The domed roof of the Pantheon in Rome was very well designed. It is 160 ft. in diameter...the largest span in the world until the Houston Astrodome. It is built of poured concrete and was built in the year 120 AD.+/- It has a large open circular hole in the center of the roof (to let the smoke out I guess). It is still standing today. I went into it last fall. The Romans built things pretty darn good.
I'd love to see some of those ancient buildings. It's amazing what they were able to build without computers and modern machinery.