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Topic:   Roof framing - 2743 visits (1 today, 3 this week)

Jeffrey Mathis
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Posts: 41
From:Grimesland, N.C
Registered: Feb 2005

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posted January 26, 2006 03:46 PM            

I did new construction today and the back side of the roof is framed with 2x8s 16" o.c.. The run of the middle rafters is right at 24 ft. (#2 yp). Pitch is 12/12. It just passed municipal code, but I swear, I hate not having a knee wall to break up that run. Granted in Eastern N.C. snow load isn't an issue, but what is anyone else's opinion?

JLMathis

Gunnar Alquist
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From:Santa Rosa, CA
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posted January 26, 2006 04:44 PM            

Nothing in the rafter tables of my UBC indicate that 24' unsupported span is acceptable for any lumber. I suppose that the IRC might be somewhat different, but not likely significant.

Tony Keith
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From:Aurora, IL
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posted January 26, 2006 04:52 PM            

Jeffrey, are you sure that you are asking about a 24' run with a 24' rise,(which would make this 2x8 about 34', and make for one heck of a tall roof) or are you asking in regards to a total length of lumber of 24'

Jeffrey Mathis
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From:Grimesland, N.C
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posted January 26, 2006 06:06 PM            

Your right, I misspoke. Run is horizontal. Rafter length was 24 ft.
JLMathis

Tony Keith
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posted January 26, 2006 06:42 PM            

Jeffrey, by all accounts, the span in question would be around 17', probably less due to an overhang. Are the rafters attached to ceiling joists? Yes?, IRC says ok. No?, then some more information is needed........dead load psf, and excact span measurement.

Jeffrey Mathis
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From:Grimesland, N.C
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posted January 26, 2006 07:01 PM            

No, the length of the rafter not including overhang is 24 ft. Normal shingle loading for 12/12 pitch. I've framed for a long time and I always did a knee wall just for safety sake. I remember building my own house and using TJI's for rafters. They sagged before the framing was done. THe sales rep. said it wasn't sag. It was deflection and OK. I made them buy me an I beam and designed it into the room. So I know what will pass for OK may not do well down the line.
JLMathis

Jim Baird
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From:Danielsville, GA
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posted January 27, 2006 07:13 AM            

Jeffrey,

24' rafter not counting overhang means a piece of lumber 24+ ft in length?

In yp(yellow pine)? I know NC has lots of wood, but down here in GA yellow pine in that length is really hard to get.

In that kind of length around here what we see is that finger jointed spruce stuff.

I agree with your intuition that some kind of braced purlin would be a good idea, but don't know if you could call what they have a defective arrangement.

Jeffrey Mathis
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From:Grimesland, N.C
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posted January 27, 2006 03:30 PM            

Well, Jim, I kind of agree. I thought it might be finger joint, but not. Was stamped #2, but spruce is possible. I get around an argument from a seller or seller's agent like I did this evening:
A different house was 4 years old and already deflecting at an unsupported ridge board intersection. I noted there were no vertical supports anywhere and that while it did pass a municipal inspection, I reminded them that all that was was the least you could get away with and municipal inspectors around here are over worked and underpaid.
JLM

Michael Greenwalt
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From:Junction City KS
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posted January 27, 2006 08:14 PM            

Ok, 2003 IRC, tables R802.5.1(1) to R802.5.1(8), minimum dimensional lumber needed to span 24' was 2x10. The maximum length a 2x8 can span is 23'-9" assuming they were 12" OC, Douglas fir-larch SS grade, (Roof live load 20 psf, cieling not attached to rafters) assuming that ceiling joist are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the vearing walls, such as rafter ties, is provided at that location.
At 16" OC the maximum span found in any table is 21'-7" and all the above applies.
So, in short, IAW IRC 2003, 2x8's cannot span 24' under any circumstances.

Michael Greenwalt
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From:Junction City KS
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posted January 27, 2006 08:16 PM            

Ummm, typing skills need improvement.
vearing is meant to be.....bearing.
scuze pleeze.