Deflection values for floors

[KDurkin]

I'm very new to tiling, and don't plan on doing much other than around the house and helping friends. (Classic DIYer) I understand the idea of deflection based on span, joist size, etc, and have found your calculator helpful, BUT have a question: What loads are the deflections based on? Example: Ceramic tile is recommended to have a max. deflection of L/360. Based on the span, joist size and spacing, but there's never a mention of weight on the floor (load). The more load, the more the floor will deflect. So I guess there must be some assumption of load when calculating deflection and whether a floor is beefy enough. Sorry if this is rather rambling, I'm just curious if there is an assumed "standard" live load and dead load on the floor factored in.

[Jason_Butler]

I've emailed our injuneer' to give you the specifics...

Hopefully he'll drop in soon.

Jason

[cx]

Welcome, KDurkin, give us a first name, eh?

I'm reasonably sure that when Injineer Bob and DaveM built the Deflectolator (is that a great tool, or what?), they used the minimum residential spec of 50 pounds per square foot; 10 pounds dead load and 40 pounds live load. That's assumed to be an evenly distributed load and is usually an adequate spec for most residential applications.

Keep in mind that we're talking minimums here, but I know Bob was fairly conservative with his calculations.

For tile work, the deflection of the subfloor between joists, especially with engineered-wood joist systems with spacings frequently greater than 16 inches on center, is sometimes of even greater concern. For those values we generally defer to the testing data reflected in the TCA handbook recommendations.



I think maybe Bob's AWOL, but I'm sure he'll comment when his wife can come up with the bail money. Again.

[John Bridge]

Hi and welcome.

The ceramic tile industry standard of L/360 assumes a 300 lb. concentrated load. In my book I've equated that to two persons or one somewhat rotund person dancing across the floor. That's really all we're worried about. The dead load of a tile installation will never exceed the designed strength of the floor system. Floors are offen calculated to support 50 - 60 lbs. per square foot (dead load). A typical tile installation might amount to 10 or 12 lbs. at most.

Engineers tend to take weekends off. Bob usually shuffles back in here late Sunday evenings or early Monday mornings.

[bbcamp]

(Yawn...shuffle-shuffle...scratch...)

Yeah, CX and John covered the basics: joists are sized based on a 50 psf loading, and the subfloor is based on 300 lb concentrated weight. I don't bother with calcualting the subfloor, the underlayment manufacturer specifies the minimum requirements, and I don't argue with them.

Bob

[John Bridge]

Bob,

That argumentative CX says I'm way off on my 50 to 60 lb. statement above. We want the truth. Don't worry a bit about my fragile feelings.

[bbcamp]

You're off. 40 psf live load, 10 psf dead load.



Unless you are talking about commercial floors, but you ain't, are you?

[cx]

Quoth CX:

10 pounds dead load and 40 pounds live load

Quoth Injineer Bob:

40 psf live load, 10 psf dead load.

Quoth JB:

calculated to support 50 - 60 lbs. per square foot (dead load)

This here is like one of them aptitudinal tests, eh? Which one of these statements don't fit?

Don't mind us, KDurkin - if you're still out there, that is. Just stick with ol' bbcamp on stuff like this and you won't go wrong.

[KDurkin]

I'm still here, don't have a great deal of time to check thread. Was running elec last night in prep for drywall. After drywall, hardi going down, hopefully this weekend.

Thanks for the info. While I'm not a structural ingineer (I'm mechanical), I figured there must be an almost accepted standard (sorry John) and I had to know.

I probably should start a new thread for a new question, but I'll risk asking it here: I'm tiling my kitchen, have 3/4" thk x 3" wide planking subfloor, put 1/2" BC plywood down and am getting ready to put 1/4" hardibacker then 1/4" 12x12 ceramic tiles, should I put down some kind of vapor barrier? The kitchen is over a 3' high crawl space w/ 2 foundation vents and insulation with a vapor barrier facing the interior on it. Since it's a kitchen, I don't expect much (hopefully no) water to deal with.

Thanks again!
Kevin

[John Bridge]

Well, I'm completely devastated. I won't be imparting any more of my vast engineering knowledge. You guys will just have to get along with Injineer Bob and CX from now on.



But a tile install alone can easily weigh more than ten pounds, and I presume that's dead load. It ain't gonna move.

[bbcamp]

Kevin, assuming the floor joists are adequately sized and spaced no more than 16 inches apart, then your floor prep seems acceptable. No additional vapor barrier is required, except one to cover the dirt in the crawl space. If you want waterproofing, apply a membrane (either sheet or paint-on) to the top of the taped and mudded backerboard, or use Ditra instead of the backerboard. I don't think you'll need one, though.

Don't forget to bed the hardibacker in thinset as you screw it down!