Maximum Allowable Deflection for Substrates

Discussion in 'Ceramic and Stone Q&A' started by Bud Cline, Jun 11, 2006.

  1. Tandy Reeves

    Tandy Reeves Resting In Peace Charter Member I Support TFP Senior Member

    Re: Who Is Responsible?

    I think we should have a committee investigate it first and like the senate we must have members on it that do not have a clue of what is going on.
     
  2. Peter Kodner

    Peter Kodner Inspector Floors Charter Member Senior Member

    Re: Who Is Responsible?

    Tandy, you writing a job description for me? Or better yet, maybe I should run for Congress! If Jesse can get to be governor, why not me for Senator:eek: :D
     
  3. Jerry Thomas

    Jerry Thomas Charter Member Senior Member

    Re: Who Is Responsible?

    This will require extreme measures. Why don't you guys hook up on the phone one night and then you will get a better understanding of one another. I'm dumb, so that usually works for me. :)
     
  4. Bill Vincent

    Bill Vincent Charter Member

    Re: Who Is Responsible?

    Plywood WILL offer structural strength in the direction the grain runs, which should be ACROSS the joists. THis is why it offers no structural strength running along the joists, if installed properly.

    Chuck, I don't know where you're getting your info, but this is only the latest in many things the TCA has tested and posted on which you've decided are bunk.

    I'd be interested to be a fly on the wall during a conversation between you and Dave Gobis.
     
  5. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    I haven't decided anything is bunk, Bill. I am just asking questions.

    I don't quite follow your logic whith respect to direction. Are you saying plywood is rigid in the long dimension and not in the short? Ever try to roll up a sheet of plywood? :)
     
    Last edited: Aug 2, 2006
  6. Tileman

    Tileman Remove the Vinyl Charter Member

    Re: Who Is Responsible?

    I roll it all the time, how else do you get it up through tight openings.:D
     
  7. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    How about a simple example? If I take one piece of plywood and set it on a pair of horses, it will bend a certain distance when I place a certain amount of weight on the unsupported center. Now let us imagine that I take another piece of plwood and screw it to the first. If I place an identical amount of weight at the same place will the degree of bend be indentical to that in the former instance? If plywood offers no rigidity to the assembly, the answer is yes. If, however, plywood does add rigidity, the answer is no.

    Ya folla?
     
  8. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    p.s. Bill, there is no single grain direction in plywood. That is what makes it plywood. Ever work with 4x4 plywood?
     
  9. Bill Vincent

    Bill Vincent Charter Member

    Re: Who Is Responsible?

    I DO folla. However, lets take your same example, and set that single piece of plywood lengthwise between the 2 horses, set 3 1/2' apart. Put your weight on it. Now, turn that same sheet of plywood perpendicular to the horses, so that the grain SHOWN ON THE TOP LAYER runs between the horses. That same sheet of plywood will bend much further.

    One way or another, we can argue this back and forth. I understand the point you're trying to make. But when it comes down to it, the people who've done the r&d on deflection, have determined that the subfloor does NOT make enough difference to where it can be considered to stiffen a questionable joisting system. I'll talke their word for it.
     
    Last edited: Aug 2, 2006
  10. Bill Vincent

    Bill Vincent Charter Member

    Re: Who Is Responsible?

    Chuck-- one thing you need to understand about me-- For alot of things, I'm very much like you. I don't hear something and then follow it blindly-- in most cases. A perfect example is my use of lightweight thinsets over Kerdi, in spite of the fact that it voids their warranty. I know it works, and I get better results than following their directions to the letter, using an unmodified. Who knows-- I may have to eat an installation one of these days, but I don't think so. There are very few people or organizations in this industry I'll follow blindly. However, there ARE three-- one person, and two organizations-- Dave Gobis, TCA, and CTEF. Not one of the three have steered me wrong yet. For anyone else, though, anything I hear, I'll put it up against my own experience and see how it stands up. I'll also think it through logically and see if it makes sense, and then I'll make my decision as to whether or not I agree with what I've been told. Only once or twice I've questioned what Dave has told me, and in those cases, once he's explained everything out, it's made alot of sense.
     
  11. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    Bill,
    I will readily concede that plywood is (more) rigid in the length than in the width. The plies running the length always outnumber those running the width. That, however, is a difference of degree, not of kind.

    Are you really going to tell me that 3/4 over 3/4 is no more rigid than CBU over 3/4?
     
    Last edited by a moderator: Aug 8, 2006
  12. Bud Cline

    Bud Cline Tile Expert Charter Member Senior Member Published

    Re: Who Is Responsible?


    SCHOOSH!!!

    I'm glad that's settled. I was about to offer that information when Chuck beat me to it. Thanks Chuck!:)

    I think some of the confusion here is that the statement made by TILEMAN that got the Playground Monitor riled was a statement being used throughout the tile industry and not necessarily the building industry. Any given floor structure comprised of let's say 2X10 joists installed at (or even slightly beyond) their recommended length required to maintain a deflection maximum of L/360 will in fact have some movement under a load. The building industry deals with "pounds of load per square foot" while the tile industry is dealing with "point loads" primarily. In a test scenario a three hundred pound concentrated point load is used.

    To add a layer of 3/4" plywood would in fact slightly increase the overall load range (per square foot) of the given floor structure while at the same time not offering much (if any) noticeable improvement to the overall deflection. This slight improvement would of course be realized in either direction.

    Now take that same floor structure rated for L/360 but this time install the joist members at 19.2" on center or 24" on center. Of course depending on whether trusses or I-Joists were used the depth of the joist would have to be increased to accomodate the L/360. Nevermind that. The floor is still rated at L/360. BUT, now deflection between the joists becomes a more serious issue and this is where the tile community gets involved. And this is why a common statement in the industry (for those that understand it) is that "adding an additional layer of plywood offers no improvement to overall deflection and it only improves the deflection between the joists.

    None of this is a matter of absolute science unfortunately. Other considerations in floor structure members also must include the lumber species used. In I-Joists OSB as opposed to plywood can be a consideration since they are made both ways. Trusses assembled with metal nail-scabs for plates as opposed to trusses assembled with plywood scab plates and glued and nailed. Many considerations go into determining why a floor system will and will not deflect and how much.

    So.. to sum this up, in my experience in the tile industry the statement made by TILEMAN is in fact an accurate statement being spewed all over the land by knowledgeable tile industry participants.:)
     
    Last edited: Aug 2, 2006
  13. Bill Vincent

    Bill Vincent Charter Member

    Re: Who Is Responsible?

    No, I'm not, and I'll even concede that it DOES stiffen the floor up. But I will NOT consider it when figuring the deflection of the floor, if for no other reason than people who've done alot more research and testing in the area than I have, have put out the word that considering the subfloor in figuring the deflection of the floor is asking for trouble.
     
  14. Bud Cline

    Bud Cline Tile Expert Charter Member Senior Member Published

    Re: Who Is Responsible?

    Something tells me this thread is officually OFF TOPIC. Shall we get back to the rerason for this thread to begin with?:)

    Yes we should.

    THANK YOU.:)
     
  15. Tileman

    Tileman Remove the Vinyl Charter Member

    Re: Who Is Responsible?

    Yes, I folla, now add the whole pallet of plywood to stiffen it even more, guess what, the horses it's on will callapse, picture the horses as the joists below, there are 2 systems here that need to work together, adding too much above is not always a better idea.:coffee:
     
  16. Jim McClain

    Jim McClain Owner/Founder Administrator

    Re: Who Is Responsible?

    OK, moving right along...
     
  17. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    Is another layer of plywood a panacea? NO

    Is it worthless? NO

    My work here is done.
     
    Last edited by a moderator: Aug 8, 2006
  18. stullis

    stullis Charter Member Senior Member

    Re: Who Is Responsible?


    Thanks Bud.

    Can't argue with much here sad to say.
    I've said it before the "hacks" make more money in this industry than the guy who does the job correctly. :(
     
  19. hookknife

    hookknife Hard Surface Installer Charter Member Senior Member

    Re: Who Is Responsible?

    And that my friends is the sorry truth of it:eek: Dang Hacks:D
     
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