Simple Round Roof
This guide will show you how to build a simple round roof to cover a circle of 1×2 stone blocks.
Build a circle of stone blocks.
Place a pole on every other corner.
Add beams sticking out both ways from each post, perpendicular to the stone blocks.
Add a roof section to each beam, and then another roof section directly to each roof section on the first ring. I prefer doing it in sections.
Add the middle roof section to one of the roof sections on the second ring, and then add another at 90 degree angle to create a + as the highest point of the roof.
There are many ways to create a more interesting round roof, but this will get you the basic shape.