rafter size for kitchen bumpout on Victoria's Cottage

[MikeT]

I will be framing the kitchen bumpout on the VC this weekend at the coast.  But I have deviated from the plans in that I will be changing the pitch of the bumpout portion of the roof.  That shed roof will be a 4:12 pitch.  The reason is that it sits on the uphill side of a slope and I will have a parking area right in front of it, and I do not want the wall to appear to be too low.

Anyway, my question concerns the size of the rafters for this bumpout.  I framed the main portion of the roof with 2x12's at 24" o/c.  For the bumpout, what size should I use? 

Thanks,
mt

[MountainDon]

Are there ceiling joists with insulation on top, or is the ceiling vaulted and the rafters insulated?

[MikeT]

I was planning on insulating between the rafters and then covering with wallboard.  Does that help?

[MountainDon]

If you're doing that then you'd probably want to size the rafters for their ability to contain all the insulation you desire, rather than sizing the rafters to the minimum required for the span. (not sure what that is). 2x12's would let you use R-38 fiberglass batts and end up with an R-37 effective rating.

insulation chart here

[John Raabe]

You could structurally use the 2x6 @ 24" o/c shown in the plan (for the 5' span) and just lower the pitch. However, that would require foam insulation to get the required R-value and Don's suggestion may be less expensive if you aren't using foam anywhere else.

[MikeT]

Thanks, John.  I went ahead and opted for the 2x12 rafters.  I posted a question in the General discussion area about how to handle the small gap in rafter height at the transition between roof pitches that results from having my seat cuts/birds mouths in the rafters.

mt