Expert: Dan Griffin - 7/18/2008

Question
QUESTION: adding a 16 X 20 room. plan to tile floor using 2X6, 16'ctr. should i/can i put down cememt board dir. on joists? if so, how thick? Or, should i lay plywood and then layer of cement bd? if so, how thick for ea? or can i just lay tile on plywood? (and how thick?)  thnx!

ANSWER: Duane, a bit more information.

a 2x6 can only span about 9 feet based on load, deflection limits, type and grade of lumber.  Here is a fairly simple span calculator that is easy to use:
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp
in southern yellow pine, you will need 2x10s.
If you are talking about ceramic tile, you better limit deflection to L/480 or more.  L/600 will require 2x12w with 1 1/8 plywood.

Durock, wonder baoard, and most cement boards have no span ability and will require all rigidity from the joists and subfloor.

Ceramic tile can be installed on plywood with appropriate mastic - NOT THINSET - though I would not recommend it.  The deflection is still critical to the installation.

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

QUESTION: Dan; ans very helpful! I failed to say that my 2X6 joists will be on an existing 5" slab and will be shimmed every 2' for min. deflection. Ques: how thick should the cement bd. be if put over plywood?  thnx again!

Answer
Durock comes in 4x8 sheets and I would use 1/2", set in thinset with screws.
Hardie makes 3x5 AND 4X8 in both 1/4" and 1/2".  If the floor is rigid enough, either will be satisfactory.
Custom has a new product out called easy mat which looks good, though I have not used it:
http://www.custombuildingproducts.com/ProductCatalog/SurfacePrep/WaterproofingAn
They also have regular and light weight cement boards:
http://www.custombuildingproducts.com/ProductCatalog/SurfacePrep/Backerboards/?u

Rigidity is crucial to good tile work.  Wood and concrete move at different rates based on temperature and humidity, the cement boards help make the trasition.