8.3 Michigan's Ice Barrier Requirement: Coverage, Materials & Detailing (R905.1.2)

Why Every Michigan Roof Needs It

Ice dams form when heat from the house melts snow on the upper roof and the meltwater refreezes at the cold eave. Water then backs up behind the ice and under the shingles, which shed water but do not seal against it. The IRC leaves the ice barrier decision to each jurisdiction's climate table. Michigan's Table R301.2(1) marks "Ice Barrier Underlayment Required: Yes" (R 408.30516). Footnote (h) ties that entry to areas where the average daily January temperature is 25°F or less, or where there is a history of ice-dam damage. For Michigan homes, treat the ice barrier as required.

The Rule (R905.1.2)

In areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2(1), an ice barrier shall be installed for asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes. The ice barrier shall consist of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet shall be used in place of normal underlayment and extend 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 with slope equal to or greater than 8 units vertical in 12 units horizontal, the ice barrier shall also be applied not less than 36 inches measured along the roof slope from the eave edge of the building.

Exception: Detached accessory structures not containing conditioned floor area.

The individual covering sections repeat the requirement, including R905.2.7 for asphalt shingles, R905.4.3.1 for metal shingles, R905.5.3.1 for roll roofing, R905.6.3.1 for slate, R905.7.3.1 for wood shingles, and R905.8.3.1 for shakes. Photovoltaic shingles have their own version in R905.16.4.1.

Measuring the Coverage

The code measures to a point 24 inches inside the exterior wall line. The exterior wall line is the outside face of the wall where it meets the roof. A practical method:

1. Measure the horizontal eave overhang from the fascia (or drip edge) back to the exterior face of the wall.
2. Add 24 inches to get the horizontal distance.
3. Multiply by the common slope factor to get the distance along the deck.
4. On 8:12 and steeper roofs, use the larger of that result and 36 inches along the slope.
5. Divide by the membrane's usable width, subtracting side laps, and round up to whole courses.

Going past the minimum costs little. Many Michigan contractors run two courses at eaves as standard practice and add membrane in valleys, around chimneys and skylights, and at low-slope transitions.

Approved Materials

• Self-adhering polymer-modified bitumen sheet (ASTM D1970 is the standard for these underlayments, and the R905.1.1 exception allows D1970 membranes). This is the common modern choice. It seals around nail shanks.
• Two layers of underlayment cemented together, the traditional method of solid-mopping or cementing two felt plies.
• Either one is installed in place of normal underlayment in the ice barrier area.

Valleys and Other Details

• Valley lining rules are in R905.2.8.2 (Section 9.3). ASTM D1970 self-adhering membrane may be used in place of the listed valley linings.
• For metal valley flashing under tile or metal shingles, R905.3.8 and R905.4.6 add a rule for areas where the average January temperature is 25°F or less. There, the metal valley flashing underlayment must be solid-cemented to the roofing underlayment on slopes under 7:12, or be self-adhering polymer-modified bitumen.
• Wood shingle and shake roofs in those cold areas need solid sheathing under the ice barrier portion of the roof (R905.7.1.1, R905.8.1.1).

Drip Edge Sequence

R905.2.8.5: underlayment goes over the drip edge along eaves and under the drip edge along rake edges. At the eave, install the drip edge on the deck, then run the ice barrier over its flange. Water that backs up behind an ice dam then flows over the metal instead of behind it. At the rake, install the underlayment first and the drip edge over it.

Cold-Weather Application

Self-adhered membranes depend on temperature for adhesion. Follow the manufacturer's minimum application temperature and any primer, storage, or back-nailing instructions. Keep rolls warm before use, and do not apply over frost, snow, or wet decks. Never use an open flame to warm a self-adhered membrane on a wood deck.

Replacement Jobs

On a replacement, an existing ice barrier bonded to the deck may stay if it is covered with a new layer (R908.3 exception). On a legal recover, the new covering must still meet Chapter 9, including the ice barrier requirement (R908.1).