Originally Posted by nealtw
As an engineer yourself you should know better than giving the quick easy answer especially when the ex-ray eyes aren't working. We don't know that the rafter aren't
2x4s joined in the middle.
Keep in mind, the home handyman type often know just enough to be dangerous and will often just take the easy answer and go for it. Adding to the problem is the fact that the answer came from an engineer.
I think it's great for people like Mick to come to a place like this for more info and I beleive it is up to us to give him as much info as posible so he can make his own decisions.
Sorry, but I'm having trouble understanding your "quick and easy answer" remark. How you read that into my refusal to guess at a load-carrying member's size or span based on pictures is beyond me. Could it be possible you were confusing my comments with those of InspectorD? He mentioned in Post #7 that "this is an easy DIY project" (which is a statement I'd be reluctant to make, as there are too many variables and unknowns that could quickly complicate matters). And to further clarify, any suggestions or answers I make as an engineer are never intended to encourage anyone to do something that's beyond their limits and capabilities.
But getting back on point--Mick, with your rafters only being 2 x 6s, they could well be at their maximum span limits when unsupported at an interior point. American Forest Products span tables for 2 x 6s @ 16" centers shows them only good for a 10'-0" span (assuming Fb of 1800 psi, allowable deflection of L/240, liveload of 50 psf and deadload of 10 psf).
As an aside, I noticed in one of your pix that someone drilled through several adjacent rafters for the purpose of running Romex (electrical wire). Doing so near the center of a span (where its bending moment is greatest) is bad practice, especially when the holes are located in the lower half of the member. In effect, this has turned each 2 x 6 into something considerably weaker. Better to locate holes closer to the neutral axis (center) instead of the tension zone (bottom) of a member, and to make them as small as possible. And better yet to not put any holes in the middle third of any rafter or joist span.