Bowed Rafters

[tbent]

I live in Donnellson, Iowa (South-East corner) at the Northern-most section of what we call Tornado Alley. We are planning to open up the attic and use open cell foam insulation. It is a Cape Cod style house with no dormers. Plain Jane but nice. We have 2x4 rafters on 16" centers with an 8/12 pitch, plumb cut at the top, no ridge beam. The diagonal span (in the newer half)- from peak to outer edge of sill plate is 13' 4". The floor to peak height is 7'8" and the horizontal span (floor center plumbed down from the peak) to end tip of the joice/rafter marriage) is 10' 10". After reading all the threads on bowed rafters, I'm wondering if there is a lightweight solution to fur-out the rafters over the living space to the knee wall, making more room for the insulation.

[Don_P]

It looks like the majority of Iowa is 30psf snow load, if that's correct for most species and grades they run out around 7'6" horizontal measure on rafter span, 2x6's would be the modern minimum. I would sister something deeper than the 2x4 alongside of them from heel to ridge if at all possible.

[tbent]

Should I be concerned with adding the weight of 62-65 2x6x14's onto the outer walls? :confused:

[Don_P]

It is worth considering, that would add perhaps 30 pounds per lineal foot to the wall. If by "opening up" you mean changing use from storage to living area you are potentially adding 200 pounds or more per lineal foot to those walls. You do need to be able to follow all loads safely to adequate footings. Architects and engineers do perform a valuable service.