Underlayment
A water-resistant or waterproof barrier material installed directly on the roof deck before shingles or other roofing materials.
The Roof's Backup Barrier
Underlayment is the water-resistant sheet installed directly on the roof deck, beneath the shingles. Shingles are the roof's primary defense against rain, but they're not fully waterproof on their own — wind-driven rain, ice dams, and normal wear can all let some moisture reach the deck. Underlayment is the layer that catches it before it reaches the wood.
Why Quality Matters
Two roofs can look identical from the street with very different underlayment underneath — thin felt paper versus a heavier Synthetic Underlayment, and whether self-adhered ice-and-water shield was used in vulnerable areas like eaves and valleys. Because it's completely hidden once shingles go on, underlayment quality is one of the easiest places for a lower bid to cut corners without a homeowner ever knowing, until a leak reveals it years later.