# Sizing a Roof Rafter in the IRC International Residential Code



## jar546 (Jun 24, 2013)

I want to try this out for fun and educational purposes.  I have, what I believe is the correct answer, but you never know.  I am proposing the anyone who is interested post whether or not you would approve this installation.  This is something at any code official would have to do when plans are submitted.  There are some rules for this so that we ensure it is educational and don't get of topic.  Sorry to be a bit rigid on this:

1) Either you would approve it on the plans or you would not.

2) The question is as is, your local variations, if applicable are not relevant.  This way everyone is in the same situation.

3) We are looking at this from a structural standpoint for the size of a rafter only, energy, and other items not applicable.

4) If you say no, please explain why and cite not only the code reference but how you determined it is not.

5) This is a prescriptive question, referring to an engineer/RDP is not an option.  If you are certified under the IRC with the ICC, you certainly should be able to.

6) If you say yes, please explain how you arrived at your answer.

I really hope this becomes a good topic of discussion and we all learn something from it.

Disclaimer:  This is not an actual situation that I or anyone I know currently has, this is hypothetical for discussion/education.

You are given plans for a home with a simple gable roof.

The roof will have a 6/12 pitch.

They will use asphalt shingles and the total dead load will be 10psf.

The ground snow load will be 40psf

The rafter span is 16'.

The height of the peak is 8'.

They will not be placing the ceiling joists at the top of the support walls, instead they will be raising the ceiling by placing the ceiling joists 24" up from the supporting walls tying into the rafters.

Framing will be with #2 SPF 16" O.C.

The framing details call for 2x10 SPF.  Is this allowable and why or why not?  Again this is prescriptive.  Let's have some fun!!!


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## fatboy (Jun 24, 2013)

I'll have to wait till tomorrow and back in the office.........sounds like a fun assignment! Knee jerk, I think it would fly...........

"height of the peak is 8'" concerns me, what is the height of the heal cut? Will the ceiling joists still function as a tie?


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## jar546 (Jun 24, 2013)

8' rise as posted, not 8"


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## ICE (Jun 24, 2013)

jar546 said:
			
		

> 8' rise as posted, not 8"


Fatboy has mustard on his screen.  A corn dog just isn't worth eating without mustard.  Sorry fatboy but a corn dog isn't worth eating no matter what you put on it.  You can't make a corn dog worth eating even if you wrap it in bacon and bacon fixes anything.

2' up is still in the lower third and I recall reading somewhere that ceiling joists in the lower third can qualify as rafter ties.

I haven't found that in the IRC.  That doesn't mean that it's not in there....just that I can't find it.  A bunch of the forum members are up for writing a new building code....that should be in there....if it works that is ....and so far all you've got is my poor memory....so probably not huh.

Oh crap, there I go again.  This was supposed to stay on topic.  Okay I'll say yes it works and shut the Hell up.


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## Uncle Bob (Jun 24, 2013)

Just woke up but providing they meet these requirements, sokay:  2009 IRC, R802.3.1 & Table 802.5.1 (9).  Gotta go to work, ya'll be good to yourselves.  Uncle Bob


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## steveray (Jun 24, 2013)

0.75 reduction in span for 1/4 the way up (raised)...my CT specific book only has 30# load....18-5 there, times .75 eqals something like 13.8? with 30# so no way for 40.....

Sorry not super accurate but the path is there....


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## jar546 (Jun 24, 2013)

Steveray, I am surprised that CT taps out at 30psf gsl seeing how you are north of PA where our snow loads vary anywhere from 30-55 (that I am aware of).  The 2009 IRC has the 30 and 50 gsl in it and we always have to interpolate which is what is needed here.

BTW, I agree with your assessment.  This will require a 2x12 to prescriptively meet the code.  A 2x10 won't make this span.  That is my opinion of course......am I right???


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## RJJ (Jun 24, 2013)

I would agree! 2x12 for 16' span @ 40lbs snow load. they could meet the code if they went to 12" o.c.


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## jar546 (Jun 24, 2013)

RJJ said:
			
		

> I would agree! 2x12 for 16' span @ 40lbs snow load. they could meet the code if they went to 12" o.c.


I agree but don't think they will want to frame the rafters and ceiling joists at 12" O.C. due to costs but you never know (insulation issue too but that's another story)

So what is the maximum rafter span for this question?:

2x10 @ 16" O.C.?  __________

2x12 @ 16" O.C.?  __________

2x10 @ 12" O.C.?  __________

Let's see what kind of answers we get.  I have mine!


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## globe trekker (Jun 24, 2013)

2x10 @ 16" o.c., Table R802.5.1(5), 30# snow load, 2006 Edition of the IRC,

..max. span = 18' - 5"

2x10 @ 16" o.c., Table RR802.5.1(6), 50# snow load, 2006 Edition of the IRC,

..max span = 15' - 1"

extrapolation = 3' - 4" (difference) / 2 = 1' - 8", added to the 15' - 1" = 16' - 9"

span capability for a 40# snow load.

.


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## jar546 (Jun 24, 2013)

globe trekker said:
			
		

> 2x10 @ 16" o.c., Table R802.5.1(5), 30# snow load, 2006 Edition of the IRC,..max. span = 18' - 5"
> 
> 2x10 @ 16" o.c., Table RR802.5.1(6), 50# snow load, 2006 Edition of the IRC,
> 
> ...


Great job but I think you are forgetting about the location of the ceiling joists......................  So according to your calculations, a 2x10 would work in this instance and you approve it?


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## RJJ (Jun 24, 2013)

From my calculation at 40# snow load I got 15' 3"


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## jar546 (Jun 24, 2013)

RJJ said:
			
		

> From my calculation at 40# snow load I got 15' 3"


Is that your final answer?  I assume it was for 2x10 @16" O.C.

BTW, I disagree with your answer.


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## globe trekker (Jun 24, 2013)

Alrighty then, ..let's try this again!

From the 2006 Edition of the IRC, Table R802.5.1(3), *..ceiling not attached to rafter,*

#2 SPF, *30# snow load*, 2x10 16" o.c. = max. span = 18' - 5"

From Table R802.5.1(4), *..ceiling not attached to rafter,* #2 SPF, 2x10 16" o.c.,

*50# snow load* = max. span = 15' - 1"

extrapolation = 18' - 5" minus 15' - 1" = 3' - 4" / 2 = 1' - 8" + 15' - 1" = 16' - 9",

assumed 40# snow load.

I still vote for the 2x10 @ 16" o.c.

.


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## jar546 (Jun 24, 2013)

Why are you using the "ceiling not attached to rafter table"?

In this case, the ceiling to rafter connection is present so you don't have to use that table.  For this application, it makes no difference.  You are still missing a calculation that makes all the difference.  Did you read the entire table including the end on the next page????


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## globe trekker (Jun 24, 2013)

..about the Rafter Span Adjustment Factors?

When using the 1/4 adjustment factor (2' up / 8' peak height),

a 2x12 @ 16" o.c. would be required!

.


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## jar546 (Jun 24, 2013)

globe trekker said:
			
		

> ..about the Rafter Span Adjustment Factors?When using the 1/4 adjustment factor (2' up / 8' peak height),
> 
> a 2x12 @ 16" o.c. would be required!
> 
> .


You win!  Nice job.  Yes, 2x12 and now you know why.

OK, who else is as smart as Globe trekker?


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## RJJ (Jun 24, 2013)

Yes 2X10 @16 OC. max span 15' 3" that is what I stated 12 OC. If factor in the 24' attachment for ceiling joist you have Globes answer.

You need 2x12 @ 16 or 2x10 @12" which ever is cheapest


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## RJJ (Jun 24, 2013)

I did not have a code book for the factor of raising the ceiling joist. I did this on the fly with a rafter calculator.


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## jar546 (Jun 24, 2013)

The factor for 24" up of the 8' rise is 1/4 which is .76 reduction.


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## RJJ (Jun 24, 2013)

Ya that in the code book. I am on the road!


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## High Desert (Jun 24, 2013)

jar546 said:
			
		

> You win!  Nice job.  Yes, 2x12 and now you know why.  OK, who else is as smart as Globe trekker?


Apparently not me!


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## jar546 (Jun 24, 2013)

OK, let's keep this going.  Does everyone agree with this?  Does it sound like overkill?  Does everyone know how to do this?  Did anyone learn anything?  Did I let the cat out of the bag too soon by confirming who got it right?  Does anyone care?


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## Rick18071 (Jun 25, 2013)

How about rafter spans for cathedral ceilings with a ridge beam? Could you use the tables. Would the span tables be over kill or too weak?


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## jar546 (Jun 25, 2013)

The IRC has tables for that.  Ceiling joists NOT attached to rafters.


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