Expert: Dan Griffin - 8/25/2008

Question
QUESTION: Hello Mr. Griffin -
We just remodeled a bathroom on the second floor supported by 2x8's with a 1/2" plywood subfloor, 1/4" concrete board and a marble tile on top. We installed a clawfoot tub that is 379 lbs empty and 769 lbs filled. The house is about 20 years old. Will our floor support this weight without additional support? Does it meet standard building codes? I am getting conflicting reports and I cannot tell if it is actually needed or if someone is just trying to get another job out of me (and more money!)
Your expert advise is very much appreciated.
~ Kevin

ANSWER: I will be surprised if the tile job does not fail.  Ceramic tile demands limiting deflection to L/360 and I would suggest much stiffer than that.  1/2" plywood cannot provide L/360 stiffness with 16" o.c. joists.  The concentradted load of a claw foot tub would be out of the question in my opinion.  Here is some plywood loading information:
http://www.apawood.org/pdfs/download_pdf.cfm?PDFFilename=managed/TT-006.pdf

TCNA (tile council of north America) does not show any application using  1/2" plywood and very few using single layer plywood.

Kevin, I can give you no information about the 2x8 unless I know the center to center distance and the length of the span involved.

---------- FOLLOW-UP ----------

QUESTION: concrete board was put on top of the plywood for added strength and waterproofing.
the 2x8's were installed on 16" centers.
thank you for your reply.

ANSWER: The 1/4" cement board helped provide an even and tile compatible substrate, it really didn't add any strength or waterproofing.  The 2z8 are good for 10-3 span if you used #3 Southern yellow pine or better, 16" centers, 40 PSF live load, and limit deflection to L/600 which will deflect about 3/8" at mid span.

---------- FOLLOW-UP ----------

QUESTION: so if the room is 168sf x 40psf then it can support 6710lbs live load weight, right?

Answer
Kevin, I used 40# per square foot as a typical residential required minimum load.  The big issues for ceramic tile are the spam of the joists which you still have never shared and the stiffness of the plywood crossing the joists.  If either deflectw (bendw down) too much, the rigid tile will pop loose, crack, or lose the grout.  The probllem is worse as the tile gets bigger.

The deflection of the joists involves a combination of span, lumber species, center to center distance, and  design load required (both uniformly distributed and point loading).  Your joists may meet the requirements, but my concern is the plywood flooring.  The  lightest design recognized by the tile council is two layers of 19/32 (nominal 5/8) exterior grade plywood.  Your 800 pound tub with a 200 person will impose a 250# point load on each claw foot that is resting on about 4 square inches.  It may last forever, but I suspect there will be too much deflection in the materials to provide a long lasting installation.