If sparks carry farther than the usual 35-ft protection area, the fire watcher can stop and alert the hot worker to the greater danger and reposition the work or add protection to the work area. That could mean extending the fire protection area to 50 ft or 75 ft, he says.

Or they can get the agency that issued the permits to reevaluate the hazards— maybe even going back to square one—because of the greater spark distance, he says. That reevaluation is due to on-site observations, not wind speed, he says.

"You've got to be cognizant of ignition sources relative to unprotected wood" and separate it with distance and the creation of protective barriers, such as a fire- protection cloth hung vertically from rafters or spread horizontally like a tarp, Colonna says.

Of course, the fire protection area means the absence of combustible materials such as oily rags, scrap lumber and sawdust, he added.

As the type of weld gets more complex, the danger of heat and sparks tends to increase, Colonna says.

More complex work such as tungsten or shielded arc welding "puts a blast of air into the welding process and generates a greater spray of slag or sparks," he says. Sparks falling on combustible material are an issue, he adds, but "a bigger concern" is work on material behind insulation that holds the heat energy inside: A fire may not ignite for 30 minutes or an hour, when everyone has gone home.

"That's why the fire watch remains behind at least 30 minutes [or even longer] after work is done," Colonna says.

T.J. Lyons, construction safety consultant with Gilbane Co., says a one-hour after-work fire watch is "now a common convention." A fire watch on duty for one hour "after the exposure has ended" is required for contracts issued through the U.S. Army Corps of Engineers, Lyons points out.

"More progressive companies put up temporary fire standpipes," he says. "They not only protect workers. They also protect property for the client."

A jobsite fire can "tarnish your firm forever, even for big fellows," he says.