Final answer:
The acceleration due to gravity is the primary factor determining the time it takes for a ball to reach the roof when thrown vertically upward.
Step-by-step explanation:
The primary factor determining the time it takes for a ball to reach the roof when thrown vertically upward on a flat roof is the acceleration due to gravity. The acceleration due to gravity is the primary factor determining the time it takes for a ball to reach the roof when thrown vertically upward.
This is because gravity acts as a downward force on the ball, causing it to decelerate and eventually reach its highest point before falling back down. The mass of the ball, initial velocity, and air resistance do not significantly affect the time it takes to reach the roof in this scenario.