same wood as the hardwood floors, will make the height difference less noticable.

08-09-2004, 08:21 AM
Thanks Bob for letting me know what has to be done.

Is Ditra a product I can get at a big box store or is it something I have to
special order. Is is glued down or what do I need to do to use it.

08-09-2004, 08:31 AM

You can get Ditra at, there is a link to them on this site, just click on the link to take you to the TYW store. From the store you will find a link to tile-experts. They have Ditra and Kerdi at very decent prices.

Home Depot should carry some of the Schluter products (mostly the trim pieces in my area) and can probably special order the Ditra if you care to go that route. I've ordered from tile-experts and wouldn't hesitate to do so again.

Check out Schluter's Site ( for information on how to install Ditra. It's pretty easy and sure beats lugging around cement backer board!