Deflection Limitations By Dale Kempster

Deflection Limitations
By Dale Kempster

The TTMAC (Terrazzo, Tile and Marble Association of Canada) released the completely revised 1997 "Specifications Guide 09300" which succeeds the 1989 edition.

In the new "Tile Installation Manual" the reader will find several references to ".... deflection under both dead and live loads does not exceed 1/360 of span." This reference can be found in details; 310F-98, 311F-98, 312F-98, 313F-98 and others. For many individuals this criterion of deflection may not be easily understood; both for the calculation and the actual ramifications for the installation in question.

First, a definition of deflection should be made; "A variation in position or shape of a structure or structural element due to effects of loads or volume change, usually measured as a linear deviation from an established plane rather than an angular variation". In other words, when a load is applied to a surface such as a plywood floor, that floor will often bend downwards in relation to the amount of weight applied and the structural capability of the plywood and the support ( i.e. joists) underneath.

It should be noted that the deflection is measurable both as the length of the span, as well as between joists, in the case of a residential application. In actual practice, what this means to the installer is that they have to know if the sub-floor will be able to support the tile and not exceed the deflection criterion of L/360.

In practical terms this means in a residential application with joists 16" o.c. (400 mm) the maximum deflection that can be allowed when using L/360 is 16/360 which equals 1/25"= .04"or (1 mm). Now how does this relate to a single layer of plywood sub-floor. When looking at the span tables published by the American Plywood Association, it will be found that a single layer of plywood 19/32" (15 mm) on joists spaced at 16" o.c.(400 mm) the maximum amount of weight that can be applied is 368 lbs (167 Kg) before the deflection succeeds L/360 (remember 1/25" or 1 mm)

Now, this may seem like a lot of weight, but this is a uniform load distributed over a wide area that is supported by the joists. If a concentrated load is applied between the joists using a 3"(75 mm) wide disk with 200 lbs (91 kg) applied to the same piece of plywood, the plywood can deflect, up to approximately .078" = 1/12.5" or 2mm. This calculation indicates that the single layer of plywood can have approximately twice the deflection acceptable for the installation of tile. In real terms, a concentrated weight of 200 lbs (91 kg) over a 3" (75 mm) wide area, is not all that difficult to attain in a situation such as a fridge or a stove being brought across a floor on a dolly.

With all of this being said, it can now be understood where the recommendation for two layers of plywood 5/8" (16 mm) thick or the use of other underlayments, is required by the TTMAC manual, Detail 313 F-98. Other underlayments such as ½" 13 (mm) cementitious backer board, or a pre-formed sheet applied membrane such as Schluter Systems, Ditra Matting can be considered.

Another consideration as well, is post-tensioned and pre-stressed concrete are often engineered with only L/240 deflection criterion. This means a slab 30 ft long, supported only at the ends, (L/ 360 = 360/360) could have deflection up to 1 " (25 mm). See TTMAC Detail 309F-98 for recommendations on installation over pre-cast concrete systems.

To give more food for thought, recently the Marble Institute of America has increased the deflection criterion to L/720, which is double that of tile. It is with all this in mind, that one should make sure that they are well acquainted with the new manual and its practices. Installation techniques and systems should be evaluated and be guaranteed to respect the L/360 criterion.

Dale Kempster, CTC, has been in the Tile industry for over 11 years and is employed by Schluter Systems. Currently, he is an active member and Chairman of the TTMAC Specifications, Technical Research Committee. In the United States, Dale is a member of the MMSA (Material, Methods & Standards Association) and is currently working on a sub-committee creating an ANSI standard for Crack Suppression Membranes. Schluter Systems is a manufacturer of transition profiles, prefabricated movement profiles, pre-fabricated stair-nosing and waterproof, crack-suppression membranes.