Kivas, or ceremonial houses, were used not only for religious ceremonies but may also have served governmental and social events. They varied in size and had either flat or cribbed roofs. Cribbed roofs were constructed of logs laid horizontally and built up to form a dome with an access hole. Aztec's Great Kiva has a flat roof supported by four massive pillars of mortar and wood, and the roof was estimated to weigh 95 tons.