# Flooring -- what is 1/360 deflection?

• posted on January 19, 2005, 6:12 pm
I have read how to install tile over plywood flooring at various websites. They say the maximum floor joist should be 16" on center with 'a maximum deflection of 1/360 of the span, at midpoint.'
What does this mean? 1/360th of an inch movement (up & down) between joist?
I saw this at Super-Tek.com
Thanks, Dave
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• posted on January 19, 2005, 7:17 pm

...
What it says...max (vertical) deflection (inches) = Span (in inches)/360
Span is unsupported length of the joist.
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• posted on January 19, 2005, 7:20 pm

If the span is 20' that is 180". If you divide that into 360 segments, you get 1/2"
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• posted on January 19, 2005, 7:32 pm
Edwin Pawlowski wrote:

It means the floor joist is not suppose to bend more than 1 degree at the mid point. The industry claims greater deflection will cause the tiles/joints to crack if more than that. This can be true when you get into larger tiles.
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• posted on January 19, 2005, 8:03 pm

Nope...
Length of the joist divided by 360.
So, if the joist is 240 inches (20 feet), you are allowed to have up to 2/3 inch (.666666") of movent at the center point of the joist. 240/360=.6666...
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• posted on January 19, 2005, 11:21 pm
If deflection is the vertical movement of the floor joist span measured at midpoint, there needs to be a set (known) weight to obtain this measurement.
A small child standing on the center of the joist will give a different reading than an elephant.
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• posted on January 20, 2005, 1:58 am

I am sure not an expert on these matters. From what I can read in my reference books the deflection factor is based on live load (as in a floor) or dead load (as in a roof).
Children are permitted in residential construction. Elephants are not likely.
I'm lazy or I would get the book and quote the "load per square foot" factor.
Colbyt
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• posted on January 20, 2005, 3:30 pm

Most building codes have the following limits for residental construction. Total deflection 1/240 (for both dead and live load) Live load deflection 1/360
The dead load is the mass of the floor and fixed structure. The live load is what can come and go such as people, furniture, etc.
A typical building code is 40 lbs/sq ft under live load for residental construction. For some other types of construction the live load requirement can increase quite a bit. For example a library under current codes has a maximum deflection of 1/360 with a live load of 150 lbs/sq ft..
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• posted on January 21, 2005, 2:45 pm
I thought that this might boil-down to the usual fire code/building code type of mumbo-jumbo... So many people within so many square feet of space... bla,bla,bla
But still.... a 120 inch span (10') of a single floor joist would be about 1/3rd inch up-and-down play... that seems (to me) like a heck of a lot of movement... tile fooling or not!
But, I guess, that's withOUT adding the strength of the subflooring that will tie the joist together.
Am I right or wrong? David
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• posted on January 21, 2005, 3:24 pm

Although what they're talking about is deflection, not bounce, it's still a lot of movement. But that's why many builders work to L/480 or higher. Like most of the rest of the code, L/360 is a MINIMUM standard. If you build to just barely meet code is all respects, you should end up with a crappy house that's livable, and not excessively dangerous to you or your community.
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• posted on January 21, 2005, 4:50 pm

Slightly wrong. the 1/360 deflection is for the floor as a whole. But with that said, the live load is *everything* that is not part of the structure itself. This includes such items as entertainment centers, furniture, bookshelves, and of course people and animals. It is highly doubtful that you'll ever encounter a floor that's moving 1/360 every day as people walk about on it. But like another poster stated, the 1/360 is a minimum requirement and a lot of builders build to 1/480 or even stiffer.
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• posted on January 19, 2005, 8:11 pm

What it means is that the allowable deflection of your floor is 1/360 of the span.
Example: span = 360"
360 / 360 = 1"
allowable deflection at the center of the span is 1"
--
Robert Allison
Rimshot, Inc.
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• posted on January 19, 2005, 10:07 pm
I think they mean 1/360th of the span of the floor joist, length-wise. Tom Work at your leisure!
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• posted on January 20, 2005, 5:46 am
Re: Flooring -- what is 1/360 deflection?
allowable deflection is the span/360
example a 15ft span would have 1/2" alloable deflection