PORTLAND – Maine’s largest city has adopted a new system to protect firefighters by identifying vacant and dangerous buildings.

Firefighters on Thursday began installing color-coded placards on unsafe structures that might contain rotten timbers or traplike holes in the floor.

“In the past, we have had people get hurt in buildings that have been either abandoned or were vacant,” said John Beatty, Portland Fire Department spokesman.

“It’s not like going into a regular building when you know you have to deal with furniture,” Beatty said. “The floor might not be there.”

Nationally, 6,000 firefighters are injured in vacant or abandoned buildings each year, and firefighters are three times as likely to be injured fighting a vacant building fire than a typical fire, Beatty said, citing National Fire Prevention Association statistics.

An ordinance allowing the department to post vacant buildings was approved this week by the City Council.

After inspecting buildings that appear vacant, the department will use a color-coded system of 2-foot square, reflective placards, each with a large “V”, to identify the buildings.

A green placard means there are minimum interior hazards in the building. Yellow indicates some hazards exist and work inside should be done cautiously. A red placard instructs firefighters to stay outside the building.

Firefighters still will enter buildings with red placards if there is a clear indication someone is inside, Beatty said. Often, fires start in abandoned buildings as a result of young or homeless people who have entered them.

Beatty said the system is especially important for firefighters from outside a particular neighborhood who might be responding to an unfamiliar building.