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


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10-04-2006, 02:17 PM
I am about to tile my master bathroom on the 2nd floor. The bathroom is above the garage so the beams running under the floor are 2 2x4 laid flat with other 2x4 running in triangles between them. I think the name for these are either trusses or i-beams. not sure. They are spaced 24 inches apart and have 3/4 plywood, the kind that is notched and they fit together. Is this a suitable substrate for 12x12 travertine with 1/2 harder backer on top of the plywood.

Thanks a bunch in advance,


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10-04-2006, 02:31 PM
Welcome, Chris. :)

Short answer: No. :shake:

First you gotta tell the size and unsupported span of the joists (they are called web-trusses). And you really need to tell that to the manufacturer whose name is stamped somewhere on one of the trusses. They will be able to tell you the engineered deflection of the truss system. Our own Injineer Bob may be able to calculate it for you if you give him enough information, but the manufacturer is your first choice.

What you're looking for is a deflection less than L/720. They'll know exactly what that means and, unfortunately, are most likely to tell you they only designed for about L/480; quite good for ceramic tile, not good enough for natural stone.

If your joists are satisfactory, you still need to add another layer of plywood on top of what you have (what you have is called Tongue & Groove), then your CBU or uncoupling membrane. But first let's do the joists.

It's not a simple thing this natural stone over wood construction when done properly.

My opinion; worth price charged.

10-04-2006, 02:44 PM
Oh CX, I knew I shouldn't have asked this question. Not exactly the answer I wanted. So assuming the trusses are ok, what size of plywood should I put down? When I do my hardibacker, do I need to put it under the new kerdi shower I am putting in?

thanks again,

10-04-2006, 07:44 PM
If the joists span the width or length of a two-car garage, I'm willing to wager very large dinero they are not suitable for a natural stone installation.

With 24" centers, I would install a minimum of half-inch plywood over the existing. Install it according to these guidelines. (

Install the plywood in the area that's to be a shower, but not the CBU. You do not need the half-inch CBU, quarter-inch will serve the same purpose.

Then go out and find a nice porcelain tile that looks just like the travertine that your floor structure won't accommodate. :shades:

My opinion; worth price charged.

10-05-2006, 07:14 AM
So your saying that no matter how much plywood I put down, there is no way my floor will support travertine?

10-05-2006, 07:37 AM
I'm saying your joist structure almost certainly doesn't meet specs for natural stone tiles, Chris, no matter how much plywood you install over what you have.

Can you install travertine anyway? Sure. Will it work? Maybe. Will it fail in a short time? Likely. Can I tell you when? No. Have I been wrong before? Yes.

But with a 20+ foot span of joists (you haven't given us that information yet), you're pushing your luck.

You buy Lotto tickets? :)

My opinion; worth price charged.

10-05-2006, 08:59 AM
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Here is what my joist look like from the side, they are composed of 2x4s, the total height is about 2 feet. They are on 24" centers. They run about 20 feet across the span of the garage.

10-05-2006, 09:05 AM
Yep, I'm familiar with what you've got there, Chris. You need to get a manufacturer's name and call them. But first you must get accurate measurements, both of the size of the truss and the unsupported span.

It's important. :shades:

10-05-2006, 12:43 PM

Plugging these figures into deflecto here is what I get.

Joist Material: Douglas Fir
Joist Height: 16", Mine are actually 24"
Joist Width: 3.5" (flat side of 2x4)
Joist Spacing: 24"
Joist Length: 18 feet

Now, I know deflecto is only for dimensional lumber, but with these calculations, my floor has a deflection rating of at least L/1216. If we compared a truss of 18 feet to a piece of dimensional lumber 18 feet, the truss will deflect less over the same distance. Since my truss is actually 8" taller than the joist height that I plugged into deflecto, I am having a hard time understanding why my floor wouldn't at least have the L/1216 rating.

Thanks again for the help, it is much appreciated,


John Bridge
10-05-2006, 01:10 PM
Hi Chris, :)

The trusses are very often engineered at L/480. Our defleco is not geared for anything manufactured.

I think you're okay for ceramic tile if you add a layer of 3/8 plywood and Schluter Ditra.