The party, which had at least 1,500 attendees, began early Saturday morning and was still ongoing when fire crews responded to the incident in the 300 block of Hathway Avenue at about 6:21 a.m.
Witness Hanna Brand called the situation "surreal."
"We show up at the party. It's so crowded. You're bumping elbows. You can't move really. Everyone is way too drunk. It's insanity," Brand said.
Courtesy of Hanna Brand
A mass casualty incident was declared as three fire paramedic-staffed engines and the Battalion Chief arrived on scene.
A video posted on the college's newspaper's YouTube page showed the roof collapse.
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"It was difficult because we were on the roof and there were too many people coming on the roof, so we couldn't get off," witness Hayley Westphal said.
One victim was impaled in the thigh by a piece of wood as the roof collapsed. Seven other victims were treated for minor injuries, according to the San Luis Obispo Fire Department.
"I think we are very, very lucky there weren't more serious injuries or fatalities," said San Luis Obispo fire Chief Garret M. Olson.
Update from Hathaway incident- eight patients, 4 of which transported to local ERs, mass casualty incident declared. pic.twitter.com/z1PwRxWDgW— SLO City Fire (@SLO_City_Fire) March 7, 2015
Cal Poly released a statement following the roof collapse Saturday: "Cal Poly is also in close contact with local public safety agencies as they work to determine exactly what occurred. Their findings will help inform our own efforts to ascertain what led to this incident and decide how we will respond."
Police said three people were arrested and charged with misdemeanors.
ABC News contributed to this report.