A year ago, I wrote a column about Rachel Wood, an appropriately named New Mexico forester, who is working to bring back local forest products as viable materials in the construction of our homes.
She recently received a grant through the Collaborative Forest Restoration Program to further those efforts. The federal program, passed in 2000, is administered by the U.S. Department of Agriculture and was the brainchild of former New Mexico Sen. Jeff Bingaman.
Her efforts are two-pronged. One is a chain-of-custody protocol called Good Wood that seeks to certify New Mexico forests as equally sustainable to those certified by the Forest Stewardship Council, the recognized national standard.
Because of costs, and because of our relatively small market, no New Mexico forests are recognized by the council, but all of our forests, whether state, federal, tribal or privately owned, are sustainably managed.
Wood’s second effort is to bring back viga span charts. Prior to changes in the 2003 national codes, these charts were used by builders, architects and engineers to determine how far a viga of a given diameter, spaced a certain distance apart, could span.
An 8-inch viga, for instance, spaced 30 inches apart, could span 16 feet with a presumed load of 40 pounds per square foot. A 12-inch viga could go 30 feet with similar spacing.
Code changes in 2003 said vigas and rough-sawn beams could only be used as structural members if they were graded and stamped by certified inspectors. Because New Mexico had none, and still does not, it effectively outlawed vigas and beams to hold up roofs. That, as we know from 1,000 years of New Mexico history, was ridiculous but went unchallenged.
Historically, vigas used in roofs were relatively small in diameter because it’s easier to haul a skinny log out of the forest than a big one. Also, before flat planks became available from sawmills, skinny branches called latillas were laid across the vigas. Then a material, most likely corn husks, was laid over the latillas and then dirt was piled on top that was tamped and shaped to funnel water to canales.
The thick layer of dirt provided nominal insulation and absorbed light rains and snow without too much dripping into the home. Later, with pumice mined from the Jemez Mountains, that rice crispy-like material provided similar characteristics but was lighter and more insulating.
Pumice also provided a sturdy base for layers of tar paper sealed with hot liquid tar topped by gravel to protect tar and paper degradation from our intense sunshine. That method was still employed into the 1970s, but both it and dirt roofs are now illegal and must be removed when historic structures are re-roofed.
The bottom line: Those old skinny vigas held up a lot more weight than what would be common today, which is spraying lightweight, closed-cell foam directly on top of covered planks or latillas.
Wood’s grant provides for complimentary engineering from Los Alamos National Laboratory. She has recruited August Mossimon, a lab engineer experienced with New Mexico forestry products, to lead the development of new span charts. His initial calculations indicate old span charts were conservative. Instead of 16-foot spans for an 8-inch viga spaced 30 inches apart, his charts show over 20 feet for a Ponderosa log and over 23 feet for a Douglas fir log.
With lumber prices going through the roof and the push to thin forests to mitigate fires and protect our watersheds, bringing back vigas and beams for roofs and eliminating the redundancy of two roofing systems is an obvious win-win situation. State officials should support these efforts.