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Unread 02-24-2012, 09:24 AM   #1
Tile Contractor, North East MD. Retired ATC
Join Date: Aug 2007
Location: Eastern Shore Maryland
Posts: 39
Natural Stone Bathroom Floor Sub-Floor and Heat

Hello Mudslingers,

I'm working with a local builder who is remodeling a good sized bathroom. The clients have specified large format, (18 x 18) natural stone tiles, and want the floor heated electrically. The construction in this approx. 16 year old house is of good quality, but the span of the floor joists in the area of this bathroom is about 13' 6". I have plugged the numbers into the Deflecto with these results:

Thank you for using the John Bridge Forums Deflect-O-Lator :-)

For joists that are SYP or Douglas Fir, in good condition, 11.25 inches tall, 1.5 inches wide, 16 inches on center, and 13.5 feet long between supports, the deflection calculated is 0.241 inches.

This translates to a deflection of L / 672.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!

Doubling up the floor joists doesn't seem like a practical solution due to the wiring and plumbing that already exists. I am "assuming" that the Deflecto assumes a single 3/4" plywood subfloor. Would the addition of a second layer of 3/4" plywood, glued and screwed, provide enough benefit to accommodate stone?

The ultimate height of the finished floor is somewhat of a concern, as the master bedroom that adjoins the bath will be carpeted. I have always used the method of embedding my heating elements in self leveling cement in the past, as opposed to the "plastic trowel, thinset" method. My concern here, again, is for the ultimate thickness of all of these components, plus the requisite anti-fracture membrane. My tile distributor swears that the use of the leveling cement alone over the doubled up plywood subfloor would be sufficient, absent ANY anti-fracture product but.............

Whaddya Think?
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Unread 02-24-2012, 09:43 AM   #2
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Location: Boerne, Texas
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Keep in mind that Deflecto calculation is only valid if you've verified that your joists are of good specie and grade.

You could consider sistering the lower portion of your joists with smaller (2x8) lumber to achieve the required L/720 deflection.

The Deflectometer does not consider any subflooring at all. Adding more subflooring, which is mandatory for your natural stone, makes no appreciable change in the joist deflection. Two completely different areas of deflection and each must be addressed separately.

The use of an uncoupling membrane over radiant floor heating systems is optional, but I would install tile over such a system without one. Up to you how you deal with yours.

My opinion; worth price charged.

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Unread 02-24-2012, 02:07 PM   #3
Tile Contractor, North East MD. Retired ATC
Join Date: Aug 2007
Location: Eastern Shore Maryland
Posts: 39
Thank You

Thank you kindly Dr CX for your timely reply. The builder will sister up the joists to the maximum extent practical. We will verify the markings, if any, on the joists when they are thus exposed.

I appreciate your expertise and your willingness to share.....
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