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Old 08-28-2008, 03:43 PM   #1
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Best subfloor and material thickness


We want to put porcelain tile in our tiny bathroom (roughly 7' x 5 1/2'). We can see that what is currently there is the original yellow pine tongue and groove flooring, then plywood, then some tar-like vinyl layer, then tile. This all sits on 2 x 10 joists that span 13 feet, spaced 16" apart on center. We have ripped out the ceiling in the room below and will sister these joists, hopefully with 2x10s along the full length, to support a 300+ pound cast iron clawfoot tub. With the ceiling gone, we can see that there are some spots near each of the fixtures where the some of the original tongue and groove boards have been pulled out to make way for plumbing.

My first question is: Should we rip out everything including the yellow pine hardwood and start fresh from the joists, or rip out just the tile and plywood and build on top of the yellow pine?

My next question is that I am reading a lot about people having two layers of plywood. Why is this? I have always understood that we should use a plywood subfloor, then a cement board (Hardibacker, Durcock) underlayment on thinset, and then the tile. Am I wrong? Please advise as to what thickness of the different materials I need depending on whether or not we rip out the hardwood and start fresh from the joists.

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Old 09-05-2008, 05:01 PM   #2
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Best subfloor and material thickness


Check out the John Bridge web site for tile. Ton's of good information.

Also, it seems that for most floor joist framing that has used solid 2 x 10's, 16" OC, if tile is to be laid, most tile people seem to want to have at least a 1-1/8" to 1-1/4" thick subfloor to ensure that the subfloor will not settle, or deflect, from the weight of the tile, especially if you are using natural stone tile. Two layers of 5/8" plywood will give you a good strong subfloor. Just run them all in the same perpendicular direction to the floor joists with the second, or top, plywood underlayment layer, offset from the firt layer so the joint do not line up. Generally it's a 1/2 width offset for long edges and two joist bsys for ends. The Thinset material that is often used to set the tile, is water based. The effect of all that water on top of an OSB subfloor can be disastrous, which is there is some resistance from a lot of tile people regarding the instlaation of tile over OSB subfloors, unless a plywood underlayment is installed over the top of the OSB subfloor. Hope that helps.

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Old 09-05-2008, 06:48 PM   #3
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Best subfloor and material thickness



Quote:
randallS1 (newbie)Check out the John Bridge web site for tile. Ton's of good information.






By all means - no one here knows anything about tile.
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Old 09-05-2008, 06:50 PM   #4
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randallS1, Why not fill out your profile so we can learn about you then sit back and relax for a few weeks until you learn the ropes of this website.
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Old 09-05-2008, 11:30 PM   #5
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Randall,

I know John, I don't think he needs rookies like you visiting other forums to direct people to his. He has a lot more class than that.

As for your attempt to give advice here. It is obvious to me you're learning by the things your wrote. Problem is that others may actually believe what you wrote.

You should not just waltz in and give advice. This is not a big box store after all. You should introduce yourself give us a brief bio and other contact info so that what you may write could be verified. Or maybe someone might send you a message or something? This will add credibility to what you say. See you again here and at JB's place.

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Old 09-06-2008, 10:57 PM   #6
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Nugentcn:

You CAN sister your floor joists. Doing so will double the strength and rigidity of the floor, and that will halve the deflection of the floor under any given weight.

However, you can strengthen your floor much more easily and at much lower cost if you're willing to sacrifice some headroom in the space below.

Take a look at the drawings on this web site:

http://www.uoregon.edu/~struct/cours...lecture40.html

They depict beams with both uniform and concentrated loads supported in different ways, and the formula for "DeltaMax" under each drawing tells you the maximum deflection of the beam under those conditions.

Notice that in EVERY situation the maximum deflection equation always has the group "EI" in the denominator.

E is the modulus (or a measure of the strength) of the material the beam is made of. For a steel beam, E would be about 30,000,000 psi, whereas for wood it would be a lot lower (I don't know what it is).

I is the "Moment of Inertia of the beam, and this is entirely dependant on the shape of the beam's cross section. For a rectangular shape like a joist, I = b*h*h*h/12, or the Moment of inertia of a rectangular beam is equal to it's width multiplied by the cube of it's height, all divided by 12.

The following web site includes the sentance: "Two useful examples, especially for wooden beams, are the rectangular beam of height h and width b, and the circular beam of diameter d. The moment of inertia of area of the rectangular beam about a centroidal axis parallel to the width is I = bh3/12, and for a circular beam, I = πd4/64."
http://mysite.du.edu/~jcalvert/tech/beam.htm

So, increasing E or I or both of your floor joists will strenthen your floor, resulting in less deflection of your floor joists.

We can't change E unless we change the material the joists are made of, like sistering the joists with joists made of stronger wood, or stronger material, like sandwiching the existing joists between steel plates.

But, we can easily change I.

By sistering your joists, you effectively double the width of the floor joists, and that doubles the value of b in that bh3/12 equation, which in turn doubles the value of I. That will reduce the deflection by 50 percent.

However, we can double the value of I by increasing the height of the joists too. If you have 2X10's now (which are 9 1/2 inches high) then h cubed is 857.4 cubic inches. If we double that to get 1714.75 and take the cube root, we get 11.97, or 12 inches.

That means that if you were to increase the height of your joists to 12 inches by adding 2 1/2 inches of wood to their bottoms, you would have the same increase in strength as sistering the joists.

All that you need to ensure is:

A) that the wood you add to the bottoms of the joists is as strong or stronger than the wood your existing joists are made of, and

B) the bond between the original wood and the wood you add has to be as strong or stronger than the original wood, and

C) there can be no slippage between the new wood and old wood. That is, the bond between them has to be rigid so that they behave like a single piece of wood. That means you need to glue the new wood to the old wood, not just nail or screw it.

If you do that, it's essentially the same as rebuilding the house with 2X12 floor joists instead of 2X10 floor joists.

And, if you don't believe me, just print off this post and take it to the Faculty of Engineering at your local University, and stop any 3rd or 4th year mechanical or civil engineering student and ask him/her to confirm. Or, consult with any engineering or architectural consulting firm, even over the phone.

LePage's boasts that their PL Premium construction adhesive not only achieves a bond strength 3 times higher than competitive construction adhesives, it achieves that bond strength within 24 hours. If you were to remove all the furniture from the floor so that there's no initial bending of your floor joists, glue and screw wood to the bottoms of your floor joists, you could double the strength of your floor much more easily than sistering all your joists.

Maybe engage the services of an engineering or architectural consulting firm since your house is your single biggest investment, and you don't want to screw it up just because you followed the advice of someone you've never met on a DIY forum.

PS1: If you can find out the value of E for the kind of wood joists you have, you can determine the actual deflection of the floor under a 800 pound concentrated load (350 pound tub, 200 pound person and 250 pounds of water) with the equations on this web page:
http://www.engineersedge.com/beam_be...m_bending3.htm

PS2: I think we should keep in mind that Randall was simply trying to help, and that by ridiculing him we are discouraging other people from posting when they think they can be of help as well. Everyone who's tiled their bathroom floor figures that they've learned a lot from the experience and wants to share their knowledge with others.

Last edited by Nestor_Kelebay; 09-06-2008 at 11:26 PM.
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Old 09-07-2008, 08:15 AM   #7
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Best subfloor and material thickness


"The moment of inertia of area of the rectangular beam about a centroidal axis parallel to the width is I = bh3/12, and for a circular beam, I = πd4/64."

OH...now I see.

Nestor, that stuff is nice - but do you really think it's relevant to the OP?
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Old 09-07-2008, 01:48 PM   #8
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Carlisle:

The original poster wrote: "We have ripped out the ceiling in the room below and will sister these joists, hopefully with 2x10s along the full length, to support a 300+ pound cast iron clawfoot tub."

It's definitely relevant. The poster might not have even been aware that they can strengthen their floor as much or even more by adding wood to the bottoms of the existing joists as they can by adding more joists.

It was a long post, but most of the text was needed to explain WHY increasing the height of the joists would also work. This is not intuitive like sistering floor joists is, so the OP may have even otherwise dismissed the idea as bogus.

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