(Reuters) — Wineries damaged by wildfires tearing through Northern California are starting insurance claims, and at least some of the smaller vintners are likely to find limits in their policies mean payouts fall short of rebuilding costs.

Gaps in coverage and a spike in rebuilding costs, typical after disaster, may come as a shock to many small wineries, favorites of Napa and Sonoma county tourists, said Tom Pagano, who heads the vineyard insurance practice for insurance broker Aon P.L.C.

"The easy part of insurance is buildings burning down,” Mr. Pagano said, describing the complicated claims process. Crops are covered, but not vines, and policies often impose quirky limits, such as when grapes spoil due to electrical failures instead of fires. The blazes came as harvest was ending and production was underway at many wineries.

Even with best insurance protection, vines themselves can take years to grow and mature.

Insured losses from the California wildfires will total billions of dollars for vintners, home owners and other entities, said Mr. Pagano.

Catastrophe risk modeler Risk Management Solutions Inc. calculates the region sustained $3 billion to $6 billion of insured and economic losses as of Oct. 12. The figures do not include automobile or crop losses, and RMS wrote in a blog post that long-term business interruption to the wine industry "could result in a higher total loss.”

The fires north of San Francisco Bay have destroyed at least a dozen wineries among more than 5,000 structures, as well as killing more than 40.

About 30 wineries in the Napa Valley Vintners trade group reported "some degree of damage" including to wine-making facilities, vineyards and tasting rooms, said Patsy McGaughy, a spokeswoman for the group, which has surveyed half of its 550 members. About a half dozen wineries reported significant losses and are part of an industry that contributes $57.6 billion to the state's annual economy, according to industry figures.

Vineyards, which mainly occupy the valley floor, appear to have been largely unscathed, as the fires in Napa County burned mainly in the hillsides, McGaughy said.

Smaller wineries, especially, are likely to find "the limitations that were not expected" in their policies said Robert Gall, managing director for the national property claims practice of insurance broker Marsh L.L.C., a subsidiary of Marsh & McLennan Companies Inc.

Paradise Ridge Winery in Santa Rosa, California, is one of the better prepared and is moving fast. It has hired a forensic expert to assess damages and a builder in order to stay ahead of the demand for construction services.

"It’s a maze of information and things to keep track of," said Sonia Byck-Barwick, who runs the winery with her brother, Rene.

The winery has already started the insurance claims process with a specialty lines unit of Allianz S.E., which covers its buildings and property, and Lloyd's of London, which covers wines in production and storage, she added.

Fire destroyed its winemaking facility, tasting room, event center and 7,500 cases of wine. The winery is still shipping wine, thanks to 10,000 cases it stored elsewhere. About 12 weddings, scheduled on the property during the coming weeks, have been canceled, Ms. Byck-Barwick said.

"Luckily we have great insurance which covers everything," Ms. Byck-Barwick said. "Some things might be a little low, but I feel very confident that we'll be able to rebuild."

Other U.S. winery insurers include the Travelers Cos. Inc. and Chubb Ltd.

Smaller wineries may lack resources or expertise to negotiate additional coverage for issues such as spoilage caused by utility failures, which is typically subject to far lower limits than other parts of a policy, Aon's Mr. Pagano said.

For example, a clause for the coverage may be limited to $100,000 and hidden deep in a policy that otherwise covers up to $2.6 million in damages.

Wineries that try to cut corners by underinsuring must also often pay co-insurance, a type of penalty that is equivalent to a percentage of the underinsured amount, based on factors such as coverage the business should have had, and deducted from the final payout.

"It's going to be a big issue," Mr. Pagano said.