Ice-Dam on Cold/Ventilated Water Shedding Steep-Slope Roofs

During the past few winters, WSRCA Member Contractors in varying areas of the western U.S. have reported an increasing number and intensity of winter-time ice-dam formation and resultant damage at various buildings in the western states.

This Bulletin’s discussion of ice-dams, along with guidelines and suggestions for mitigation of ice dams, is focused on steep-slope roof systems in general and primarily on ventilated (i.e., theoretically vented, so “cold-in winter”) watershedding steep-slope roof systems. These ventilated/cold watershedding steep-slope roofs are frequently, though not exclusively, found on single-family and multi-family residential construction. These ventilated steep-slope roofs are also found on light commercial (e.g., dental and medical clinics, etc.) and some institutional buildings (e.g., schools, community centers, etc.). This document also considers ice-damming on warm (i.e., non-vented, but compact insulated) watershedding steep-slope roofs, which are frequently found with cathedral-ceiling or vaulted-ceiling construction on multi-story townhouses, other split-level residential, smaller scale commercial, and on many churches, and some other institutional projects.

The Ice Dam Cycle

Icedams are the naturally occurring eave edge refreezing of snow-melt water, typically at or upslope from the exterior wall line of a building. The subsequent repetitive back-up and refreezing of snow-melt along downslope roof perimeters and eaves is typically the initiator of melt-water (i.e., snow and/or ice melt) intrusion through layer(s) of watershedding roofing systems.

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