As their sodden couches and carpets rot at the curb, Long Islanders are coming to a grim realization: Flood insurance only covers a fraction of what it will cost to replace furniture ravaged by the storm.

While most flood policies do cover dining sets, televisions, clothing and other household goods, they won’t necessarily pay for brand-new replacements.

Instead, the National Flood Insurance Program gives homeowners what adjusters determine their stuff was worth just before the storm hit. Furniture depreciates. And that means Long Islanders washed out by Sandy may need to receive loans or grants or dig into their own pockets to fully refurnish their homes.

“It’s definitely a rude shock for homeowners, and it’s buried in the legalese of their policies,” said Amy Bach, executive director of United Policyholders, a San Francisco nonprofit consumer advocate group that helps homeowners file disaster-related insurance claims.

In Massapequa, the storm gutted Joseph Demeo’s three-bedroom ranch house, which backs up onto an arm of Great South Bay. His recliner is wrecked. His china hutch is stained. And the upright piano he played since childhood stands, waterlogged, on the front lawn, its notes reduced to grunts and plinks.

“I had La-Z-Boy furniture in here,” said Demeo, 62, standing on his warped living room floorboards. “My adjuster is telling me it’s all going to be depreciated.”

It is difficult to predict how much homeowners will get for their furniture, according to industry trade groups. Furnishings depreciate at different rates, depending on a litany of variables, including how it was used and where it was stored.

Before filing claims, consumer advocates suggest that policy holders make detailed lists of everything lost, including photographs and receipts whenever possible.

Ultimately, it will fall to private insurance adjusters to decide how much homeowners receive for the berms of watery sofa cushions, lifeless electronics and other ruined furnishings lining South Shore neighborhoods. Some won’t fetch a dime. Anything kept in a basement, for instance, isn’t covered by flood insurance.

Flood protection for home furnishings — or “contents coverage” — is optional under federal insurance plans. Private companies don’t offer flood insurance at all.) Nearly 75 percent of those with flood insurance on their homes, schools or businesses also had contents coverage as of August, according to the Federal Emergency Management Agency, which runs the federal flood insurance plan.

Those policies only pay “actual cash value” for damaged furnishings. Even if homeowners are willing to shoulder higher premiums, there is no option for broader coverage to pay the full cost of replacing furniture. That means all of it is subject to depreciation.

There are programs, however, to help make up the difference. The U.S. Small Business Administration, for instance, offers low-interest loans of up to $40,000 to help people in disaster areas refurnish their homes.

Back in Massapequa, Demeo sorts his possessions room by room, separating the ruined from the salvageable. The house smells of firewood, which keeps Demeo warm at night as he leans over a laptop and compiles a list, assigning dollar amounts to all he has lost.

“There is just so much,” he said. “I’m never going to buy anything nice again.”