In this design the tops of the rafters attach to a load bearing ridge beam that runs along the length of the roof.
Roof without ridge board.
For 2 x 6 rafters use a 2 x 8 ridge board and so on.
You may splice two boards together to achieve this length if necessary using a 14 and 1 2 inch block of the same material.
A ridge board is a non structural member that serves as a prop for opposing rafters to rest against and connect to.
While the joists commonly attach to the side of a ridge beam they can also sit on top of it as shown in the diagram below which creates an interesting look.
This can be accomplished with collar ties boards spanning the shed at ceiling height or with large gussets further up the rafter slope.
Ridge board the engineering principles of the ridge board are a bit more complicated.
You don t need a ridge beam but you do need a way to keep the walls from spreading.
This video shows how to make roof rafters with a rafter square and or a speed squ.
Rafters and ridge boards.
One collar tie would be enough in your case.
For 2 x 4 rafters use a 2 x 6 ridge board.
First off yes it is a common practice to frame a roof with no ridgeboard.
A structural ridge is the standard alternative to ceiling joists.
Ridge beams are required by code if the roof slope is less than 3 in 12 irc section r802 3.
A ridge board was not always required and the photo above of the attic in a 1920s era house shows a roof framed without one.
Its only job is to provide a connection point for the roof joists on each side of the roof.
Ridge beam sizing is based on the span of the beam between supports and the amount of roof load supported by the beam.
Because the ridge board is a smaller member typically only an inch thick sometimes even less it is not a structural member and doesn t carry the gravity load.
In most applications the ridge board is not structural and only serves as a convienience for the carpenters as they frame the roof.