The pizza oven is roofed by catalan vault. It is then plastered with a lime-soil-sand plaster, then pressed, then lime wash.
The last two nights we have had hard rain, 8cm each time and there is no leaking except one cornor over the wall where i put plain wet mortar.
This roof i struggled with personally. kind of long story but in the end it is 7cm tiles on end, then another 3 layers of tiles with mud mortar. work on the groins on the first layer and the other layers it doesnt matter so much.