What an ice barrier is
An ice barrier is a waterproof underlayment installed along the lower edge of the roof, under the shingles. The most common type is a self-adhering polymer-modified bitumen membrane, often called by brand names like ice and water shield. It sticks to the roof deck and seals around nails driven through it, so water that backs up under the shingles behind an ice dam cannot get through to the deck and into the house.
Ordinary felt or synthetic underlayment sheds water running downhill, but it is not designed to hold back standing water. Behind an ice dam, water pools and runs uphill under the shingles. That is where an ice barrier makes the difference.
What the building code requires
The International Residential Code (section R905.1.2), as adopted in many states, requires an ice barrier in areas where there has been a history of ice forming along the eaves and causing a backup of water, as designated by local jurisdictions. Under the code text summarized by state building officials such as those in Massachusetts, the ice barrier consists of at least 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 at least 24 inches inside the exterior wall line of the building. Requirements vary by code edition and local amendments, so check with your building department.
The idea behind the 24-inch measurement is that ice dams form over the cold overhang and just above the wall, where the roof transitions from unheated to heated space. The membrane must extend far enough up the roof to cover where backed-up water can reach.
How it works with drip edge and gutters
At the eave, the order of layers matters. Under the code, drip edge at the eaves is installed under the underlayment, with the ice barrier lapping over the drip edge, so any water on top of the membrane drains onto the drip edge and into the gutter. (At the rakes, the order is reversed.) Some high-wind programs, such as FORTIFIED, specify different details; follow the system your roofer is installing. The gutter then sits under the drip edge so water lands in it. See drip edge and gutter apron.
If the ice barrier or drip edge is installed wrong, water that backs up behind ice can drain behind the gutter onto the fascia, or into the soffit, even when the membrane itself is intact.
Gutters and ice dams
Gutters do not cause ice dams; heat loss from the house does. But gutters full of ice extend the dam outward and add weight. A properly installed ice barrier keeps backed-up water out of the house, strong hangers keep the gutter on the fascia, and attic air sealing and insulation reduce how much meltwater forms in the first place. See ice dams and gutters and snow load and gutters.
When it gets installed
An ice barrier is installed when the roof is replaced. It cannot be added under existing shingles without removing them. That is one more reason to coordinate gutter replacement with a new roof: the roofer can install the ice barrier and drip edge correctly, and the gutter installer can hang new gutters on sound fascia in the right relationship to the roof edge. See new roof, new gutters?
Valleys and other vulnerable areas
Many roofers also install self-adhering membrane in valleys, around chimneys and skylights, and along low-slope sections, where water concentrates or moves slowly. In valleys, the membrane protects against the heavy, concentrated flow that also challenges gutters at valley corners. See roof valleys and gutter overflow.
Signs you may lack an ice barrier
Water stains on ceilings or walls near exterior walls after ice dams, wet insulation or decking at the eaves seen from the attic, and peeling paint on soffits in late winter all suggest water is getting past the roof edge. A roofer can confirm what is under the shingles when the roof is next replaced, or by lifting a few shingles at the edge.
Warm climates
In regions without a history of ice dams, codes generally do not require an ice barrier at the eaves, though some roofers use membrane at eaves and valleys anyway for added protection against wind-driven rain. In hurricane areas, sealed roof decks under programs like FORTIFIED serve a similar purpose. See FORTIFIED roofs and gutters.
Ventilation still matters
An ice barrier stops leaks; it does not stop ice dams from forming. The dams themselves come from heat escaping into the attic and warming the roof deck. Soffit and ridge ventilation, air sealing at the attic floor and enough insulation keep the deck cold, so snow melts evenly and less ice builds at the eaves and in the gutters. A roof with both a good ice barrier and good ventilation is far more resilient than one with either alone.
What it costs
Self-adhering membrane costs more than standard underlayment per square foot, but it covers only a strip along the eaves, valleys and penetrations, so the added cost on a typical re-roof is modest compared with repairing ceilings, walls and insulation after an ice dam leak. In snow country, it is one of the most cost-effective parts of a new roof.
Questions to ask your roofer
Ask whether your area requires an ice barrier and how far up the roof it will extend, whether it will also go in valleys and around penetrations, how the drip edge and ice barrier will be layered at the eaves, and how the new roof edge will meet the gutters. If you are replacing gutters at the same time, make sure both crews agree on the sequence.