Underlayment: The Layer Under Your Shingles and Why It Matters

What underlayment actually is

Underlayment is the layer between your roof decking and your shingles. It is not waterproof on its own, but it is a secondary barrier that catches any water that makes it past the shingles (and water does make it past - wind-driven rain, small shingle gaps, and ice dams all push water under the shingle line).

For decades, the standard underlayment was felt paper (15 lb or 30 lb organic asphalt-impregnated felt). It worked, but it tore easily, wrinkled in humidity, and rotted if it stayed wet.

Why synthetic underlayment is the modern standard

Synthetic underlayment (GAF Deck-Armor, Owens Corning ProArmor, and similar) is a woven polymer sheet. It does not absorb water, does not rot, does not tear easily under foot traffic, and installs flatter and straighter than felt. It also lasts longer if the roof is delayed mid-project and left exposed for a few days.

On any modern reroof, synthetic underlayment is the right default.

Ice-and-water shield is different

Ice-and-water shield is a separate, self-adhering, fully waterproof membrane that goes in the most vulnerable areas: eaves, valleys, around chimneys and skylights, and around any penetration. It bonds directly to the deck and seals around nails driven through it.

In the Northeast, building code generally requires ice-and-water shield at least three feet up-slope from all eaves to protect against ice dams. A good roofing contractor often goes further than code.

What to ask before a reroof

Ask specifically what underlayment will be used and where ice-and-water shield will be applied. The cheapest bid is usually 15 lb felt everywhere. A proper system uses synthetic across the field and ice-and-water at all eaves, valleys, and penetrations.

The difference in cost is modest. The difference in protection is not.