Bill,
If I understand you correctly, you can do what you want with the barrel vault roof. Note in attached that how i drew my half circle before magic-wanding the barrel roof to them determined which way the curve went.
If you want a smooth curve, than create a custom profile as Gary suggested.
If this isn't what you are after, please post a screen shot.
HTH
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"