Old Canaan Building Materials

Ice and water shield is one of the most misunderstood underlayments in New England roofing. It is required at eaves and valleys in a cold climate. It is not a roof replacement. It is not appropriate under the entire roof deck. Its presence everywhere is a sign of a contractor who does not understand what it does or why — and whose work will produce a worse outcome than the correct limited application.

Ice and water shield is a self-adhering rubberized asphalt membrane, typically 36" to 40" wide, installed at the eave edge and in valleys before the primary roofing material is applied. It is a black or grey membrane, smooth or granule-surfaced, that seals around roofing nails and creates a waterproof barrier at the roof's most vulnerable locations.

Ice and water shield is required by code in cold climates at the eave edge — typically 24 inches inside the interior wall line — to prevent water infiltration from ice dams. At valleys, it provides secondary waterproofing at the most water-concentrated location on the roof. Standard products: Grace Ice and Water Shield, Henry BlueSkin, and similar rubberized asphalt membranes.

Ice and water shield is not breathable. Under a cedar shingle roof, it prevents the ventilation from below that cedar requires to perform correctly. Under a slate roof, it traps moisture and accelerates decay of the wood substrate. It is correctly used at eaves and valleys — locations where the waterproofing benefit outweighs the ventilation penalty — and nowhere else.

The code-required ice and water shield at the eave edge prevents the most common and most damaging leak in a New England roof: the ice dam backup. Water pooling behind an ice dam infiltrates under any roofing material without a self-adhering membrane below it.

From roofing suppliers. Specify the brand and product. Confirm with the contractor exactly where it will be installed — eave to 24 inches inside the wall line, and at valleys. Reject any proposal to install it under the full roof deck.