Michigan's Ice Barrier Requirement: What the Code Says at the Eaves

In the national model code, the climate table in Section R301.2 is a blank form. Each jurisdiction fills in its own ground snow load, frost depth, wind speed and, in one column, whether ice barrier underlayment is required. Footnote h of that table tells the jurisdiction to answer "Yes" where the average daily January temperature is 25°F or less, or where there is a history of local damage from ice damming.

Michigan did not leave that column blank. In the state's version of Table R301.2(1), adopted as administrative rule R 408.30516, the ice barrier column already reads "Yes" for the whole state. So you will not find Kent County, Ottawa County or the City of Kalamazoo publishing their own ice barrier answer. The answer was given once, in the state code, and every enforcing agency applies it.

The climate numbers back that up. NOAA's 1991 to 2020 normals put the average January temperature at 24.8°F at both Grand Rapids and the Kalamazoo/Battle Creek airport, under the 25°F line. Muskegon's airport, warmed slightly by the lake, averages 26.6°F in January, just over it. Because Michigan filled the column in statewide, lakeshore homes still fall under the requirement, and West Michigan's ice dam history would justify it anyway.

The 24 inches is measured horizontally from the wall, but the membrane is laid along the slope, starting at the roof edge. Two things stretch the distance: the overhang, which has to be crossed before you even reach the wall, and the pitch, because a sloped run is longer than its level footprint.

That is why one roll is often not enough. Self-adhered membranes commonly come in rolls about 36 inches wide. On a 6:12 roof with a 12-inch overhang, the membrane has to run about 40 inches up the slope to reach 24 inches inside the wall. With an 18 or 24-inch overhang, it takes closer to 47 to 54 inches. In each case a second course is needed, lapped over the first per the manufacturer's instructions.

Manufacturers may ask for more than the code. GAF's current Timberline installation instructions call for its leak barrier at eaves and wherever ice dams can be expected, extending 24 inches beyond the inside wall line. The inside wall line is the interior face of the wall, so GAF's measurement reaches a few inches further than the code's, by the thickness of the wall. Because the code also requires roofing to follow the manufacturer's installation instructions (R904.1 and R905.1), the stricter of the two is the one to build to.

When the whole roof is replaced, Section R908.3 says replacement includes removing existing layers of roof coverings down to the deck. There is one exception that matters at the eaves: where the existing roof includes an ice barrier membrane adhered to the deck, it may stay in place and be covered with an additional layer of ice barrier membrane. That is a practical rule, since fully adhered membrane often cannot come off without destroying the deck.

Shingling over an existing roof is a different story. Section R908.1 requires re-cover materials to meet Chapter 9, which includes the ice barrier, and a membrane laid on top of old shingles is not the same as one bonded to the deck. Thomas Township near Saginaw, for example, tells permit holders that an overlay must show in detail how ice dam protection will be applied. Stellar won't warranty a shingle overlay and talks people out of them, and R908.3.1.1 bars a re-cover outright where the roof already has two or more layers.

An ice barrier does not stop ice dams. It makes the roof edge tolerate them. Ice dams form when heat escaping from the house melts snow higher on the roof and the water refreezes at the cold overhang. The cure for that is in the attic: air sealing, insulation and balanced ventilation. A roof with perfect membrane can still leak when a dam grows taller than the membrane reaches, or where water backs up in a valley or behind a chimney.

Warranties draw the same line. GAF's shingle warranty, for example, treats ice damming as a condition it does not cover, apart from leaks in an area covered by its leak barrier that come from a defect in the leak barrier itself. If your house has recurring ice dams, fix the heat loss and treat the membrane as the backstop it was meant to be.