When the

hung a red sign with a white U above the door of her Southeast Portland store, Kit Ballard became nervous.

She feared it was a scarlet letter that would hurt business at

.

"At first, it was kind of like, wow, what does that mean?" said Ballard, the store's owner for 11 years. "But no one has even said anything."

Portland Fire Bureau spokesman Lt. Allen Oswalt said that's a good thing. The signs aren't meant for the public; they're for firefighters.

The signs, placed on at least 21 Portland buildings since the bureau introduced its Unsafe Building Alerts Program in December 2009, tell firefighters that if a fire were to break out in the building, it would be unsafe to battle from inside. Firefighters will still enter to rescue people, though.

"We need a way (to know) from the outside of the building that the building is structurally unsafe," Oswalt said. "The big U is the way we do it." He said the signs are similar to those that warn of hazardous materials, such as the ones on schools that have chemistry labs.

Most of the marked structures are visibly abandoned. But some house businesses, including

at 550 S.W. Park Ave.;

at 915 S.W. Second Ave.; and

at 938 E. Burnside.

Oswalt said there's no reason for the public to stop visiting the businesses; if the Fire Bureau deemed them unsafe, they would be forced to close.

That's the detail Ballard wants her customers to know.

After speaking with Mike Alderman, a Portland fire inspector, Ballard said she understood the need for the sign, but she wants the meaning to be clear to others. "He was very good at explaining ... what the risk would be to the firefighters," Ballard said. "I did not feel that it puts my workers or my customers at any risk."

A building gets one of the signs if a fire inspector and city engineer determine together that not every part of it meets fire codes. Reasons vary, Oswalt said, from structural problems to too few exits. Finders Keepers, for example, is in a 70-year-old building that has been renovated several times and has some rooms without exits. If the owner updates the building to meet the codes, the sign can be removed.

At another building, at 2021 S.W. Morrison St., Don Mutal, a representative for the building's owner, said plans are under way to renovate the top two floors into six townhouses, which should bring the 1911 building into compliance.

"For us, it's just a temporary thing," Mutal said of the red sign outside one of the doors. "It wasn't worth arguing about. It's really a good thing for the fire department."

Customers of the building's ground-floor

, a neighborhood favorite, have asked employee Gary Jondahl about the sign.

"Ninety-nine percent of them don't know what it means," said Jondahl, whose wife manages the bar. Jondahl said customers shouldn't worry about it, though, and those gathered there on a recent day didn't seem to.

Frequent patron Glenn Haneberg said he's drawn in part by the building's early 20th-century charm. "I like this bar; I like that it's old," he said.

He's never noticed the sign, he said, and doubts others have, either. "I'm on the ground floor. I'm next to an exit," he said. But, he added, the sign "might affect me if I were looking for a place to rent above."

Michael Wells said he isn't surprised the older buildings are being marked. "Some buildings might not get the attention they deserve," he said. "Now that I know what it means, I'll notice it more."

Sophie Lareva, waiting at a bus stop across the street, has noticed the sign and thought it was some kind of advertisement.

After learning its meaning, Lareva said she would keep an eye out for other ones.

"It probably would make me a little nervous" to enter a marked building, she said, "because it's more vulnerable to fire."