Residents of Stoughton and Hollis Halls' top floors returned to their dorm rooms yesterday, after two days of emergency renovations repaired the dorms' ceilings.

A ceiling collapsed in Stoughton Hall on Tuesday, injuring three students.

Fourth floor residents were moved into temporary housing in Loker Commons while engineers investigated the incident. Students on the first three floors of the buildings were allowed to return to their rooms Tuesday night.

Engineers worked late into the night Tuesday and throughout the day yesterday inspecting the buildings.

On Wednesday, an outside engineering firm hired by the University presented officials with a letter that reiterated that the first through fourth floors were safe for occupation, but said the fourth floors of both buildings needed "supplemental repairs."

The firm blamed the collapse on faulty nails that had been weakened by additional layers of the ceiling added over the years.

To stabilize the structure, engineers installed large bolts into the ceilings of fourth floor rooms. In some rooms the bolts were spaced across the entire ceiling eight inches apart.

Rooms on the first, second and third floors had additional supports added in 1992 during building renovations.

Students in Stoughton North 29, the room where the ceiling collapsed, were allowed back to their suite late last night.

There was very little damage to objects in the room, according to University spokesperson Doug L. Gavel, who said that only one chair and one lamp were broken by the failing cement.

Dean of Freshmen Elizabeth Studley Nathans said she was impressed by the speed with which the rooms were repaired and reopened.

"The expectation that even the students in whose room the ceiling collapse occurred can return to Stoughton this weekend bests by two weeks any guesses I would have made when I first saw the damage on Tuesday," Nathans wrote in an e-mail message.

Some students said the inspection and renovation calmed their fears that the building was unsafe after Tuesday's incident.

"The administration wouldn't put us in a building that they had severe doubts about," said Lauren R. Dorgan '04.

Other first-years said the incident was a blessing in disguise, as it encouraged them to meet fellow residents.

Students in both dorms gave Harvard administrators high marks for their round-the-clock efforts to search and fix the building.

And while many fourth floor residents were frustrated when they weren't allowed back in their rooms until last night, they said they understood it was for their own safety.

"I'm sure they went to great lengths to prevent any further disturbances," one displaced resident said.

The University provided the displaced students with full-sized beds and toiletries while they were in Loker--a move that drew high marks from affected students.

"They really provided good accommodations, especially on such short notice," one Stoughton resident said.