If your shingles are your roof's first line of defense, underlayment is the second. It is the layer between the shingles and the wood decking, and its job is to provide a secondary water barrier in case water gets past or under the shingles. Which it will, eventually.

BHR crew installing underlayment on a shingle roof replacement with shingle bundles staged on the roof, Park City, Utah . Bob Harvey Roofing

What it looks like

Underlayment is a sheet material that gets rolled out across the entire roof deck before the shingles go on. It overlaps in courses from the eaves to the ridge, so water that reaches it runs downhill and off the edge without penetrating to the wood.

Felt versus synthetic

For decades, roofing underlayment was asphalt-saturated felt paper, commonly called tar paper. It came in two weights: 15-pound and 30-pound. It worked, but it had drawbacks. Felt absorbs water, tears easily, wrinkles in heat, and degrades quickly when exposed to UV if left uncovered for more than a day or two.

Modern synthetic underlayment has largely replaced felt on quality installations. Synthetic underlayment is made from woven or spun polypropylene. It is lighter, stronger, lies flatter, resists moisture, and holds up significantly better if it gets exposed to weather during installation. It is also safer to walk on because it is less slippery when wet.

We use synthetic underlayment on every installation. The cost difference over felt is small relative to the total job cost, and the performance difference is significant. If a contractor is still using 15-pound felt, ask them why.

"Without underlayment, any water that gets past the shingles goes straight into the decking and from there into your attic and your home."

Why it matters

Shingles are not waterproof in the way most people think. They shed water through overlap and gravity. But wind-driven rain can push water sideways under shingle edges. Ice dams push water uphill. Nail holes are potential entry points. Flashing transitions are vulnerable. In all of these situations, the underlayment is what stands between the water and your wood decking.

Without underlayment, or with underlayment that has failed, any water that gets past the shingles goes straight into the decking and from there into your attic and your home.

What to look for on your estimate

Your roofing estimate should specify the type of underlayment being used. "Synthetic underlayment" or a specific product name is what you want to see. If the estimate does not mention underlayment at all, or if it specifies 15-pound felt, that is worth a conversation about why.