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former33t 10-11-2009 03:43 PM

2x9 deflection and tile
I know this sounds odd, but I just bought a house (year 2000 construction) that is using what appear to be 2x9 floor joists. They measure 9" exactly. I am curious if anyone knows where I can find deflection ratings for these joists since I want to install tile.

In one part of the house, they span 15' 3" on 12" centers. In another part, they span 11'6" on 16" centers. Is this sufficient for tile? I know I want to be in the L/360 area for ceramic or porcelain tile but I can't find the deflection ratings for a 2x9.

Both spans are covered with 3/4" T&G plywood. I intend to put down a layer of 1/2" plywood and 1/2" hardibacker (although I know neither do anything for deflection).

Anyone know if this will provide the desired results? Thanks.

Daniel Holzman 10-11-2009 07:25 PM

For a beam that is a full 2 inches wide and 9 inches deep, using an allowable stress of 1500 psi for the wood, the maximum deflection for a 50 psf floor load (40 psf floor plus 10 psf dead load) would be 0.3 inches, for a d/L of about 550, assuming the joists are spaced 12 inches OC with a 15'3" clear span.

For the 11'6" span at 16" OC, the deflection would be .14 inches, for a d/L of about 970.

Again, both of these cases assumed full 2 inch width, if your width is less the results need to be recomputed. Also, the 3/4 inch plywood above the joists will stiffen them up a little.

former33t 10-11-2009 08:50 PM

After reading your reply, I went and pulled a couple more ceiling tiles out of the basement to expose the joists and did some more measuring. I think these might be true 2x10's and the one I measured the first time was just a little cupped. That would lower the width to 1.5". Would I still be okay?

JazMan 10-11-2009 10:14 PM

You have 2x10's which measure 1.5x9.25. Is that right? Your joists system meets and surpasses minimum standards for ceramic & porcelain tiles and min. building specs of L360. You didn't mention their species or grade, but I'm sure you are better than L360.

The more important thing is to have enough plywood bridging the joists, and you already do, and plan to add more which never hurts. The extra 1/2" ply reduces deflection between the joists. You could use 1/4" CBU, or Ditra uncoupling membrane instead of the 1/2" CBU which does nothing but make the floor higher.

Make sure you don't buy the cheap "sheathing" plywood to use as an underlayment. I imagine you know how to install the new ply.


Daniel Holzman 10-11-2009 11:11 PM

For a 2x10 measuring 1.5 inches wide x 9.25 inches deep, at 1500 psi allowable stress, spanning 15.25 feet, spaced 12 inches OC, the d/L is about 450, so is OK. But are you sure the spacing is 12 inches, that is a bit unusual, I would have expected to see 16 inch OC.

former33t 10-12-2009 12:23 PM

Definitely 12" OC
I checked and the joists on the 15' span are definitely placed 12" OC. On the shorter span area (12') they are 16" OC.

What is wrong with using the sheathing quality plywood as underlayment? I've never heard that as a warning before. It says that it's rated for roofing and floors. If not that, then what?

Daniel Holzman 10-12-2009 12:56 PM

Sheathing quality plywood is fine as far as strength goes, only possible issue is how sensitive it is to moisture. If you use Ditramat under the tile, you have created an essentially impermeable barrier (assuming you tape the Ditra joints correctly), in which case there is no issue using sheathing quality plywood. If you do not use Ditra or equivalent impermeable barrier, just make sure that any subfloor is rated for moisture exposure.

JazMan 10-12-2009 08:17 PM

Where does it say it's OK for roofing and floors? If it's in an APA or manufacturer's brochure, then fine.

The cheap stuff is rated for "Sheathing" which will contain voids in the center plies and so should not be used on floors. Buy the ply rated for underlayment use. C/C or better, waterproof glue, or Exposure 1.

So, the joists are actually 1.5x9.25"?


former33t 10-13-2009 06:49 AM

Yeah, bad measurement the first time
I made a bad measurement the first time on the joists. The joist I picked to measure was just shy of 9" (like 8 15/16"). I have a drop ceiling the basement and the tile I popped out to measure the joists had some ductwork around it so I could only get to the one joist. I think that one was a little cupped. I've since pulled many more tiles and the sampling of others I've measured are correct (and 1.5" x 9.25").

Thanks for the heads up on the plywood. That makes sense, but I'm not sure that I've ever seen it before.

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