Maximum Allowable Deflection for Substrates

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

  1. Bud Cline

    Bud Cline Tile Expert Charter Member Senior Member Published

    Here is some information I have borrowed from the TCNA:

    Maximum Allowable Deflection for Substrates Installed by Other Trades:

    Ceramic tile installations require the floor areas over which tile is to be applied to have a deflection not greater that 1/360 of the span when measured under a 300 lb. concentrated load (see ASTM C627). This deflection limitation differs from the L/360 deflection limit under uniform load prescribed by building codes.

    It is the responsibility of the project owner (or owner’s builder or design professional), not the tile installer, to insure the substructure does not deflect more than this maximum allowable deflection criteria. As tile is a “finish” applied to and relying upon the underlying structure, an inadequate substructure can cause a tile failure. In many cases, problems in the substructure may not be obvious and the tile installer cannot be expected to discover such.

    -TCNA (Tile Council of North America - Handbook for Ceramic Tile Installation 42nd Edition)

    In my experience a problem or a failure of a tile installation has always been quickly laid at the feet of the installer. Though I can't argue that the ultimate responsibility for the "quality of workmanship" is solely the installers responsibility I will argue that an installer is not an engineer and shouldn't routinely be expected to recognize a structural deficiency. Fortunately our friends at TCNA support this theory and have published the above for all to know. Thank you TCNA, it's been a long time comming.:)
    Last edited: Jun 11, 2006
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  2. stullis

    stullis Charter Member Senior Member

    Problem is there is no way to field test for L/360. The number is based on engineering values. You as a setter still need to confirm that it meets the standard.
  3. Bud Cline

    Bud Cline Tile Expert Charter Member Senior Member Published

    I agree. The installer at the very least should inspect the structure somehow. You can't just walk on the job and start slamming tile.

    I don't have the span tables in front of me but I know that a minimum of 2 X 10's sixteen inches on center with a 3/4" t&g cover and not spanning more than 12 feet will usually meet the L/360. When the joists go to 19.2" and especially 24" on center then a guy should take a second look and do some calculating. Builders skimp from time to time as we all know, there are "hack" builders out there just the same as there are 'hack' flooring installers.

    The problem I have run into over and over and over again is when I install for a retailer that doesn't know anything about deflection and spans and doesn't want to know. The job is sold and the price is a "done-deal" when I am dispatched to install the tile. I have many times lost a days work or a weeks work because I got to a job with a structure that wasn't sufficient to have tile installed and I pulled up and wouldn't do the installation until the problems were addressed and corrected. This is what got me into trouble with one retailer and I finally quit him a year ago. It cost me every time.

    I have had jobs like this that once I bitched and pulled-off I wasn't sent back to do the job. Whether or not the corrections were made I have no way of knowing. In my mind I know nothing was done and the retailer simply sent a more agreeable (less knowledgeable) installer to do the job. End of story.

    Judgement calls have to be made from time to time and I can tell you first hand that an installer with knowledge and principals can starve while the hacks make routine trips to the bank making timely deposits into their accounts.:)

    Good to see ya Scott.:)
  4. Jerry Thomas

    Jerry Thomas Charter Member Senior Member

    Scott's always trying to get in our heads to charge accordingly. Thats a good thing :)
  5. Lo Down

    Lo Down Old as dirt member Charter Member Senior Member

    I don't do any tile installs, but I does a guy actually measure the deflection rate of 1/360 of a span at 300lbs? Do the math knowing the structural material dimensions, or do a deflection test?
    In an older home, it becomes more difficult to know deflection....... with todays building codes, there is more 'known' info to go from.
    I know the difference between the nice 'feel' of a good solid floor and one with a bit of bounce in it............... but how does one actually measure the deflection if you don't have a helper that weighs 300lbs? :D
  6. Jim McClain

    Jim McClain TFP Owner/Founder Administrator

    That is supposed to be L/360 and I think it refers to a specification in an industry guideline. Just my guess.
  7. Jerry Thomas

    Jerry Thomas Charter Member Senior Member

    I use span tables to figure if the floor meets L/360. Current codes require a floor to meet L/360 deflection be it dimensional joist or floor trusses on new work. On older houses using the span tables will give you a pretty good idea.

    What it does not tell you is the deflection between the on-centers. I try to apply common sense to address that issue, and adding more plywood is never my first choice on a new construction job.
  8. selvalee

    selvalee No one special Senior Member

    Mr. Gobis, at his school, had/has a dail guage with a plunger, he would put
    some boards across some blocks, then, put it under them, put some weight on the boards, like pine, spruce, and so on,, hey Bud, how about pine plywood, good idea? :cool: sweet sweet!
  9. floorman67

    floorman67 One of THOSE Charter Member

    Re: Who Is Responsible?

    as scott stated, it is based on the engineering ... the enineering of the substrate/subfloor system.

    In the most general terms, the MINIMUM requirements are 2"x8" joists on 16 inch centers with a maximum unsupported span of 12', with a single layer of 5/8" plywood, nailed every 6"-10" (or a minimum thickness of 1.5 inches of plywood for direct bond of tile to plywood without a membrane or suportive cbu with approved modified mortars). This gives an active deflection of L/389 which is well withing minimum approved requirement of L/360 ... but remember this is a minimum.

    Of course if you increase the width of the joists, decrease spacing of the joists, decrease the unsupported span distance, and/or use laminated or more structurally sound or engineered substrate/subfloor structure, then the deflection decreases considerably.

    Of course if you decrease the width of the joists, increase spacing of the joists, increase the unsupported span distance, then the deflection increases and would NOT fall within the requirements of L/360 !

    the engineering of the joist width, unsupported span, and placement apart form one another all play factors in the deflection ratio.

    for ceramic tile that is L/360 of the span.

    In more technical terms, it should be noted that the deflection is measurable both as the length of the span, as well as between joists, in the case of a residential application. In actual practice, what this means to the mechanic is that they have to know if the sub-floor will be able to support the tile and not exceed the deflection ratio of L/360.

    With joists 16" o.c. the maximum deflection that can be allowed when using L/360 is 16/360 which equals 1/25"= .04". Now how does this relate to a single layer of plywood sub-floor. When looking at the span tables published by the American Plywood Association, it will be found that a single layer of plywood 19/32" on joists spaced at 16" o.c. the maximum amount of weight that can be applied is 368 lbs before the deflection succeeds L/360 (remember 1/25").

    If a concentrated load is applied between the joists using a 3" wide disk with 200 lbs applied to the same piece of plywood, the plywood can deflect, up to approximately .078" = 1/12.5". This calculation indicates that the single layer of plywood can have approximately twice the deflection acceptable for the installation of tile. In real terms, a concentrated weight of 200 lbs over a 3" wide area, is not all that difficult to attain in a situation such as a fridge or a stove being brought across a floor on a hand truck or dolly.

    With all of this being said, it can now be understood where the recommendation for two layers of plywood 5/8" thick or the use of other underlayments is required. Other underlayments such as ½" cementitious backer board, or a pre-formed sheet applied membrane such as Schluter Systems, Ditra Matting can be considered.

    Another consideration as well, is post-tensioned and pre-stressed concrete are often engineered with only L/240 deflection criterion. This means a slab 30 ft long, supported only at the ends, (L/ 360 = 360/360) could have deflection up to 1 ".

    In contrast, the Marble Institute of America has increased the deflection ratio to L/720, which is double that of tile.

    Installation techniques and systems should be evaluated and be guaranteed to respect the L/360 ratio for tile.

    John bridge has an awesome tool called Deflecto this isnt the final word in deflection and it isnt scientifically perfect per se, but it come VERY close and we use it alot in questionble areas and it has never let us down.

    The American National Standards Institute (ANSI) in coordination with the Tile Council of North America (TCA/TCNA) sets and distributes the standards by which every job is judged and responsibility is determined in cases of inspected an/or litigated failures.

    I would advise everyone doing professional tile work in the United States to purchase a copy of the The Tile Council of America's Handbook for Ceramic Tile Installation ... we use it extensively as well and it has never led us astray.

    * * *​

    for portions of the information in this post, the following sources are credited:
    The Natural Stone Institute
    The Tile Council of America
    The Terrazzo Tile and Marble Institute of Canada
    Last edited: Jul 13, 2006
  10. Jim McClain

    Jim McClain TFP Owner/Founder Administrator

    Re: Who Is Responsible?

    Thank you for your contribution to this thread. I edited your post only to add links to the home pages of each of those individuals and organizations you credited at the bottom of the page. I also appreciate your direct links to pertinent information relating to your post.

  11. floorman67

    floorman67 One of THOSE Charter Member

    Re: Who Is Responsible?

    I re-read my post and wanted to clarify something that might be misunderstood.

    the substrate/subfloor plywood layer has nothing to do with the deflection ratio overall (as far as the TCA/ANSI is concerned), but instead the rigidity of the plywood layer, which is a totally separe issue from the overall substrate/subfloor deflection, so adding additional plywood layers to a existing subfloor system which doesnt fall within the deflection ratio of L/360 IS NOT an appropriate remedy... at least i dont THINK it is.

    The rigidity of the plywood layer only ensures deflection between joists, and DOES does have some secondary/tertiary effect on the overall system, but i dont think that can be used as a consideration for proper pre-installation site conditions of the substrate/subfloor in regard to L/360 deflection.

    minimums for the plywood layer are 5/8" with approved membrane or cbu.

    minimums for the plywood layer as 1.5" for tile over plywood installs without membrane or cbu.

    i know this can be a confusing suubject for many ... and if i am wrong please let me know.
  12. Jerry Thomas

    Jerry Thomas Charter Member Senior Member

    Re: Who Is Responsible?

    Good info Floorman,. My thinking is the plywood minimum is about 1 1/8" without CBU or Ditra. Not real sure because I will not tile directly over plywood without using something between.
  13. floorman67

    floorman67 One of THOSE Charter Member

    Re: Who Is Responsible?

    oh absolutely a 1-1/8" thickness is MUCH better than the minimum recommended 5/8" plywood over ditra or cbu.
  14. Chuck Coffer

    Chuck Coffer Well-Known Member

    ?? Adding plywood will most definitely alleviate deflection. It adds rigidity to the structure. Provided it is structrual(3/8 or better) adding plywood is the easiest way to beef up a floor.(CBU offers no rigidity benefit whatsoever)

    p.s. Have any of you readers had a chance to check out the 1/4 point deflection theory set forth by the folks at Schluter? Peter Nielsen and Frank Woeste Ph. d. wrote an excellent thesis on it. The logic underlying it is "smack you in the face" type stuff. It seemed contrary to everything I thought I knew until I actually came to understand it. I could probably find it if anyone is interested.:)

    p.p.s. I know some of you must have the article. Its in the binder they gave you at the skool. :)

    Here is one that confuses me. Schluter says 5/8" decking is ok under Ditra on 16" centers. That seems a bit weak. There is, however, an awful lot of thinset going down when we use Ditra. Reckon that has something to do with their calculation?

    Heck, the finished assembly is akin to a reinforced mudbed. I have never seen anything but 3/4 around here, so it is more a matter of trivia than anything else.

    Any ideas?

    I should concede that 1/2in Hardi actually does offer a nominally measureable increase in structural rigidity, but it pales in comparison to 3/8 plywood in that arena.(hell, it probably offers less than 1/4 in Baltic Birch)

    This info is free for whomever desires it. All it takes is what you already have at your fingertips. Knowing beats wondering in whom to place your trust. I beats arguing also.

    Knowing is a handy trump card. Seek and ye shall find. Go get it.

    When it comes to matters technical, there are very few disagreements among the learned. Standards are not simply made up for the sake of creating standards. Are there logical inconsistencies within every published standard? Yes, but they are incredibly rare. Is everybody in manufacturing out there trying to ensure that failures occur so they can blame them on the installer?


    As an installer who never suffered repeated failures of the materials I installed for a living, I really don't get the fixation on failure that some seem to have. I focus on excellence. Failure is the last thing on my mind when I install.The bar is where you put it. Blow them away with excellence and you will never know failure.

    The rules have not changed. They never will.

    BTW: L/360 is a value related to the deflection of the framing members. That standard was adopted in the days of 12 and 16 centers. The advent of 19 and 24 spacing has introduced another consideration; curvature. L/360 does not even begin to address the problem of sufloor curvature between the framing members. Regardless of the overall rigidity of a designed flooring module, the problem of curvature creates instability. The L/360 deal does not make for an accurate frame of reference these days.
  15. Tileman

    Tileman Remove the Vinyl Charter Member

    Re: Who Is Responsible?

    Plywood doesn't add any strength to the defledtion in the joists system, just the deflection between the joists, if spaced 19" or above, more plywood is needed, I wouldn't tile floor even with one layer over 16" spacing, if a floor needs more strength in the flex to meet L/360 or above, then sistering, adding joists, or a cross beam under them is the only way, CBUs, membranes and thinset don't help.;)
  16. Floorguy

    Floorguy The Living Dead Charter Member Senior Member

    Re: Who Is Responsible?

    Get this...

    I have a home of hardwood to do in the upstairs. It is a new home they are building with one of those fast paced slap them up as cheap as possible builders. I'm going in after closing, to do my work directly for the homeowner.

    I have ½" plywood to put down to make the substrate meet the wood flooring requirements, because ¾OSB over trusses on 24" center, isn't going to cut it.

    The sad thing is... They came in, and put down CBU and laid tile in the master bathroom and the other two baths upstairs.. I have yet to see of feel it now the tile is in, but I can just imagine. A failure waiting to happen!!
  17. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    Huh? How do you figure?
    Last edited: Jul 31, 2006
  18. Bud Cline

    Bud Cline Tile Expert Charter Member Senior Member Published

    Re: Who Is Responsible?

    Maybe we should schedule a meeting?:)
  19. Tileman

    Tileman Remove the Vinyl Charter Member

    Re: Who Is Responsible?

    Chuck, be patient, we'll agree on something sooner or later, until then, keep reading.:D

    So Bud, when is this meeting taking place, I'll bring the coffee and donuts, should take awhile.:coffee:
  20. Chuck Coffer

    Chuck Coffer Well-Known Member

    Re: Who Is Responsible?

    How is it that plywood offers rigidity in one direction but not in the other? Is this something you have tested?

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