# Rated Ceilings



## Keystone (Oct 23, 2014)

What is the primary purpose of the minimum wood floor joist depth in a rated ceiling seperating upper and lower units, would the minimum floor joist depth be for  structural integrity or assistance with heat dispersion?

Example; National Gypsum UL L505 Floor/Ceiling 2 Hour Rated Assembly and many others stipulate 2" by 10" floor joists when using nominal lumber.

http://nationalgypsum.com/products/..%5CFile%5Cgolds.pdf

USG

http://www.usgdesignstudio.com/ceiling-assembly-selector.asp?bldgSystem=&framingType=18713&fireRating=2&stc=&iic=&thickness=&orderby=


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## steveray (Oct 23, 2014)

I would assume both.....


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## Keystone (Oct 24, 2014)

steveray - I also assume both but its been one of those nagging questions.


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## tmurray (Oct 24, 2014)

It's based on what was actually tested. If 2x10 gives them a certain hourly rating, increasing the size of the lumber will only increase the rating as the charring of the lumber adds an insulation layer causing a barrier to heat attacking the uncharred portion.


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## cda (Oct 24, 2014)

Air gap helps reach the rating ???

Assembly as in all works together to achieve a goal


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## steveray (Oct 24, 2014)

I don't know if that info is available anywhere.....it's not like a code change where you could maybe find some discussion or reasoning, it's pure data and it passes the listing test or it doesn't. No further justification or discussion needed....Someone here that has spent time at UL might have better insight...


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## cda (Oct 24, 2014)

steveray said:
			
		

> I would assume both.....


Sorry ,,,,    Best answer

Has to hold up to weight

And has to hold up to fire rating


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## TJacobs (Oct 25, 2014)

I like tmurrays' answer............


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