Salt Box Roof

by Michael
(Calgary)

Rough Sketch

I want to build a 12:4 sloped salt box shed roof with over a 12 ft span. The roof will rise 1 foot over 3ft and then drop 3ft over 9ft. I'm concerned about the necessity of collar ties or rafter ties. I was thinking about tying a board off the top plate of the highest wall to a rafter, similar to how a collar tie is used, but since the tie board remains high off the low wall I'm not sure you can call it a collar tie. It's kind of a half collar half rafter tie (isn't it?) and I'm not sure that this is a structurally sound idea. Could you provide me with some advise on this? The roof will see snow in winter.