Outdoor Living & Lifestyles - Barbecue chimney

I am building a bread oven and barbecue in my garden. The design involves a central stone wall with a roof pitched to either side. The rear side will be used for storing wood and the front side will have a porch with the bread oven (3ft diameter) to the left against an existing wall and to the right a barbecue.

I plan to vent the oven fumes up it's own chimney and through the porch roof . I imagine this will be more or less effective. For the barbecue to the right of this I also plan a similar chimney but worry that it won't really work and the smoke will hang around everywhere - in the chimney and outside the chimney and under the porch roof.
Any suggestions as to how to design an outside chimney that will draw? Would a slit near the roof ridge work better than a chimney?