Your ceiling overlays (joists) are keeping your exterior walls from spreading.
If you remove them what will keep the walls from bowing out? To do this properly it needs to have load calculations and a load bearing ridge beam. The ridge will carry the load of the roof both ends of the beam will need to be posted through the house to the foundation.
There are of coarse other ways to do this, scissor type rafters going from the top plate to mid-span of opposing rafter and vise versa.

Now if you have trusses up there...don't even think about it

 

Check out this page. Towards the bottom is the "scissors truss" that could be used to replace the cross beams on your original room structure. You'll still need an engineer's help to determine the loading and required design.