How much higher must a chimney extend above the point of exit through the roof?

The requirement for how much higher a chimney must extend above the point of exit through the roof is based on ensuring safe venting of combustion gases. A chimney should extend at least three feet above the roof where it exits to prevent backdrafts and ensure that the products of combustion are dispersed effectively.

This height helps to create a proper draft, which is critical for efficient and safe operation. If the chimney is too short, it may not allow the gases to rise adequately, which can lead to potential dangers, including the risk of carbon monoxide entering the living space.

Additionally, the chimney must also be at least two feet higher than any portion of the building within a horizontal distance of ten feet. This further establishes a vertical space that maintains safe venting conditions and minimizes the risk of downdrafts that can occur if the chimney is lower than surrounding structures.

The other choices do not meet the minimum safety guidelines established for chimney height, making three feet the appropriate and necessary standard for proper chimney function and safety.