on their own exceed 20 psf, which demonstrates that IRC span tables aren’t always enough. Research indicates that an even more important consideration for tile and stone floors in wood frame construction is the thickness/stiff- ness of the subfloor, although not necessarily because of system- creep-inducing overload. Rather, the subfloor sheathing could simply deflect (bend) between joists under an applied load more than a hard surface tile can withstand, even if the sheathing is otherwise adequate within the full design scheme to support the expected loads. This industry-specific consider- ation, not addressed in IRC, is addressed in the TCNA Handbook through more stringent deflec- tion limits. Specifically, the TCNA Handbook limits deflection under concentrated loads, whereas IRC deflection limits are for uniform loads. What this means for build- ing designers is that the mini- mum subfloor thickness/stiffness required by code for strength may not be enough. A thicker/stiffer subfloor may be needed to limit subfloor bending between joists. More robust framing may also be needed, again to go beyond the strength consideration to further limit bending related to concen- trated loads. The heavier and more concentrated the load, the greater the need to beef up the floor fram- ing to limit bending. An example: the large kitchen island As an example, consider the large kitchen island scenario. With 30mm (3cm) thick stone tops and normal contents being stored inside, this popular kitchen feature could present a 40 psf dead load, calculated by using the square footage of the island’s footprint as the area. In service, the framing and subflooring directly below and around the island is subjected to a substantial sustained load that produces creep deflection, but only in that area. As such, for hard surface floors, building design should incorporate more TECH TALK –––––––––––––––––––––––––––––––––––––––––––––––––––––––––– In Method F144, the wood subfloor can be 19/32” thick or 23/32” thick and relates to whether the installation methods falls under the resi- dential or light commercial ser- vice rating. Because this kitchen island is oriented parallel with the wood joists, its weight is on fewer framing members. 66 TileLetter | February 2019