What is the condition under which a parapet is required for exterior walls?

A parapet is often required for exterior walls that are fire-resistance rated primarily to enhance fire protection measures. When a building has walls designed to resist the spread of fire, especially between different occupancies or divisions of space, the inclusion of a parapet helps to control the vertical spread of flames and reduce the potential for fire escape over the roofline to adjacent structures. This is important in urban settings where buildings are close together.

Additionally, the presence of a parapet can provide a physical barrier that limits the exposure of combustible materials on the roof to potential ignition sources or flames, thus reinforcing the fire-rated integrity of the wall. It can also contribute to overall safety by directing water runoff away from the wall, further minimizing the risk of damage that could also lead to fire hazards.

The other conditions presented—building occupancy, combustibility of the roof, or adjacency to a street—while important in their own contexts, do not universally mandate the installation of a parapet for exterior walls, making the fire-resistance rating the most relevant and definitive condition for this requirement.