When you have a morph that has both curvy faces and sharp edges, to increase the resolution of the curve and still have the sharp edges. Select the morph Command Design>Modify Morph>Modify segmentation More segments.
To create a Barrel roof please see ArchiCAD Help?: Virtual Building : Construction Elements : Roofs : Create a Barrel-Vaulted Roof. The set the number of segments created go to Options > Magic Wand Settings (so for eg. setting “segments along arcs” to 20 will create 20 segments to describe the roof.)