A coroner's inquest into their deaths last year revealed that both victims suffered severe head injuries in the incident.
Patricia Willems, 43, and 40-year-old Owen Thomas McKenna - both Christchurch residents - also died at the same intersection when their cars were struck by falling masonry.
Mr Wiersma told the royal commission today that after the September 4 quake, he commissioned a basic engineering report to see what damage had occurred and whether it was safe to occupy.
He said he had found visible signs of damage inside the building with bricks having "come down'', and the stairwell damaged to such an extent that it couldn't be used.
Two weeks after the quake, he decided that the Ruben Blades students should no longer occupy the building as it was "unsafe'' with the fire escape out of order.
Mr Wiersma's "friend from church'' Sean Gardiner - a structural engineer at firm Structex - inspected the building on December 7.
In his report passed on to Wiersma on December 20, the engineer concluded that the building was "likely to be earthquake prone'' and suggested a detailed engineers' strength assessment be requested.
But Wiersma failed to ask for the report, which would have looked at the building in greater depth.
Under questioning from Mark Zarifeh, counsel assisting the commission, he admitted failing to carry out the recommended instructions, but he defended himself by saying: "I always said, 'do what it takes. Just do it.' He (Gardiner) was the expert.''
He was also asked by Marcus Elliott, counsel assisting the commission, that with the knowledge he now had, and in hindsight, whether he should have instructed a more detailed report.
The owner said: "Yes, I should have, but I thought everything was under control.''
The building was also inspected in January, and engineers again concluded they would benefit from finding out more about the building in a detailed engineering strength assessment.
Wiersma also admitted that he was unclear over his role in determining the building's strength and safety.
He said: "It was always my impression the council (Christchurch City) would instruct me to take action.''
Mr Zarifeh asked if he was waiting for the council to tell him to carry out further action. "Yes,'' he replied.
After the 2010 Boxing Day aftershock, the council served a building act notice on the building, which instructed repair work.
Wiersma asked for an extension from January 31, 2011 until May 31 to carry out the work, claiming today: "I didn't know how much time people needed to do the work.''
Repairs were underway when the February 22 quake struck, resulting in the building's frontage collapsing across Manchester St.
The royal commission will also hear evidence later today from engineers who inspected the building and Christchurch City Council environmental policy and approvals manager Steve McCarthy.
