Some useful info. if you replace the floor (there's a June 2004 TileLetter article by the same people, called "Positioning the Underlayment to Prevent Cracked Tile and Grout" you can find if you google; I know you said you want hardwood now, but if you do the underlayment right, you could easily switch to tile down the road!):
Impact of plywood underlayment direction
By Frank Woeste and Peter Nielsen
Beginning in 2007, the TCA Handbook for Ceramic Tile Installation listed in bold print the following requirement for two-layer wood sheathing installation methods:
- face grain of plywood shall run perpendicular to joists for maximum stiffness.
Should the tile contractor or other interested parties be concerned whether or not the plywood underlayment runs perpendicular to the joists? Just how important is it anyway? Many times we’re given directives (for example, when following a method in the Tile Council of North America’s TCA Handbook For Ceramic Tile Installation), but if we’re not given the “why,” it’s difficult to know how much importance to assign the directive. You may be thinking “A directive is a directive,” so what’s to discuss?
But what if the tile contractor shows up at the jobsite and the plywood underlayment has already been installed by another trade—parallel to the joists. Calling “time out” and having the underlayment removed and re-installed is no small endeavor. It can throw the job off schedule and it can be costly. In this scenario, the tile contractor needs to know how much importance to assign the directive: run the plywood underlayment perpendicular to the joists.
The purpose of this article is to discuss the aforementioned questions and to emphasize the importance of the directive to run the plywood underlayment perpendicular to the joists, so that good decisions can be made in the planning process and in the field. We will also clarify the meaning of "stiffness" in the context of a ceramic tile installation. The questions will be addressed in the context of one two-layer system—F149-07 (joists 24-inches on center., 23/32-inch tongue-and-groove subfloor, 19/32-inch underlayment).
In general, framing contractors know that the “strength axis” or 8-foot dimension of the subfloor panels must run perpendicular to the joists. Even though the TCA Handbook does not specifically state that the strength axis of the subfloor panels shall run perpendicular to the joists, it remains a residential building code requirement (IRC, 2006). The 2007 TCA Handbook (and later) requires floor framing systems to be code-compliant as stated under the Requirements for each wood floor sheathing method. Tile contractors should be aware of the possibility that some subfloor panels may be installed incorrectly, with the strength axis running parallel to the joists. The impact of improper subfloor installation on ceramic tile performance could be severe, as the “stiffness” of an incorrectly installed 23/32-inch subfloor may be only one-fourth to one-fifth of a correctly installed code-conforming panel. (Panel stiffness data are discussed in more detail in the next section.)
Plywood underlayment direction
Knowing that the subfloor must be installed with the strength axis perpendicular to the joists for code compliance is important. But the tile contractor must also understand the importance of running the plywood underlayment perpendicular to the joists if he or she is to maximize potential for a successful installation. In fact, engineering science for the two-underlayment orientations reveals that both the subfloor and underlayment should run perpendicular to the joists (without exception).
“Stiffness” is a general technical term that quantifies the extent of a specific type of deformation due to a specific applied load. In the context of plywood/oriented strand board (OSB) installation, the type of stiffness of interest is “panel bending stiffness,” referred to as EI. EI of a wood panel is a measure of how much it will rotate from applied gravity loads such as dead and live loads. It is also used to calculate deflection of a panel between joists.
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We have reviewed the bending stiffness data for common span-rated wood sheathing panels and demonstrated how bending stiffness of two-layer wood sheathing systems can be maximized by installing both the subfloor “strength axis” and underlayment “strength axis” perpendicular to the joists.
Achieving maximum bending stiffness from both wood sheathing layers will “maximize the radius of curvature” or “minimize the bending action” of the wood sheathing/ tile assembly from gravity loads for the service life of the installation. We believe that, if a tile contractor understands the important role of the underlayment positioning as presented in this article, he’ll have the confidence to call “time out” should the situation arise.
Frank Woeste, Ph.D., P.E., is professor emeritus at Virginia Tech University, Blacksburg, VA, and a wood construction and engineering consultant. Comments are welcome and may be sent to email@example.com
Peter Nielsen is general manager and technical director for Schluter Systems L.P. (www.schluter.com
) in Plattsburgh, New York.
If you google, you can find that article on the internet in PDF format; it's too big to upload here.