What is the allowable deflection of a substrate suitable for a porcelain tile?


We’re debating the ability for porcelain tiles to accommodate deflection in a timber frame building under construction here in the UK, as we would ideally like these tiles on the first floor. I’ve looked for guidance on the internet, and you are the only expert I can find!

We have been told that a 12mm thick porcelain tile is able to accommodate a maximum deflection of 2.5mm over its 600mm length without the likelihood of cracking.

I wonder if you would be kind enough to comment.


ANSWER - The standards for a porcelain tile is that the substrate can not have more deflection than L/360.  For natural stone over a wood subfloor it is L/720.  L stands for the length of the span that you are evaluating in whatever unit of measure you are using. 

 You can add bracing to the floor joists in order to reduce the deflection.

2 thoughts on “What is the allowable deflection of a substrate suitable for a porcelain tile?

  1. Dave says:

    I’m looking for validating a floor assembly and meeting deflection requirements.
    ¾” Advantech subfloor
    Hydronic tubes
    Marble tile

    I’ve been in the TCNA handbook, ANSI and NTCA reference manual, trying to find how to get the deflection rating needed. I have also left messages with the floor truss manufacturer.
    I’ve spoken with Veteran NTCA members as well.

    • Donato Pompo says:

      RH141-22 Stone in the TCNA handbook does give you a an assembly based on a 16″oc.

      If you need to determine the rating on a certain tile assembly different from this then you need to hire a structural engineer to do the calculations. You might have trouble finding someone knowledgeable enough to figure this out.

      Ceramic tile (porcelain tile is a ceramic tile) maximum deflection requirements per industry standards is L/360 whether it is over a wood subfloor assembly or a concrete slab. For natural stone the maximum deflection over a wood subfloor assembly is L/720 and over concrete slab it is L/360.

      Floor joists give ratings for deflection but that is along the span of the joists. More important is the span between floor joists. That is why it is always good to add blocking between floor joists when in doubt.

      There is no practical way to measure for deflection. Engineers do mathematical calculations to determine hypothetical deflection limitations.

      The rule of thumb in the industry is if you put a full glass of water on a wood subfloor and jump up and down next to it then it should not spill. If it does add blocking between the floor joists and/or add an other layer of a wood underlayment. For suspended concrete slabs you have to rely on what the engineer says. So the tile installer should not take this responsibility, but rather tell their client to verify it.

      For walls, if you can push on the vertical surface and it moves, then that is too much movement. So had some bracing between studs.

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