# Can I get a quick rundown on floor deflection and large format tile concerns



## MikeGC (Dec 6, 2008)

1. Make sure the floor is solid enough if your going with a large format tile or natural stone.

2. L/360 vs L/720


These come from another post. 

Can I get an explanation on large format tile and natural stone concerns in regard to floor deflection and a minimum size that is considered large format

Can I get a quick lesson on deflection? Are there tables? If it can be measured, how is that done?


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## angus242 (Oct 20, 2007)

MikeGC said:


> Can I get an explanation on large format tile and natural stone concerns in regard to floor deflection and a minimum size that is considered large format


Ceramic and natural stone tiles are different beasts. Stone is more brittle so that's why less subfloor deflection is allowed. 
There really is no "minimum" size considered large format. It used to be 8"x8" but we all know that is ridiculous. I consider 14"x14"+ to be large format. The larger a tile is, the more perfectly flat a floor needs to be for a successful installation. Flat is not the same as deflection. You can have a perfectly flat floor with a deflection of L/420 and you cannot install 12"x12" stone over it. You can have a deflection of L/850 but if it is out of flat by 1/4" in 6', you'll have an almost impossible time installing 16"x16" porcelain. 



MikeGC said:


> Can I get a quick lesson on deflection? Are there tables? If it can be measured, how is that done?


Deflection is the concave movement of a floor joists structure under live and dead loads. There is a formula but it's not a simple equation. Species of wood also comes into play. There are deflection calculators floating around on the internet. Thing is to find one you trust. If the joists are anything other than dimensional lumber, I like to call the manufacturer to get their stats.


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## ee3 (Feb 10, 2006)

Currently, large format is any tile with a 15" side.(15" X ?" ).

DEFLECTION (more or less)
L"/360 = LENGH/ 360 or 720
that would allow you, .5 " in A 15' span @ L/360, under designed load.
also keep in mind that it should be measured in both directions so 16" spacing would be .044"
alot of people will apply a load in center of span and measure gap under a straight edge...


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## GettingBy (Aug 19, 2010)

ee3 said:


> alot of people will apply a load in center of span and measure gap under a straight edge...


Yes, and from this midspan concentrated load, the wood characteristics can be found and then the deflection of the same wood under a distributed live and dead load can be figured.


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