# Joist size and span under tile.



## Bob Mariani (Dec 1, 2008)

joist size depends on span and spacing. Together they help determine the deflection factor of the floor. This is what tells you that a floor is sufficient for tile. Post a more detailed question.


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## Clutchcargo (Mar 31, 2007)

You can use:
http://www.johnbridge.com/vbulletin/deflecto.pl
or
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
to calculate deflection.
the general rule of thumb is L/480 for tile, L/720 for stone.


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## jackson_norman (Aug 6, 2009)

Bob Mariani said:


> joist size depends on span and spacing. Together they help determine the deflection factor of the floor. This is what tells you that a floor is sufficient for tile. Post a more detailed question.


 Are 2x8 joists acceptable to tile over. Thee is a 5/8" sub-floor and 1/2 " underlayment plus 1/4" Hardy Board between the underlayment and tile. The joists are 16" on center and are 12' long. Will this be ok?


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## Bob Mariani (Dec 1, 2008)

This translates to a deflection of L / 302. you are close but not ideal. I would have removed the 1/2" underlayment and installed 3/4" exterior grade plywood. Hardi board should not be installed over underlayment, since it is installed with thinset and the underlayment does not have water resistant glue. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.


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## ccarlisle (Jul 2, 2008)

Would doubling up his 2x8's - i.e sistering two together - help here?


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## jackson_norman (Aug 6, 2009)

Bob Mariani said:


> This translates to a deflection of L / 302. you are close but not ideal. I would have removed the 1/2" underlayment and installed 3/4" exterior grade plywood. Hardi board should not be installed over underlayment, since it is installed with thinset and the underlayment does not have water resistant glue. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.


 Hi Bob, My floor as it is at the moment has linoleum covering. It is in bery good shape. Can I use thinset over the linoleum to adhere the Hardy Board to the floor? I can sister a 2x6 to the existing floor joists but it would be a major job as the room beneath the floor is finished off with suspended ceiling. Thanks for the input. Norm


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## Bob Mariani (Dec 1, 2008)

Sistering new joists will help. Blocking will help. Straping (runing a board across all joists will help a lot.) This creates an I-Beam structural floor.

You cannot use thinset over the linoleum. It will not adhere. Remove it and the underlayment.


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## jackson_norman (Aug 6, 2009)

Bob Mariani said:


> Sistering new joists will help. Blocking will help. Straping (runing a board across all joists will help a lot.) This creates an I-Beam structural floor.
> 
> You cannot use thinset over the linoleum. It will not adhere. Remove it and the underlayment.


 Thanks Bob for all you help. Greatly appreciated. Norm


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## Termite (Apr 13, 2008)

I moved your thread from the landscaping subforum to where it belongs! :thumbsup:


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