Viga-and-Latilla Roof

A viga-and-latilla roof is a flat or nearly flat earth roof carried on round timber beams (vigas, Spanish for beams) with smaller poles or split boards (latillas, laths) laid across them. It is the traditional roof of Pueblo and Spanish colonial adobe buildings in New Mexico and the wider American Southwest, including the Hopi village of Walpi, and its projecting log ends are a signature of the region's architecture.

Origins

The technique goes back to the Ancestral Puebloan peoples between about 750 and 1300 CE. Vigas, or the holes left where they have rotted away, survive in many ruins, including Pueblo Bonito at Chaco Canyon, built between about 850 and 1150 CE. At Walpi, vigas were salvaged and reused from older buildings. Taos Pueblo, built in the late 13th and early 14th centuries and still inhabited, is a well-known living example; its adobe requires regular replastering by community members.

Construction

• Vigas are usually peeled logs 6-10 in (15-25 cm) in diameter and about 15 ft (4.6 m) long, which limited rooms to under about 15 ft wide. They sit directly on adobe or stone walls, roughly 3 ft (0.9 m) apart, with about six to a room as a common standard; Spanish colonial work often spaced them irregularly.
• Latillas are laid across the vigas, sometimes diagonally, and could be peeled branches or hewn boards. In some interiors they were sorted and painted in colours.
• Fill: layers of brush or soil may go on next, followed by a slightly sloped layer of adobe that insulates the room and drains water. Vigas were laid with their small ends to one side to help create the fall.
• Parapets and eaves: parapets enclose the sides, and vigas extended past the wall can carry a covered porch (portal) on posts and lintels, often with carved corbels.

Timber

In the 17th century piñon (Pinus edulis) and ponderosa pine were the common species. Engelmann spruce was valued because it cracks less, but ponderosa pine was used more. Logs were felled in winter from dead and downed trees, hauled from the mountains by oxen, and large vigas were cut first so they could season longest.

Later changes

The arrival of the railroad in the 1880s brought dimensioned lumber and eastern building methods, and machine-sawn beams replaced round vigas for many structural uses. Later builders added horizontal bond beams to spread loads and rebar anchors through pre-drilled holes to resist uplift. In the Pueblo Revival style of the 1920s and 1930s, associated with the Santa Fe architect John Gaw Meem, vigas became largely ornamental. Santa Fe's codes, revised in 2012, give credit for structural vigas, although most new buildings use them as decoration. The reconstructed Palace of the Governors in Santa Fe contains both structural and ornamental vigas.

Performance

Earth roofs of this kind provide thermal mass suited to hot days and cold nights, but they are heavy, need regular maintenance of the earth and plaster, and depend on parapets and drainage outlets to keep water from pooling.