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Q.The plans for a new house call for saltillo tile over a 3/4-inch plywood subfloor with 1/2-inch concrete backerboard (Durock) thinset and screwed to the plywood. The floor joists are Dietrich Trade Ready Floor System 12-gauge steel joists at 16 inches on-center spanning 24 feet. In your opinion, should I use an isolation membrane under the saltillos?

A.Michael Byrne responds: I am not comfortable tiling any span over 20 feet without an isolation membrane, unless I can get an assurance from the joist supplier that their system will provide the deflection resistance required for most ceramic tiles:

Maximum deflection = L/360 (where L is the span)

Keep in mind that tiles require the L/360 for both uniform and concentrated loading (loading between the joists, see illustration). Many floor joist systems meet L/360 maximums for uniform deflection only, so make certain that the system you want to use — sawn, composite, or steel — meets the requirements for tile.

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The manufacturer of your system has an online chart (www.dietrichmetalframing.com) that can tell you if the particular steel joists you selected provide the strength you need. I suggest you log onto their span tables, email your questions and concerns to the company, and ask for an assurance in writing that the system will work as advertised.

Contributing editor Michael Byrne is an expert tilesetter and consultant in Los Olivos, Calif.