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Old 04-03-2011, 10:30 PM   #1
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how do you figure floor flex?


so how do you figure whether your floor is ok for ceramic tile? is there a formula for figuring the "flex factor"? it must have something to do with the span of the floor joists right?

tnx,

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Old 04-03-2011, 10:44 PM   #2
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how do you figure floor flex?


Hi,

Quote:
so how do you figure whether your floor is ok for ceramic tile? is there a formula for figuring the "flex factor"? it must have something to do with the span of the floor joists right?
That's right. Tell us the type and size of the joists, spacing, unsupported span to the inch measured from face to face of the supports. It would also help if we knew the species and grade too. How old is the house and style.

Then the type and thickness and number of layers of the subfloor.

Start with that.

Jaz

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Old 04-04-2011, 08:26 PM   #3
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how do you figure floor flex?


OK,the joists are 2x10 fir. the span across the room from outer wall to inner wall is 15' (that must be what you mean by "unsupported")
the subfloor looks like 3/4 ply. with a 1/4 underlayment under the current vinyl.

tnx,
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Old 04-04-2011, 08:28 PM   #4
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how do you figure floor flex?


oh yeah,the house is a 30 yr. old ranch and the joists are 16 on center

Last edited by analogmusicman; 04-04-2011 at 08:32 PM. Reason: forgot
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Old 04-05-2011, 09:53 AM   #5
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how do you figure floor flex?


You can try this site's calculator.

http://www.johnbridge.com/vbulletin/deflecto.pl

It also has a great forum for tiling questions.
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Old 04-05-2011, 01:44 PM   #6
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how do you figure floor flex?


Quote:
OK,the joists are 2x10 fir. the span across the room from outer wall to inner wall is 15'
That may be close to the right answer, (15'), but the size of the room has nothing to do with the framing or unsupported span. You need to measure from below.

Please get an accurate measurement to the inch and report back.



Blondesense suggested;
Quote:
You can try this site's calculator.
I'm sure the owners of this site really appreciate you doing that.

The Deflecto is a very useful tool, I use it to double check often. However, it only deals with two species of pine of unknown an grade. So while it's very useful, it is limited. You also can not insert load info to account for various types of room uses (live load) or flooring types, (dead load).

Jaz
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Old 04-05-2011, 02:30 PM   #7
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how do you figure floor flex?


Max deflection due to uniform loading:

Delta=5wL^4/(384EI)

w=loading
L=length
E=1600000 psi (Douglas fir north #2)
I=98.9 in^4 (2x10)

If loading is 15 dead initially plus another 16 psf for 3/4 inch tile in 1/2 mortar bed and assuming 40 psf live loading with16" spacing...

Delta dead @ midspan: .3 inches
Delta live @ midspan : .38 inches

These numbers must be increased by a creep factor equal to 1.5. This makes dead .45 inches and live .57 inches.

Typical deflection limit for dead plus live is L/240 which is .75 inches

Typical live deflection limit is L/360 which is .5 inches.

It appears that you need to stiffen the joists for deflection. It is also possible that the joist will be overstressed due to the tile and design live load.

Last edited by Talbot3; 04-05-2011 at 02:40 PM.
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Old 04-05-2011, 03:04 PM   #8
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how do you figure floor flex?


Quote:
Originally Posted by JazMan View Post
That may be close to the right answer, (15'), but the size of the room has nothing to do with the framing or unsupported span. You need to measure from below.

Please get an accurate measurement to the inch and report back.



Blondesense suggested;


I'm sure the owners of this site really appreciate you doing that.

The Deflecto is a very useful tool, I use it to double check often. However, it only deals with two species of pine of unknown an grade. So while it's very useful, it is limited. You also can not insert load info to account for various types of room uses (live load) or flooring types, (dead load).

Jaz

that 15' measurement IS the measurement of the room underneath. I'm not as dumb as I look.
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Old 04-05-2011, 05:04 PM   #9
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how do you figure floor flex?


Quote:
that 15' measurement IS the measurement of the room underneath. I'm not as dumb as I look.
LOL, Thanks for clearing that up. OK, 2x10, 16" oc, exactly 15'0" span. Doug fir. Which douglas fir? My chart shows 3 choices in a total of dozens of grade combination. It makes a difference.

The stiffest is Douglas Fir-Larch, then Douglas Fir-Larch North and then Douglas Fir-South. The typical grade used for joists is #2. Your span is 15'0" because the min. code is L360 @ 40/10 live/dead load. You're OK for standard flooring and safety as far as codes go. However that's a little shaky for tile. I like to figure L360 or better @ 50/20 loads. Your floor does not meet that standard. It doesn't mean it will not work though. I just wouldn't recommend or guarantee that installation.

OK, so Talbot gave us the engineering version of deflection. We need to use them as a guide, it's not an absolute situation. It's not a guarantee that a structure that comes in at L359 will fail, while it would be fine at L360. Doesn't work that way.

So unless you have additional info, I'm gonna vote no, but you're free to roll the dice.

Jaz
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Old 04-05-2011, 07:06 PM   #10
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how do you figure floor flex?


If you want to check bending stress :

f= M/S

M= wL^2/8
s=bh^2/6 for rectangular cross section

For the loading I mentioned above:

M= 31850 # in
s=21.39in^3
f=1489 psi

I don't have my NDS with me so I cannot check allowable stress, but I have a feeling it will be exceeded.

I agree with Jaz. This does not mean that the floor will not support the tile, but you will be at a large risk of failure if your floor ever sees the design live loads, and you will likely have subpar performance( bounces, cracks, etc.)

Last edited by Talbot3; 04-07-2011 at 02:45 PM.
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Old 04-05-2011, 07:53 PM   #11
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how do you figure floor flex?


well, this is not really a disaster since there are some nice alternatives for flooring and I really wouldn't want to take a chance. but,I and my wife are wondering why we haven't heard anybody even mention this as a reason NOT to tile a floor. the guy at Lowes who seemed to know what he was talking about didn't say one word about this.

tnx,
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Old 04-06-2011, 11:44 AM   #12
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how do you figure floor flex?


the more I read,the more I think this IS a disaster! tile seems to be the only alternative for my kitchen. from what I've read,your choices for a "high moisture" area (like a kitchen or bath or laundry room) are limited to either ceramic tile or some sort of vinyl (like sheet vinyl or vinyl tiles which are both "out of the question") to compound the problems with a "fake" wood floor is the fact that if you happen to damage a portion by say dropping something heavy on it (as sometimes happens in a kitchen) it's durned impossible to fix it without removing the whole floor because of the "tongue and groove" construction. tile on the other hand can be removed with a "grout saw".
so, my question is: "exactly what risks does one take if he goes ahead and puts ceramic tile on a floor that doesn't meet specs"? the floor won't collape will it? (with all those 2x10 joists hard to swallow) some cracked tiles? (which I could replace) I've posted the relevant figures, how far out of spec am I? what are the realistic chances that something could go wrong?

tnx,
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Old 04-06-2011, 02:11 PM   #13
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how do you figure floor flex?


I agree that a ceramic floor is the only high quality choice in rooms that get lots of traffic or may get wet occasionally. It'll also add value to the house.

Quote:
the guy at Lowes who seemed to know what he was talking about didn't say one word about this.
LOL, once in a while you find someone that can fake it well. Plus telling you the truth might cause a lost sale. I don't care about a sale.

Is there no way to stiffen the joists? You do not necessarily have to sister the joists with the same 2x10's. Are the joists exposed from below?

Even if I couldn't improve the deflection, I would still go ahead with the installation, esp. if it was my own house. The along-the-joists deflection is NOT as critical as the between-the-joists deflection. There is a thin set that is more forgiving according to its maker for one thing.

Jaz
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Old 04-06-2011, 07:17 PM   #14
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how do you figure floor flex?


well,Jaz,if I could "beef up" the joists I sure would,but the whole space below the kitchen level is finished. the only reason I was able to get those joist measurements was because I could take off the light "diffuser" on the light which is directly below the kitchen. (and I used to curse the former owner for installing those "dumb" recessed lights in the ceiling )

yeah,the guys at Lowes sometimes know what they're talking about but you DO have to be wary of what they do and don't tell you.

tnx,
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Old 04-06-2011, 07:30 PM   #15
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how do you figure floor flex?


Call and go to Bedrosians, see if they have and will sell you Hydroment's Reflex mortar. Hydroment says it'll work even at L240. (that's what they say).

Bedrosians is located on N. Citadel Dr. South of Galley Rd. bet. North Academy Blvd. & North Chelton Rd. in Colorado Springs. (719) 591-7770

Jaz

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