After most of the cathedral was built in 1296, many builders tried to complete the dome. But none could do it. No one knew what material to use. Many builders knew how to build concrete domes. However, the dome in Florence needed to be wider than any dome ever built. Also, tall buildings of the time relied on structures that supported the heavy stone from the outside. The cathedral didn't have these structures, so a concrete dome was too heavy. It would easily fall. So, the vathedral's roof was left unfinished for over a hundred years.