Pithouse construction was usually based on four corner posts positioned upright in the pit. These posts were carefully chosen and trimmed to create a branch or fork at the top as a structural support. They were joined by horizontal beams and crossed with ceiling joists. The interior sides of the pit were plastered with clay or lined with stone — either large slabs wedged upright in the soil or courses of smaller stones. The exterior of the pithouse was formed of branches, packed tree bark, or brush and grass. A thick layer of mud on the outside of the roof and walls protected the shelter from the weather. Often the initial mud layer was carefully plastered with a lighter colored clay.