IRC R905.1.2

What does the test involve?

In areas with a history of ice forming along the eaves causing a backup of water — as designated in Table R301.2(1) — an ice barrier is required. It consists of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet, extending from the lowest edges of all roof surfaces to a point not less than 24 inches inside the exterior wall line of the building. On roofs sloped 8:12 or steeper the barrier is also applied not less than 36 inches measured along the slope from the eave edge.

Common misconceptions

• The trigger is a LOCAL designation in Table R301.2(1), not a national rule. Whether your address needs one is a question for your building department.
• The 24 inches is measured inside the exterior wall line, not from the roof edge. On a house with a deep overhang that is considerably more than 24 inches of membrane.
• The 2024 IRC changed this provision. Before relying on the figures above, confirm which edition your state has adopted.