How high must rain caps be above grade and horizontally from operable openings?

The requirement for the height of rain caps above grade and their horizontal distance from operable openings is based on ensuring adequate safety and effectiveness in preventing rainwater from entering structures and allowing for proper ventilation.

The correct choice indicates that rain caps must be positioned 15 feet above grade. This height provides a sufficient buffer to mitigate the likelihood of rainwater intrusion, as water is not typically projected to such heights. The horizontal distance of 4 feet from operable openings helps prevent any potential for water to be inadvertently directed toward windows, doors, or vents that may be operable, optimizing the performance of the rain cap in preventing ingress of moisture.

In this context, the other options specify heights and distances that either exceed practical requirements or do not provide enough of a barrier to effectively protect against rainwater. The balanced specifications of 15 feet above and 4 feet horizontally in the correct answer make sense according to both safety practices and operational guidelines for installations designed to manage rainwater drainage.