Vigas are basically poles used as beams in the ceiling. They can extend through the walls and show on the outside of the home, or not. They have two purposes, support and decoration.
Normally, they are created from the trunk of a tree with the bark peeled off. Ponderosa pine is one of the most popular trees to use, but Englemann spruce is also used. They can support the roof, but more often than not they are simply used to create a Southwestern rustic look to the room.
The peeled poles can be used with or without latillas placed across them. They are used inside the home, sometimes with the timber extending through to the outside, or as supports for porches (called portales) and
When visiting the Old El Paso Adobe Company in Tucson, we found that they are often hand-peeled and can be produced with or without small sections of the bark still attached. This depends on how rustic the homeowner wants it to look.
They also were kind enough to explain to us that vigas tend to shrink over time, so theirs are aged a year to a year and a half before being sold. This prevents later problems with shrinkage in the home.
When speaking with our chosen builder, Aaron, he mentioned another point. In Southern Arizona, vigas are not normally allowed to extend outside the home. This is due to the heat in this area, which causes the wood to decay within a few years.