Installing porcelain planks 6"x 36" with 11mm thickness, in main floor Living room that was previously carpeted. House is 2 story town, 13yrs old.

Room size across W 18' x Length joist span of 11.8'(below shows end of joists are sitting directly over steel support beam, directly under in room below shows ceiling joists span of 11'4"rest on steel beam) ,S.P.F. 2x8 joists, 16"oc, cross braces. According to-Deflector Cal. is L337

Subfloor is 5/8" OSB possibly T&G, some unevenness, dips and joint lipping more on large area on one side by patio door entry. No bounciness but does not feel as stiff as should be for large tile. Alot of squeaking and creaks even though applied several screws on all seems only lessened somewhat. I am now at this point.

I will add 1/2 or 5/8 plywood perpendicular. Then follow with a membrane like Ditra and then tile.

But advised now of possible Deflection L337 but I can not get to floor joist thru basement to sister any joists, as room is finished ceiling. How do i accomplish this correctly. Am I on borderline deflection or do I need to do more to fix this, other than stiffen with another layer plywood running perpendicular?

Appreciate any input, thx Jake

  • Sanity check from someone who hasn't seen this issue before: Is that really intended to be l/360 (letter l), or a 1/360 ratio (number 1)?
    – keshlam
    Nov 23, 2014 at 22:44
  • Deflection is 'bounciness' or bend in middle of floor with weight. The deflector Calculator came up with L/337 but minimum is L/360….. LETTER, thx
    – Jake
    Nov 23, 2014 at 23:03

1 Answer 1


Typical requirement for US building codes is l/360 live load deflection limit. So it is not quite accurate to classify this as a correction if the design deflection is l/337.

If the finish material requires a stiffer floor, then a structural change is required for aesthetics and or performance, but not for structural safety. Structural modifications will normally require selective demolition of finishes whenever structure is not exposed.

The options are:

  • install the tiles with l/337 deflection and live with the possibility of aesthetic and or performance issues.

  • expand the project scope to modify the structure.

  • select a different floor finish.

  • Thank you Ben for your response If I decide to 'expand the project scope to modify the structure' How would I go about doing that. Since I have the floor clear would I remove the subfloor and access there but do they have to go from sil to sil or can they be sistered only in middle section of entire floor or do they have to run full width and full span? Are there any exception if there may be obstructions. Are there other alternatives than 2x8's. thanks.
    – Jake
    Nov 23, 2014 at 21:38
  • Or would it be possible if I am able to put a wall maybe a closet on each side near the exist steel support beam ,of room below which will then shorten the span?
    – Jake
    Nov 23, 2014 at 23:11
  • 1
    @Jake Engineering analysis of the structure is not necessarily a do it yourself project. Likewise, the design of an engineered solution. Thus, expanding the project scope may entail hiring an expert or substantial additional research on your part. A proper design for a site specific installation is a non-trivial exercise.
    – user23752
    Nov 24, 2014 at 1:11

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