You don't necessarily need engineering, although it is a good idea nevertheless.
Your local code office or knowledgeable framing contractor can also assist with sizing the new joists and rafters and spans.
Here's your problem:
When ceiling joists are fastened directly on top of bearing wall plates to resist the outward thrust from roof loads and fastened along side the rafters at that location, they are in the best poisiton to create a stable roof assembly.
Once one moves ceiling joists vertically above the bearing walls...the design not only reduces the ability of the rafters themsleves to resist those loads and keep the bearing walls from spreading....the design also reduces the ability of the rafters to resist loads placed upon them from things like snow and wind.
The result is, the higher you raise the joists above the top plate of the walls, the weaker the rafters become, to point that, depending on how high the joists are raised, the rafters themselves may also need to be replaced or increased in size or spacing to accomodate the loads.
Bottom line is, a design professional or code offical will usually need to assess the loads by your proposed design to see if it will work before you do anything.