The early mission buildings used dry tule thatch for the roofs. In 1776 San Luis Obispo was attacked by Indians shooting flaming arrows. A fire started on the roof of the padres quarters and soon spread, consuming all the buildings except for the church and a granary. There were other attacks in the following years.
The missionaries at San Luis Obispo remembered the tiled roofs in their native Spain (tile was used for hundreds of years in Europe) and began to experiment with making tile locally. By 1790 the use of tile had spread to all the missions. The tiles were made by taking local clay and working it in pits under the hoofs of animals. Squares of clay of the right thickness were patted over curved wooden forms on top of sand that kept it from adhering. The edges were then trimmed and the clay put in the sun to dry/ Clays were then baked in a kiln.
I am attaching a picture of a display set up on the grounds of Mission San Buenaventura. This includes an olive press, and to left, some of the mission tiles and part of a kiln like those used to bake the tiles.
The early mission tiles were about 22” long and tapered from 12” to 20” across.