TileLetter is the industry's leading tile magazine

Tuesday, May 21, 2024

The industry’s leading tile installation magazine

HomeTechnicalAsk the ExpertsUsing L/620 and L/720 on stone floors

Using L/620 and L/720 on stone floors


I’ve got a quick technical question. I know natural stone tile requires a second layer of subfloor. Does that apply to natural stone mosaics as well? Specifically a 3/4″x3/4″ marble mosaic. The substrate (based on the John Bridge deflection calculator) was L/620. I’ve poured self leveler over the whole floor ranging from 1/8″-5/8″ thick. Ideally I would just add DITRA Heat Duo (5/16″) and then KERDI DS (20mil) (Schlüter rep approved this method for small tiles over DITRA). I feel like this would result in at least a rating of L/720 but I wasn’t positive and I also didn’t know if the L/720 rating was even necessary for such a small mosaic. Thanks in advance!


I will break down my response to your question in a few components.

1. Background: For horizontal stone installations on wood framed floors, per TCNA Handbook Method F250, a layer of plywood underlayment is required on top of the subfloor to achieve a minimum L/720 deflection rating for stone.

a. Question: Is the second layer required for all dimensions of stone including mosaics such as 3/4″ x 3/4″ marble mosaic?
b. Short answer: Yes. There is no size description listed for stone in smaller sizes over wood framed structures therefore the second layer is required.
c. Longer answer: In mosaics it is quite rare to see cracking of the stone – if something has to give it will nearly always be the grouted joint between mosaic pieces.
d. Please see the accompanying graphic used with permission of the Natural Stone Institute. This is the basis for the two-layer floor sandwich mandate. The bending moment over the joists is actually greater and sharper than the bending moment at midspan. With a single-layer floor, or a floor with a wimpy underlayment layer, the discontinuity resulting from a seam in the subfloor over a joist causes a “hinge effect” at that location and we see cracked stones as a result. Ceramics tend to be both stronger and more flexible than stone, so those types of tile can get by with the deflection, but stone can’t.

2. Many underlayments used as substrates (CBU, FHCU / SLU, etc.) are not designed to remove deflection. The total system weight, included underlayment, must be included when engineering the wood structure and calculating deflection for dead and live loads.

3. I am not certain about the effectiveness of applying a second layer of wood over the SLU (now officially known as FHCU for Flowable Hydraulic Cementitious Underlayment) that you have already poured at the thicknesses you’ve described. The FHCU (SLU) may or may not have the ability to improve the deflection rating of the system. FHCU products I am aware of that have been designed for use over a wood structure contain fibers or other materials to add strength. Only the manufacturer of the FHCU product you use can answer this for you. Please contact the FHCU manufacturer to ask whether the product you used at the thicknesses it was poured will increase the deflection rating. Let me know if you need help in contacting your FHCU/SLU manufacturer and I can get you connected.

4. As a general rule, all membranes should be installed directly beneath the tile/tile bond coat layer. Manufacturer instructions for certain products like FHCU/SLU; uncoupling membranes that allow for pouring SLU/FHCU on top of them will take precedence.

5. The Natural Stone Institute Dimension Stone Design Manual and the TCNA Handbook for Ceramic, Glass and Stone Tile Installation require L/720 for all installations. This is measured for the entire floor and between supporting/structures (i.e. floor joists). It is very important to consider the deflection between joists. As illustrated and described above, problems tend to occur on top of the joists where load deflections ratings are inadequate to support the live and dead load. Again, this is more critical for stone than ceramic/porcelain tile and problems can occur in individual stones or in the grout joints between the stones. This can include grout joints between mosaic-sizes stones.

6. More resources on this topic include:

  • ANSI A108.5 (installation instruction for Setting Ceramic Tile with Dry-Set Cement Mortar)
  • TCNA Handbook Method F250
  • TCNA Handbook Natural Stone Tile Selection and Installation Guide
  • NTCA Reference Manual Chapter 2 – Substrates –Section: General Contractor’s Guide for Ceramic and Stone Tile Installations over Engineered Wood Structural Floor Assemblies.
  • NTCA Reference Manual Chapter 2 – Substrates – Tile Over Wood Sub-Floors – Letter

NOTE: NTCA was supported in this reply by the Technical Director of the Natural Stone Institute. My use of their verbiage and diagram are with their permission.

I hope this helps!

– Mark Heinlein

NTCA Training Director

NTCA Technical Team
+ posts
- Advertisment -

Most Popular

- Advertisment -