I have no experience in timber framing, although I have experience in other contruction.
I am planning to build a small post and beam barn (more like a large shed). I was thinking of using wood shingles or shakes for the roofing. Pictures I have seen, show 1x4 as the nailers attached to the rafters. I am concerned about this. In a conventional roof, roof sheathing prevents the roof from "sliding" or collapsing. I don't see how this works in the pictures I have seen.
Any one can explain this to me?
thanks
Jonathan

Plywood or OSB Roof Sheathing is used to stabilize the rafter system. It spaces the rafters, and stiffens the roof members. Historically, regular 1x's (or perhaps shiplap boards)were used to perform this function.
With cedar or wood shingles, it is important to 'vent' the back side so that the shingles are kept dry and last longer. Hence, the 1x4's space the shingles away from the roof sheathing and or insulation. If the shingles are placed directly onto plywood or OSB sheathing, the shingles do not dry out properly and their live expectancy is reduced accordingly.
Here in Beautiful BC, Building code says to include Roofing Felt every two rows of wood shigles/shakes to allow an additional layer of water protection.
Typically, Peel-n-stick Roof membrane is applied at least 3 ft (1 metre) up from the edges and throughout the valleys if plywood is used for the roof sheathing, and then the 1x4's on top of that. In rainy or cold elevations, it is recommended to Peel-n-stick the entire roof regardless of the roof covering (asphalt, wood or metal)
Hope this helps...

Forgot one other note... use solid wood to build up from the roof edge and/or up the valleys to support the Peel-n-stick as this is were ice damming will occur...

Thanks Jim,
I know that in the plywood situation you have a lot of rigidity, and that is what prevents the roof from just collapsing like an accordion. In an old section of my house there was roof planking. But they were 8"-10" wide and had no space between planks.

It would seem that 1x4" s with a 4" space, wouldn't provide a lot of resistance to racking of the rafters. That is my question. Are you saying that this is OK?
This size of the shed is going to be in the
10' x 12' -> 12' x 16' range. I may put on a front window dormer for interest.

I would suggest the 1x4's are fine in a building this size but also I would suggest you let in some wind bracing on the rafters. This is done with 1x4 or 1x6. Tack the brace board onto the rafters(outside surface) on a 45 deg angle, starting from a corner of the roof surface. Then take a pencil and trace the sides of the brace board onto the rafters, remove the brace board and set a circular saw to the thickness of the brace board, score out the area inside the lines you traced with the pencil.... then take a ripping hammer and clean out the cuts. The brace board should now fit into the rafters reasonably tightly and will add plenty of stiffness to your roof. Generally speaking I use two brace boards per side on a small building.

Mark,
This is what I had thought about doing. I've seen it done on corners of buildings to prevent the walls from racking.
thanks
Jonathan

Yup, what Mark says...