Does anyone know..

There is a rule of thumb method but I'm not going to tell you how to do it as it should not be used by amateurs. Building Control will require you to demonstrate that the joists are adequately sized for dead and live loads. This is a very simple calculation for an engineer which should take him all of 10 minutes. He will calculate the joist sizes and check that deflection is within acceptable limits. There is also the need to ensure that the structure is adequately tied and connections are suitable. My advice - get a professional engineer involved. Alternatively, you could commission a truss roof manufacturer to design and construct the trusses for you. This is a cost effective way of doing things.