Using 6x6 beams for roof joists

Question

I'm designing my new home. I live in the desert southwest (Nevada). The house will have an adobe style, i.e. it will be 8" cmu's covered with stucco.

I will have an essentially flat roof (1/2:12). I would like to use 6x6 yellow pine beams as joists for the roof with about 18" of each beam protruding from the walls.

The floor plan is rectangular. with the rooms being 12x12, 16x16, 12x16, 8x12.... I'd like to place these 6x6 roof joists 48" OC with spans of 12' and 16'.

From an engineering perspective, is this feasible? The county I live in goes by the 2006 IRC, which does allow for unconventional spacing IF properly engineered.

Answer 1

Probably not.

Without any particular input from you on actual required loads in your area, I tossed a 16 foot span and 48" spacing at the Wooden Beam Calculator here:

https://courses.cit.cornell.edu/arch264/calculators/example8.1/

And it spat out 4x16 and 6x16 as options (for defaults of 50 PSF live (snow, etc) and 20 PSF dead (structure). Thus, a 6x6 is going to be FAR less beam than needed, probably.

Beam strength goes as the CUBE of beam depth, so that's in the range of 12.5-19 times less beam that you'd need. Going to a 30 lb live load (which is the ICBO minimum if memory serves) only shrinks it to a 4x14, (a beam 8X stronger than a 6x6, roughly) and at a 1/2:12 slope you will need to be very concerned with roof deflection in rain events, however rare they are - once the roof deflects enough that it does not drain, the roof load can become quite extreme on the day it does rain, if it's a gully-washer.

You also add the problem of what you are spanning the 48 inches with - plywood alone won't be happy, so you'd need additional framing (purlins) to get the deck span in range, or you'd need to use a material that can span that distance at load, such as stressed skin panels.