What is the correct top lap measurement for single underlayment when starting at the eave line?

The top lap measurement for single underlayment when starting at the eave line is 2 inches. This measurement is critical for ensuring that there is adequate overlap between the sheets of underlayment. Proper lap measurements help to prevent water infiltration, which can cause significant issues like leaks and damage to the roofing structure beneath the underlayment.

By maintaining a 2-inch top lap, it ensures that any water that may seep through the shingles has a minimal chance of entering the building, as the upper layer overlaps sufficiently to redirect it. Understanding this measurement is essential for anyone involved in roofing installations, as it contributes to the overall integrity and durability of the roofing system.