Some 22 residents of retirement flats had to be rescued after a "significant fire" in its roof.
Emergency services were called to Ashley Court, Wellfield Road, in Hatfield, in the early hours, Hertfordshire Fire Service said.
Residents were evacuated from the flats, with two treated in hospital for "minor smoke inhalation", according to police.
Wellfield Road and Stockbreach Road remained closed.
Parts of the roof had collapsed.
Twelve engines attended to bring the fire under control and investigations were ongoing to determine the cause. Police said the fire had started in a laundry room.
Darryl Keen, chief fire officer, said it was a "significant fire in the roof space of the building".
"A reception centre has been opened for everyone who has been affected by this fire," he said.
Sanctuary manages the flats and described them as a "retirement scheme". The company said it was assisting the fire service with its investigation.
"Our thoughts are with everyone affected by this devastating incident and we are working closely with Welwyn Hatfield Borough Council to support the residents, including arranging emergency accommodation for any who cannot immediately return to their homes," a spokesman said.
Hertfordshire Age UK said it was urgently collecting men's and women's clothing of all sizes at its head office in Welwyn Garden City.
*URGENT HELP NEEDED*— Age UK Hertfordshire (@AgeUKHerts) December 15, 2020
A fire broke out at Hatfield sheltered accommodation and male & female clothing donations are needed! Any clothing for men and women, all sizes, needed ASAP. Drop any donations to our head office today – 1 Silver Court, Watchmead, Welwyn Garden City, AL7 1LT pic.twitter.com/n7z7wewZQm