- 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.
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.