I take all criticism as constructive but yes I did design this for the most part by myself. I looked at dozes of other designs, took lots of pictures and designed with their strengths in mind and worked around their areas that I saw as week. "Quite interesting" will make me a little defensive though.
I will defend the home designer here and say that I have never called Chrysler, Toyota, Cummins, GM, or anyone else when I am going to drastically modify a vehicle either. Do we all call an engineer when we build a work bench or a vice stand?
That being said it is 12' x 14' with a peak height of 9.5'. The requirements for the permit was pretty lax at best. Submit plan to the county 2 people look at it and and I have a permit. The sheething will be 1/2" OSB with arcitectual shingles. Nothing really to mention that will be hung, a few 4' lights (10lb) and a weed eater because they are always in the way. No insulation or drywall. The decking and shingles will weigh nearly as much as any snow load I figured 6-800lbs for the roof itself and 18" of snow over that area would be less than 1000lbs IF we ever had 18" of snow. That is still less than 11lb per square foot. All construction is 2x4 stud or better grade and is what, spruce/pine/fir? Everything is 16" OC, the ridge is 14'. 3.5" ring shank nails for the framing and 2.5" for the sheething with screws mixed in where a nail gun is hard to get in place.
I understand your hinge analogy and that is what I am trying to address. I believe both green and red will address this tendency and I believe green would be better. It will however create less head room near the walls so is less desirable.
Ok,
Now I understand more of the parameters. Here's what I would do.:
Provide more than just a couple of nails as fasteners on each end of each collar tie.
Create a plywood gusset at each connection between upper and lower rafters. Glue and fasten with numerous ring shank nails or screws.
Install metal plates at lower rafter tails.
The first item will assure the collar ties do their job and triangulate the upper rafters into a truss.
The second will keep that joint from rotating by triangulating the two rafters into a truss.
The third will prevent uplift of the roof in high winds by suction.
It's just a shed, but structure is structure. Small details like fasteners can have a large result. Bridges fail from one connection. Your shed probably won't fall down, but I've seen many garages built with builders "rule of thumb" that have sway backed roofs and bowed out side walls. This is especially true in hip roofed and gambrel roofed structures with no or few ceiling joists. A lot of them eventually end up with steel cables keeping them from collapsing. The purpose of engineering is to know that the result will perform as expected. Building codes design for loads that you will probably never see with a safety factor just to be safe.
No, the above suggestions were not engineered. They come from experience based on the spans and sizes you gave me and will assure that the structure won't deflect too much.
A gambrel is a difficult beast to properly design. The almost flat upper portion holds a lot of snow. The connections are critical and the lower rafter portion can easily be under sized. The collar ties and any gussets are the only thing triangulating it and preventing the tail of the rafters from thrusting the top of the wall out.
One way to help with a gambrel is to provide a ridge beam rather than just a ridge board, and allow this to support half the roof loads. These loads then need to be carried down to the floor on columns at each end. The rafter to rafter connection and some out thrust still need to be accounted for though.
