Fire Marshal Michael Leong said the fire traveled up the pipe chase, the vertical space used to house pipes in buildings, all the way up through the roof of the four-story building.
The fire was first reported around 10:45 p.m. Saturday at a hotel at 330 N. Bayshore Blvd. Firefighters had it controlled by 1:10 a.m. Sunday, a fire dispatcher said.
Leong said the blaze caused about $800,000 in structural damage to the hotel. The building was empty because it is being converted from a Howard Johnson hotel to a Holiday Inn, he said.
When workers use soldering torches on pipes, they are required to take precautions to protect the wood behind the pipes from the torch's flame. Leong said the proper precautions had not been taken in this case.
The fire will be officially classified as an accident, Leong said. However, he said, crews were performing the work without permit.
"They had a permit for the exterior work, but did not have a permit for the interior work," he said.
The city has halted the hotel renovation project until the necessary permits have been granted, Leong said.