No offense intended to the contractors out there that do know
their way around structural engineering, but I would be very
leary of relying on just any contractor to do the structural
design of a reinforced wall and concrete roof.
There's a lot of contractors that know their stuff, and a lot
of them just think they do. It's generally the contractor's job
to build things according to plan, and the engineer's or architect's
job to do the plan design.
I'd get a design by a professional engineer. Regulations may
vary by area, but in many locations, you need a design by
a certified engineer if your concrete wall is higher than 4',
and certainly if you're putting a concrete deck overhead.
(If you're concerned about things like building codes and permits.
But you may not be.)
You can find small engineering companies in just about any
town, just check your yellow pages. You'll probably want a
small engineering company. The large companies typically
don't want to screw around with a small project like that,
and have too much overhead costs to provide you a
reasonable cost on your design. I'd figure on your engineering
costs running you maybe 15% of your total project costs.