The roof structure is a reciprocating roof. It's called that because all the beams sit on top of each other like a cardboard box with all it's flaps tucked into each other.
Beam number one was held up out of the way by a large forked pole.
The other 12 poles were then laid on top and tied to each other with baling twine until we had completed the circle (13 poles in all – one for each lunar month 'cos I was building my own roof, and I could) with the last pole coming underneath the first pole in a large spiral. We then carefully lowered the first pole down onto the last one and tied it in place.
It held together with just baling twine, but we strapped it with steel strapping, just for good measure.
We got instructions for how to build this from Tony Wrench's book ' Building a Low Impact Roundhouse. ' ISBN 1-85623-019-8
Highly recommended it if you're ever contemplating making a roof like this. His website is thatroundhouse.info.
We fixed plywood to the edges of the hole to make a frame for the skylight. This is custom made, a double glazed, toughened glass unit.