I finally bit the bullet and went ahead this week with a "new" technology for me. Timbrel or Catalan Vaults.
Where I want to use this technique is on the top half of the stairs in the groined vault house. Where the brightness is up to the top floor.
I made a post about this technique, and I tried some trials and found the gypum plaster works for the first layer.
I know how to do timbrel vaults but the catenary shape can vary, so I kept putting it off and off and fretting about it. I bit the bullet a weekend ago and traced the shape on some reclaimed chipboard. The load on the arch will vary so I worried about that until i realized when someone is standing on it it would also change the line of thrust so i figured at any time the LT will stay within 1/3 of the arch.
So here is from the side, the first of the three layers. tomorrow we remove that chipboard.
And what it looks like on top of the arch.
Edit 7 November 2012:
This is a good link about Timbrel vaults. It explains more and better than I can.
You too could have a roof/ceiling like this in your house.