Hello,
The ceiling of my first workshop I built this way, so that was not the roof exactly. The form it took was individual arches spanning the space between the beams of a woven matrix using 5 mm thin strips of wood 30 mm wide. I filled these spaces in with the clay mix from above even with the top of the beams and laid the floor planks. Then plastered from underneath with another clay mix. The arch makes it self-supporting. Of course it leaves a varying insulation value across the arch where it is thicker along the beams and thinner at the pinnacle. Overall it worked well for me and was an enjoyable work technique. The book, A Pattern Language explains it well enough.

Greetings,

Don Wagstaff