What size of water and ice dam protection should be used on a roof with a slope less than 4:12?

The correct approach for water and ice dam protection on roofs with slopes less than 4:12 is to use a minimum of 36 inches in from the inside exterior wall. This requirement stems from the increased risk of ice dam formation on low-slope roofs, where water can pool and freeze, potentially causing leaks and damage.

In areas prone to cold weather, ice dams can form when melted snow runs down the roof and refreezes at the eaves where it is colder. This scenario necessitates more extensive coverage of water and ice barrier materials to ensure that the potential for water infiltration is minimized. The 36-inch requirement effectively provides a buffer zone to protect against potential damage from these ice dams.

Choosing any lesser dimension, such as 24 inches, 18 inches, or even 46 inches, would either compromise the effectiveness of the barrier or exceed what is standard for the situation. Therefore, adhering to the minimum 36 inches maximizes protection and adheres to best industry practices for roofing in Illinois and other similar climates.