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l/360 for dummies

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  • l/360 for dummies

    Ok, I admit it, I dont understand what L/360 is. Or should I say I dont understand how to determine if a floor with joists x spaced y over a span of z meets or exceeds L/360 or greater.

    *** If you care to answer you can stop reading here, the rest is pretty much ramblings of my experience and search for the answer to my question ***

    I have tried, lazily I'll admit, to research the subject but it seems everywhere I look including several tile books, ansi and tcna all only say to make sure the floor meets or exceeds L/360 720 etc, nothing explains how it works. Also everytime someone asks if a floor will support tile or marble the responses are that the floor needs to meet L/360 bla bla bla.

    This may be something simple that I do know but dont know I know it. When I was taught tile in the union here in NJ that isnt something we were taught or something that anyone was worried about although I wish it was. After the union I worked for a contractor remodeling bathrooms, the more experienced I got and the more I learned the less impressed I was with his quality standards so I started my own business doing the same type of work.

    While Im not stupid and usually understand concepts fairly easily, I have yet to find a good explination of L/360 and anyone I have asked in person has either brushed it off or didnt seem to know or be able to explain the answer either.

    So if anyone would care to explain it to me and anyone else who doesnt understand it, or could post a link to a site, or reference a book where I should be able to get a good understanding of it I would greatly appreciate it.

    Additionally I have "Setting Tile" and Michael's 3 dvds and John Bridge's book, as well as ansi and the tcna, so if anyone can reference any of those possibly I missed it.
    Last edited by magna111; 01-07-2009, 04:36 AM. Reason: To spare you from reading the rest when the question was in the first 2 sentences.

  • #2
    Re: l/360 for dummies

    I know I've seen a formula a long time ago but since 99% of my work is on grade I didn't pay attention at the time. Now I'm seeing more older remodels and simply use the Deflecto calculater.


    • #3
      Re: l/360 for dummies

      Alright, did a little looking and I think I understand the 1st half of my question, which I think I new before but still am unsure how to apply it so feel free to advise me on that, and also let me know if my understanding so far is correct.

      L/360 is the length of the unsupported span in inches divided by 360, so for a 10 foot span, L= (10feet x 12inches per foot) so L is 120, 120/360 is 1/3 so the max deflection is 1/3" for that span.

      Now to the second part of my question, how do I apply that to my joist size and spacing, or where can I find a chart or calculator that tells me how much deflection a particular species of a certain size joist will have over any given unsupported span.


      • #4
        Re: l/360 for dummies

        Article from FCI. No formula yet though. last one. Halfway down next to the pic of the guy using 9235.
        Last edited by scuttlebuttrp; 01-07-2009, 05:25 AM.


        • #5
          Re: l/360 for dummies

          Ok, did a little more research and I think I have most of what I need.

          From that link you can view tables which tell you how much span is allowed to meet L/360 by different variations of grade size and spacing at different loads.

          So given the above example and 30psf live load 10psf dead load grade 2 2x6 spaced 16oc exceeds L/360.

          I guess at this point all I need to know is how do I determine what my live load and dead loads are for a 1st floor kitchen vs a 2nd floor bathroom in Maine vs in Texas?

          *** I will try to answer this myself, but if anyone else beats me to it thank you, and also please correct me if Im wrong with anything above or if you have better links or suggestions on books/charts to purchase vs what I can find free online. As a note I didnt get too much sleep last night so confirmation that all my research thus far will make me feel a bit better about its accuracy. ***
          Last edited by magna111; 01-07-2009, 08:25 AM. Reason: psi to psf


          • #6
            Re: l/360 for dummies

            In reading one of the links provided by Scuttlebuttrp, the author recommended using a laser in the center of the span to mark the wall, then load the center of the span (about 300 lbs) and measure the deflection and compare the results to the allowed L/360.

            Well even though I did most of the work myself, I now have a much better understanding of deflection, how to find it and apply it.

            Hopefully others more experienced and who have a greater understanding can pitch in and confirm or expand on what I've learned already.


            • #7
              Re: l/360 for dummies

              Hate plug Mike's rival, but... there's a handy calculator over at Bridge's:


              Truth is just one man's explanation for what he thinks he understands. (Walter Mosley)


              • #8
                Re: l/360 for dummies

                Careful Magna, loads are usually psf, not psi. The equation you are after is here:

                If you play around with this, you can also figure out the depth of the member needed for a given deflection limit.

                also, the best span calculator I have seen is


                • #9
                  Re: l/360 for dummies


                  30-foot span, divided by 360 = 1-inch of uniform deflection.

                  or 360-inch span, divided by 360 = 1-inch of uniform deflection.

                  Joist spacing:

                  24-inch spacing:
                  22.5-inches divided by 360 = .0625-inches of concentrated deflection.

                  16-inch spacing
                  14.5-inches divided by 360 = .040-inches of concentrated deflection

                  Use 300-lbs. for weight


                  • #10
                    Re: l/360 for dummies


                    I’ve been studying and teaching “L/360” subject matter for 38 years and I too did not understand the (past) tile industry’s reference to L/360. The problem with the industry’s use of the L/360 deflection criterion was analogous to the following case. Imagine a person saying, “The sum of three numbers is 110. The first number is 26. What are the other two numbers?”

                    For L/360 to have meaning to all parties involved, several issues needed to be defined or clarified. For starters, the “span” needed to be defined. Industry standards used the word “areas” which did not define the applicable “span” needed for any recognized structural deflection calculation. The type of load was specified, however, no relationship existed between the vague tile industry specifications and what engineers and architects use to design floor systems per the local building code. In summary, the tile industry specification of L/360 did not relate to others outside the tile industry.

                    After study by an ANSI committee, the deflection language in the 2005 ANSI A108.01 was changed to read as follows:

                    "2.3 Deflection
                    Floor systems, including the framing system and subfloor panels over which tile will be installed, shall be in conformance with the IRC for residential applications, the IBC3 for commercial applications, or applicable building codes.

                    NOTE: The owner should communicate in writing to the project design professional and general contractor the intended use of the tile installation, in order to enable the project design professional and general contractor to make necessary allowances for the expected live load, concentrated loads, impact loads, and dead loads, including weight of the tile and setting bed. The tile installer shall not be responsible for any floor framing or subfloor installation not compliant with applicable building codes, unless the tile installer or tile contractor designs and installs the floor framing or subfloor."

                    The deflection issue was addressed in a building code magazine:


                    The new deflection language replacing the “L/360 language” is now contained in the TCA Handbook.

                    Frank Woeste
                    Professor Emeritus
                    Virginia Tech University


                    • #11
                      Re: l/360 for dummies

                      Dr. Woeste:
                      Thanks for the information regarding L/360. Perhaps you could help the installers who really do not have any access to a design professional or even a knowledgeable GC in regard to assurance that a particular floor will meet or exceed the desired standard.

                      Many installers face floors that cannot be accessed without destroying a portion of the floor to check for sizes, spans, etc. For years, I have been using transits, dial indicators, lasers, and varying amounts of weight to determine both concentrated and uniform deflection. I have been using weight stacks around 300-pounds to test for both and have had good success. I have used this test even when tiling over floors that had been cleared by the architect or SE.

                      For new construction, there are load tables that can be easily consulted, but for remodeling or add-ons, especially where no architect or structural engineer has been budgeted, is 300-pounds (borrowed from the ASTM C627 test) a reasonable force to apply for a field test of deflection?

                      Any light you could shed on this would be greatly appreciated.


                      • #12
                        Re: l/360 for dummies

                        It seems that the new wording is simply a "out " for some tile contractors to be able to say "not my problem". To just say, must meet Building code, gives the contractor (no one here) no responsibilty to be sure the floor will handle the loads . Its a shift ,for who to blame if it fail's.And as Michael questioned ,what about renovation work..


                        • #13
                          Re: l/360 for dummies

                          Last year I had to quote an Addition job for a master suite. I was supplied with very inadequate drawings and I thought the owners were under-served by their "designer" I ended up not even being in the ball park as far as price because I said that I could not do the tile work in the bath unless the structure was beefed up. They wanted 16" granite tiles on the floor, a separate shower and the tub was a 2+ person jacuzzi style. In addition, there were two good sized vanities with granite counters, a toilet and a bidet. I tried to educate them about how much load they were going to have in the room, but I guess the "designer" was a great BS artist. I am very glad to not have persued this job because I knew that I would be back endless times for cracked tiles, sheetrock issues and caulking problems all due to an insufficient structure. L/360 is all relative. The bigger the tile the more stable a structure you need, natural stone even more so. Having joists at 12oc for the bath would be cheap insurance. the floor should also be calculated for at least 60lbs /sf as well to start with. Every body in the chain has to think like a team for a good outcome. Good luck.

                          Last edited by philthegreek; 01-16-2009, 09:57 AM.
                          It's better to try and fail, than fail to try.


                          • #14
                            Re: l/360 for dummies

                            You hit it right on the head with your comment about the team approach to tiling (roofing, trim, foundation, plumbing...). There are many designers out there who are smart enough and secure enough with themselves to never hesitate to consult with their tile contractor - PRIOR to the client presentation - to ensure that (1) the structure is adequate, (2) the design can be accomplished with tile, and (3) the budget is large enough to do things right.

                            Unfortunately, though, there are also designers, some of them quite famous, who arrogantly loft themselves above such mundane issues as the laws of physics, and blithely go about designing tile installations, on flimsy foundations, that ultimately crack, leak, and deteriorate.

                            Up to a point, I agree with tile industry people who feel that determination of structural integrity is not the responsibility of the tile contractor. On the other hand, I think any installer worth his salt should be able to tell, by observation, whether or not a structure is robust enough for tiling. A good installer should also be able to counsel builders on how to upgrade the structure with closer spacing and heavier joists, studs, subflooring, and underlayments, and tighter and stronger reinforcing and finishing specs.

                            Any installer can educate himself about structure, without having to attend night school, by closely reading the current ANSI A108 standard specifications for tile. With the addition of an elementary understanding of carpentry and concrete, it should not be too difficult for an installer to see that a particular construction is not strong enough for tile.

                            I have no particular interest in doing any type of framing or concrete, but I can write precise and detailed floor or wall specs for a general contractor or architect, and as a tile installer, I want complete control of the installation of underlayments, membranes, sub-drains and some other fixtures, in addition to the tiling. Some have accused me of installation overkill, but when you realize that the ANSI A108 standards are minimums, not recommendations, I look at the upgrades I use as giving the customer A+ work instead of D+.

                            A good installer needs to be able to know when to walk away.

                            I guess you prefer sleeping well at night to getting every job.


                            • #15
                              Re: l/360 for dummies

                              Subflooring spans joists and joists span between essentially solid points - be it girders, columns, or foundation walls. Joists are laid in rows, in common fashion, with some fixed on-center distance between (12in,16in,24in,etc.)

                              Framers or designers usually consider deflection of joists *between bearing points*, along the length of the joists, not based on deflection *between joists*. Joist deflection between bearing points depends on the span between those points and the on-center distance between joists.

                              robc's link to performs such calculations. These calculations typically assume spans between solid bearing points. They are purely theoretical numbers based on the properties of the wood joists, and vary with species and other conditions. They are based on the presence of an evenly distributed load - typically 30 or 40 lbs/sqft.

                              However, when considering deflection for tile, one needs to also consider deflection between joists. A heavy, concentrated load can exceed l/360 between joists. Joists are almost always laid on standard spacings - 12in, 16in, 19.2in, or 24in. The deflection of subfloor spanning between these distances is the same no matter the joist span or other floor configuration.

                              Thus, one can use "rules of thumb" to meet deflection criteria between joists, and check joist-span tables to meet deflection along joists.

                              Moreso, if you find your existing joist span insufficient, I would hazard a guess that a substantial amount of subflooring would be necessary to meet your needs. That is, where joists greatly deflect along their length, the remedy is with the joists, not the subfloor.