Stern,
The responses talking about wind load causing a parachute or umbrella uplift are dead on.
Don't depend on $1.39 hurrican clips, they just keep the pieces together after the wind moves the roof someplace else.
You need really good connections between your post bottoms and their piers. If you lag screw the principle rafters to the top plate you help make the lift problem go away. (assuming good joinery between the plate and the posts). By all means include braces in each wall plane so racking is not a factor. Slightly oversize the timbers will add weight and stability. A pitch steeper than 6/12 has better down force characteristics than a common "garage" roof at a shallow pitch.If you enclose the gable end above the end wall plate be sure to put vents on each end at the peak to reduce the lift effect.
