Wooden L platforms that were rebuilt over the past few years as part of the $530 million Brown Line renovation are already deteriorating at some stations.

Officials with the Chicago Transit Authority began noticing the decaying wood in August 2008 — two years after the first stations were completed, the C.T.A. said.

Since then, about $300,000 has been spent replacing an estimated 10,000 square feet of Southern yellow pine decking at eight Brown Line stations — Chicago, Sedgwick, Armitage, Diversey, Montrose, Rockwell, Francisco and Kedzie — according to records and interviews.

No injuries have been reported, and there is no imminent safety threat, C.T.A. officials said.

The agency said preservatives applied to the wood were to blame for the deterioration.

For decades, the C.T.A. used creosote to protect wood on platforms, but the agency said it did not use the substance for the Brown Line project because it is increasingly hard to find, too sticky when applied to wood and less environmentally friendly than other applications.

Instead, the agency chose Flame Safe X-T, a material that is supposed to protect wood from fire as well as from the weather, C.T.A. officials said. But some of the treated wood planks soon started aging, they said.

“It was expected that the exterior fire-retardant material would perform better than it has,” Sheila Gregory, a C.T.A. spokeswoman, said in an e-mail.

Another C.T.A. official, Noelle Gaffney, said: “It’s not that the preservatives deteriorated the wood, it’s that they didn’t protect the wood” as well as previous products.

Louis Jacobini, principal owner of the company that makes Flame Safe X-T, said the product was not ideal for long-term weather protection. Fire prevention is its main benefit, Mr. Jacobini said.

The transit agency hired Lee Gjovic, a wood expert and former employee of the federal government’s Forest Products Laboratory in Madison, Wis., who recommended a weather-resistant preservative called CedarShield. It was applied to the wood planks from April 2009 to August 2009.

But a spot check of the Brown Line in October found platforms at a half-dozen stations with holes, splintering and what appeared to be mold.

Paul Beaty/Chicago News Cooperative; examples of deterioration by Emily Jurlina/BGA

The Chicago Avenue stop was among the worst. More than 80 boards were identified as needing replacement, according to internal C.T.A. e-mail obtained under the Illinois Freedom of Information Act.

Dave Glassel, president of the company that manufactures CedarShield, said he was “appalled” that the C.T.A. had not used a weather-protection material first. Mr. Glassel said CedarShield did not work as well when it was applied over materials like Flame Safe X-T.

Ms. Gregory said that less than 10 percent of the square footage “of all the wooden platform stations that were included” in the project had to be swapped out. The project lengthened platforms to accommodate more L cars and improve access.

Crews are replacing the bad boards as quickly as possible to prevent customers from tripping and falling, transit officials said. The agency is absorbing the costs.