As floodwaters receded Thursday, a week after torrential rains decimated parts of Louisiana, communities throughout the region turned themselves inside out: people dragged carpet, furniture, sheetrock into their yards, in a race to air out their homes before mold sets in.

“Smell it?” said Justin Martinez as he and family members rushed to tear out anything wet – which is to say everything down to the studs – and throw it outside. The unmistakeable odor of mold rode the damp air. “It comes that fast. It’s unbelievable.”

Martinez lives on the Amite River in Denham Springs, and thousands of people in every direction worked in the same frantic way.

Livingston Parish, and those around it, looked like history’s most hapless yard sale.

The death toll from the state’s historic floods had risen to 13 people by Thursday. Torrential rains that began a week ago brought an astounding 26m cubic metres (6.9tn gallons) of water down, with more than 76cm (2.5ft) of water to parts of the state.

A mobile home drifted on the floodwater. Photograph: Matthew Teague/The Guardian

The weather has damaged some 40,000 homes in the state and, as of Thursday, left approximately 4,000 Louisiana residents in temporary shelters, down from a high of more than 12,000 earlier in the week.

In the course of the storm, more than 30,000 people have been rescued from flooding, and more than 85,000 people have registered to receive federal disaster aid.

“Thousands of people in Louisiana have lost everything they own and need our help now,” said Brad Kieserman, vice president of Disaster Services Operations and Logistics for the Red Cross, in a statement late on Wednesday. “This disaster is the worst to hit the United States since Superstorm Sandy and we anticipate it will cost at least $30m, a number which may grow as we learn more about the scope and magnitude of the devastation.”

Superstorm Sandy barrelled into the East Coast in 2012, leaving more than 100 people dead across several states.

Housing and urban development secretary Julián Castro announced Thursday that his agency would be offering assistance to homeowners and low-income renters affected by flooding damage. And Homeland Security secretary Jeh Johnson was expected to visit the state Thursday to survey the damage.

At Amite Baptist Church, members dragged chairs into the parking lot. On a road near the river a tractor broke down with a trailer full of waterlogged furniture, and motorists stopped to help the owner push it off the road. Everywhere a gray silt covered trees and shrubs and walls, marking the water’s historic height.

Lanette Watson and her daughter make hundreds of peanut butter and jelly sandwiches. Photograph: Matthew Teague/The Guardian

People whose homes didn’t flood worked in other ways.

Lanette Watson, who would normally be working at her audiologist office, ripped open bags of bread and spread the pieces on her kitchen counter. She had mixed a vat of peanut butter and jelly, and now glopped it onto the bread while her children stuffed potato chips into sacks.

“How many left?” she asked without slowing her assembly line. She and her husband, Mickey, are making a thousand meals a day and distributing them at their church.

She received an automated call from the Livingston Parish school system. Instead of the usual monotone recital of holidays and lunch menus, the school official spoke with a wavering voice.

“In these difficult times, we need to stay strong and keep our faith,” said assistant superintendent Steve Parrill.

Schools are closed, he said, and there’s no sense of when they may reopen. Maybe weeks. Maybe months. “We pray for our students and their families, that they will find the strength they need to persevere.”

There were signs of life, though: people walked through the drive-through lane of a McDonald’s for lunch. Ambulances ran, and garbage trucks made their rounds. And everywhere people helped their neighbors begin a task that will take many days: cleaning out the debris that nature left in its wake.

Although many regions had entered recovery mode, local authorities in at least one region were warning residents to evacuate due to rising waters. An area outside of Vermillion Parish, which lay outside the local levee system, was told to evacuate by local fire chief Evans Bourque, who said the evacuation order concerned some 60 to 70 homes and scores of residents in total.