JAY CRANDELL WROTE the book on residential load bearing. Literally. It's called the Residential Structural Design Guide. About five years ago, Crandell and some of his associates at the NAHB Research Center in Upper Marlboro, Md., noticed a dearth of engineering information on modern housing. Most homes, they came to realize, are built from experience and instinct—not from a body of documented science.

“House construction has never fully been described in engineering terms,” says Crandell, now a private consultant with a firm called Applied Residential Engineering in West River, Md. “We just know what works. We wanted to develop a method describing why certain methods work.”

That method of looking at residential structures, he says, was modeled after what happened in Australia in the 1970s, when the government began to crack down on “under-designed” homes, ultimately leading to research on residential engineering.

In writing the book, Crandell discovered that one of the more challenging aspects of modern residential engineering is the hybrid nature of modern construction. It's not uncommon to combine insulated concrete forms, light-gauge steel framing, solid sawn lumber, and engineered I-joists in a single design. But for Dana Bres, a research engineer for the Partnership for Advancing Technology in Housing (PATH) in Washington, the complexity of connecting and applying different materials should not deter builders from using them.

“You're no longer forced to do 2x14 [dimensional lumber] headers,” Bres says. “You can do it with engineered products such as glulams, and we're seeing an incredible use of these alternatives. They give the builder a lot of flexibility.”

He adds that because new products are pushing their way into the mainstream of home construction, the manufacturers have a huge vested interest in seeing them installed properly. “The builder and the product manufacturer and the supplier all have skin in the game,” he explains. “They are truly partners in delivering a good product.” He adds that when dimensional lumber was the only material, the “guy with the hammer” was ultimately responsible for all load-bearing considerations. Now, with building so much more complex, framers need help to avoid mistakes.

Why Worry? Studies of homeowner warranty claims are one place to look for evidence of load-bearing mistakes. Considering their lack of formal engineering knowledge, home builders do surprisingly well, at building structurally sound homes. That's because margins of error are built into most building codes. A study by the Canada Mortgage and Housing Corp. of Quebec back in 1994 found that about6 percent of defect claims fall under the structural category.

The most dramatic result of load-bearing errors is not catastrophic collapse of a home under normal use but excessive damage to the home during a high-stress event, such as a hurricane or an earthquake.

Again, that's where close adherence to local codes and manufacturer specifications can save the day. This is where small mistakes that may seem unrelated to heavy matters like the building loads can be greatly magnified.

“Often, it's something as simple as whether or not nailing was done properly,” notes Bres. “I have seen cases where the roof is peeled back on a home, and you can see where the row of nails missed the rafter. That's something that goes all the way back to the worker. If you miss with a nail, you nail it again.”