2024-25_NTCA_Reference_Manual_dig_dwnld

General Statement on Deflection For Informational Purposes Only

Deflection as it applies to the ceramic tile industry is the curvature induced by live and dead loads on the flooring structure of an elevated/suspended concrete slab or other substrates that will receive tile as a finish. The normal structural design criteria for allowable deflection is L/360 where L = the span in inches. These criteria must be met by the substrate prior to the underlayment or finishing material application. An example of how this measurement is used is as follows: If you have a suspended slab that measures 30 feet across with no intermediate supports, the “L” or length of the area is 360 inches. When this measurement is placed over the number 360, the result is 1 or 1” (360/360 =1). This means the slab or flooring structure can “deflect” or bend 1” without negatively impacting the structural integrity of the tile installation. If, for example, there was an intermediate support in the center of that same area, the “L” would measure 180; (180/360 = .5). The allowable deflection of this structure is now 1/2 in. The L/360 criterion applies to the full length of a room but it also applies to any segment within that room such as the span between floor joists in a wooden structure. This shorter span between joists is of great concern especially when the joist spacing is increased from the standard 12 in. or 16 in. spacing to 19.2 in. or 24 in. on center.

With this information in mind, we can say that deflection is the amount of vertical movement or curvature permitted by a substrate that will receive tile, and that this movement shall not exceed L/360.

It should be noted that measuring deflection is very difficult and should not be calculated by the tile contractor. It is the design professional’s responsibility to design a substrate that meets or exceeds the minimum stiffness criteria and to measure and evaluate deflection in actual assemblies.

When installing stone tile and other tile, the level of permitted deflection is decreased to L/720 according to the Natural Stone Institute (NSI).

A108.01-2.2.7 Deflection All substrates over which tile will be installed, shall be in conformance with the IRC for residential applications, the IBC for commercial applications, or applicable building codes. Note: Glass tile may require more stringent deflection ratings. Consult tile manufacturer for additional requirements.

CALCULATION EXAMPLE: L/360, where L = the span in inches.

On a suspended slab that measures 15 ft. across with no intermediate supports, (L) or length of span is 180 in. When the measurement (L) is placed over the number 360, the result is 0.5 (180/360=0.5). The deflection measurement therefore is 1/2 in. If a support were placed in the center of the span, (L) would be 90; (90/360-.25). That deflection measurement is 1/4 in. Typically, this measurement is made before the subfloor is installed on wood systems and on the concrete floor for concrete systems. This is a very rudimentary example and other factors must be considered for an accurate calculation. For example, the tile industry references a 300 lb. concentrated load in its calculations. The criterion for measuring deflection is not a settled issue in the industry and you should always consult with your tile and setting material manufacturer for their specific deflection recommendations. When installing stone, refer to Natural Stone Institute (NSI) requirements regarding deflection.

44

NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates