How Ice & Water Shield Works
Ice & water shield is a peel-and-stick membrane made of rubberized asphalt backed by a polyethylene film. When the backing is removed, the sticky underside adheres directly to the wood roof deck. Its most important feature is that it self-seals around roofing nails: when a fastener punctures the membrane, the rubberized compound grips the nail shank so water can't sneak through the hole.
This creates a continuous waterproof layer in the spots most likely to leak. Standard underlayment sheds water but does not seal around fasteners, which is why ice & water shield is reserved for high-risk areas rather than the entire roof.
Where It's Installed on a Roof
Roofers concentrate ice & water shield where water tends to pool, back up, or be driven by wind. Typical locations include:
- Eaves — the bottom edge of the roof, where ice dams form and trap meltwater
- Valleys — where two roof planes meet and channel large volumes of water
- Around penetrations — chimneys, skylights, vent pipes, and dormers
- Low-slope sections — porches and additions where water drains slowly
Building codes in cold regions generally require it along the eaves, extending up the roof to a point beyond the home's interior wall line so it covers the area most prone to ice damming.
Why It Matters in the Spokane & Inland Northwest Climate
The Inland Northwest puts roofs through a brutal cycle of heavy snow, freeze-thaw swings, and ice dams. When attic heat melts snow on the upper roof, that water runs down and refreezes at the cold eaves, building a dam of ice. Meltwater then pools behind the dam and can be forced backward, uphill, under the shingles, where ordinary underlayment offers no protection.
Because ice & water shield is fully bonded and self-sealing, it stops this backed-up water from reaching the wood deck and leaking into your home. Combined with wind-driven rain and hail common to the region, it's one of the most important defenses against the leaks, rot, and ceiling stains we see on Spokane and North Idaho homes. Proper ice & water shield is standard on every roof replacement we perform.
Ice & Water Shield vs. Standard Underlayment
Every roof needs an underlayment layer between the shingles and the deck, but not all underlayment is the same:
- Synthetic or felt underlayment covers the whole roof, sheds water, and resists tearing — but it does not seal around nails and is not fully waterproof.
- Ice & water shield is fully adhered and self-sealing, giving true waterproof protection — but it's more expensive, so it's used only in vulnerable zones.
A quality roofing system uses both: ice & water shield at the high-risk areas and synthetic underlayment across the open field of the roof. Skimping on the shield to save money is a common shortcut that leads to costly leaks during our snowiest winters.