deflection 4x6 post/beam with 2x6 T&G [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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05-27-2004, 10:28 AM
I have an 18 year old house with 4x6 beams spaced at 4'. The 4x6 posts are spaced at 5'. Laid 90 degrees to the beams is 2x6 tongue and groove planks. I have calculated deflection to be >1440 by assuming simply supported 2x6 spaced at 4'. I am going to install 18" travertine stone.

Does this sound correct?

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05-27-2004, 11:57 AM
Welcome to the forum!

I don't get your number, but I can slice this a couple of ways and get different anwers. The one I like is L/547. :D

To get that number, I took a nominal 2x6 ( 1.5 x5.5) and calculated the section properties laying on its side. Then I assumed 25 pounds per foot (1/2 of 50 pounds per square foot) and used the best wood properties for douglas fir or southern yellow pine (1.7E6 psi) and got L/801.

I then looked at a true 2x6, and got L/2119.

I then looked at a concentrated weight of 300 lbs and asssumed that 3 nominal planks will support it (the T&G distributes the load). Neglecting shear deflection (which the deflectolator and the above calculation include), I get L/547.

Looking at the above case with a full dimension 2x6, I get L/1416

I use the L/547 number since I don't know for sure that you have full dimension lumber and that with travertine, you can't be too carefull. Someday, you'll move something very heavy across the floor. By using the 300 lb load case, you'll be safe.

Don't despair! Your floor will still be fine. You have to add plywood over the planks anyway. Make it 3/4" thick, and your floor will make it to L/700-plus.

While I had your floor on the rack, I looked at the joists. I assumed that they are simply suported on 5 foot span, and support a 4 foot wide section of the floor. They are fine (L/835 or so)


Steven Hauser
05-28-2004, 02:26 AM
:D Welcome.

Looks like Bob got the deflection parameters set.

Do you have any other questions about the installation?

05-28-2004, 09:08 AM
I did not plan on installing plywood prior to hardibacker installation. Other than concentrated 300# loads, the numbers appear ok for stone. Comments????

05-28-2004, 09:24 AM
I had planned on using hardibacker 1/4" board and have just now become aware of ditra due to this site. Should I consider returning the hardibacker and using ditra instead? Again, I have the 2x6" t&g flooring over 4x6 post and beam construction and had not planned on installing plywood over the t&g. Thanks for all your help!!!

05-28-2004, 09:33 AM
You have to use something to isolate the expansion of the planks from the underlayment. The TCA, all the CBU manufacturers, and Schluter (Ditra) require plywood for this purpose. If you skip the plywood, I believe your floor will be doomed to failure sooner, rather than later.

I would use Ditra in this case, since the plywood thickness hadn't entered in to your plans and Ditra is only 1/8" thick.

Schluter is a little vague on the thickness of the plywood, so if you are thinking about using Ditra, give their tech support folks a call.


05-28-2004, 10:06 AM
I do have 3/8" particleboard now on top of the 2x6 t&g planks. Will this work to decouple the floor from the planks, or do I have to use plywood? How do I find a supplier in Sacramento CA???

05-28-2004, 10:14 AM
You can't leave the particle board in there, it'll swell badly, and that's bad news to tile. If you really have OSB, and it's stamped Exposure 1, then you can leave it, and install Ditra over it.

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