More than 55 firefighters battled flames as they ripped through a terrace house threatening to destroy adjoining homes.
Residents were evacuated from their homes in the road, in St Austell, after the blaze broke out at 3am yesterday.
Fire crews stemmed the raging fire from spreading to other houses in Holmbush Road in the town.
The occupants of the house were out at the time and no-one from the neighbouring properties was injured.
Fire crews and appliances from across the county were drafted in to beat the flames.
Eight fire engines attended the scene manned by fire crews from St Austell, Fowey, Mevagissey, St Dennis, Lostwithiel and Bodmin.
Police and paramedics attended too, along with engineers from Transco and Western Power who isolated power supplies.
Steve Halstead, the incident commander from Cornwall Fire and Rescue, said an investigation into what triggered the blaze had been launched.
He said: "We're very pleased the fire has not spread across the roof.
"At first we thought that it might. We managed to stop it.
"We're doing our best for the residents to get them away to temporary accommodation.
"Then we will get the crews back and start an investigation."
The A390 at Holmbush Road was closed overnight.
Lee Hall, 35, a local resident, said he saw the aftermath of the fire while on his way to work at around 9.30am yesterday.
Mr Hall said: "There were still fire engines all over the place and the scene looked pretty busy. The roof of the house was completely destroyed. There were scorch marks around the windows at the front of the house. It was in a real state."