Synthetic Underlayment: The Layer Under a Hail Roof That Nobody Sees

Shingles shed water. They don't seal. Wind-driven rain gets pushed up under the laps, ice backs meltwater under the courses at the eaves, and every nail is a small hole in the roof. The underlayment catches whatever gets past the shingles and walks it down to the gutter instead of into the decking.

It also protects the deck while the job is underway. On a big spring hail replacement, a crew might strip a slope at eight in the morning and catch a thunderstorm by two. A papered deck can ride that out. Bare plywood soaks it up and swells.

Asphalt felt, the old 15- and 30-pound paper, still works. Plenty of roofs in older Midwest neighborhoods have it under them right now. It's also heavy, it wrinkles when it gets wet, and it rips when a boot slides on a steep pitch. Wrinkled felt telegraphs through new shingles as ridges you can spot from the curb.

Synthetic rolls weigh a fraction as much, so a crew hauls fewer of them up the ladder and covers more deck per roll. The sheet doesn't absorb water, so it doesn't buckle. Most are printed with lap lines and a fastener pattern, which keeps courses straight. Many carry an ultraviolet exposure rating measured in weeks or months, so a roof left papered over a rainy weekend isn't a crisis.

It has drawbacks too. Some bargain sheets get slick when they're wet or dusty, and that's a safety issue on a steep roof. Synthetic is also less permeable than felt, so attic ventilation has to be right or moisture from the house has nowhere to go. That's one more reason we climb into the attic on every replacement.

Underlayment is invisible the day the crew leaves, which makes it the easiest line to shave. Two shortcuts show up most. One is the thinnest synthetic on the supply yard shelf, light enough that you can see daylight through it, held down with staples. The other is skipping it on part of the roof, laying new shingles straight onto a section of bare deck where the old felt tore away.

Fasteners matter more than people guess. A synthetic sheet held with plain staples can tear loose around each one in a gust before the shingles are on, and once it's flapping it does nothing. Most manufacturers specify plastic or metal cap fasteners. Ask which your crew uses.

A hail replacement estimate should carry its own underlayment line covering the full roof area, separate from the shingle line. Sometimes it's priced as felt when the shingle manufacturer's instructions call for something better. Sometimes it's missing altogether, especially on estimates written fast after a big storm when one adjuster is juggling dozens of houses.

An absent or undersized line is a normal thing to add through a roof claim supplement. Whether the carrier pays for a higher grade that local code demands depends on the ordinance or law coverage in your policy, if it has any. We go over every line item with you before work starts. Our hail damage insurance claim page covers the rest of the process.

Underlayment never works alone. Along the eaves in cold country, a self-adhering membrane goes down first and the synthetic laps over it; our page on ice and water shield explains that membrane. At valleys, sidewalls and chimneys, each piece has to lap under the flashing in the right order, upper over lower, so water runs across a seam instead of into it. Most leaks we chase on newer roofs start at those transitions, not out in the open field.

We won't lay new shingles over old felt left on the deck. And we won't skip checking the wood just because the underlayment would hide it. If a sheet of decking has gone soft or started to delaminate, paper on top only buries the problem. We replace bad wood first, photograph it for the claim, then cover the roof. To see how that fits into a whole job, read our hail damage roof replacement page.