No timber structures survive (roofs, floors etc.), so our knowledge of how these were put together is limited. The spans are, in the main, limited and suggest very simple beam and post structures spanning stone walls. For the longer spans, it is uncertain if the Greeks or Romans invented the truss but the Romans certainly used timber roof trusses. Before 650 B.C.E. the now famous ancient Greek temples were built of wood, but after this date began to be built of stone. The process of a timber structure being repeated in stone is called petrification or "petrified carpentry".