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Do rafters become joists at low pitches?

M

MikeC

Silver Member
Joined
May 17, 2012
Messages
230
Location
NW Pennsylvania
It makes sense to me that a rafter should be treated as a joist when dealing with low pitch roofs. I quickly scanned the 2009 IRC and didn't find anything that points me in this direction. Are they still rafters at a 1:12 pitch? I know that ridges, valleys, and hips become beams at less than 3:12, but can't find anything about the rafters.
 
R802.2 Design and construction.

The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater. Roof-ceilings shall be designed and constructed in accordance with the provisions of this chapter and Figures R606.11(1), R606.11(2) and R606.11(3) or in accordance with AFPA/NDS. Components of roof-ceilings shall be fastened in accordance with Table R602.3(1).

It does not require engineering per say, you just cannot use the prescriptive tables in the IRC for rafters. there are many after market programs that will calculate the roof framing members regardless of the pitch. Struc-Calc is the one we use in the office.
 
R802.2 Design and construction. The framing details

required in Section R802 apply to roofs having a minimum

slope of three units vertical in 12 units horizontal (25-percent

slope) or greater. Roof-ceilings shall be designed and constructed

in accordance with the provisions of this chapter and

Figures R606.11(1), R606.11(2) and R606.11(3) or in accordance

with AFPA/NDS. Components of roof-ceilings shall be

fastened in accordance with Table R602.3(1).
 
Thanks. Not sure how I missed that. I was looking for low pitch rafters to be addressed as something other than rafters and missed what was right in front of me.
 
MikeC said:
It makes sense to me that a rafter should be treated as a joist when dealing with low pitch roofs. I quickly scanned the 2009 IRC and didn't find anything that points me in this direction. Are they still rafters at a 1:12 pitch? I know that ridges, valleys, and hips become beams at less than 3:12, but can't find anything about the rafters.
Click to expand...
Yes they become Roof Joists

Definition

The rafters of a flat roof..
 
And don't forget 802.3.........Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.
 
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