Shingles shed almost all the water — underlayment catches the rest. Rolled out across the decking before any shingles go on, it backs up the roof against wind-driven rain, protects the deck during installation, and buys time if a shingle ever blows off. The old standard is asphalt-saturated felt ("tar paper," #15 or #30); modern synthetics are lighter, far more tear-resistant, and grip better underfoot when wet.
Why it matters in Douglas County
A backup layer means more in a place with roughly 130 rain days than almost anywhere else. Our winter storms drive rain sideways under shingle edges, and our wet-then-bone-dry cycle ages the top layer fast — so the layer underneath gets called into service more often than homeowners think. There's a hidden local benefit, too: because Douglas County prohibits shingle-over-shingle overlays, every legal reroof is a full tear-off — which means every reroof gets brand-new underlayment instead of hiding 25-year-old felt under new shingles.
When it fails, what it costs you
Old felt gets brittle, cracks along deck seams, and tears in wind. Once the shingles above it start losing granules and lifting, brittle felt offers little backup, and small leaks find the nail holes. You can't inspect it without opening the roof — which is why the underlayment spec on a replacement estimate deserves a close read.
Felt or synthetic underlayment for Oregon rain?
We generally recommend synthetic here. It resists tearing in wind, tolerates longer exposure during the job, doesn't wrinkle when damp, and holds up better over a 20–25 year shingle life in our climate. Felt still works, but on a roof you plan to keep, synthetic is cheap insurance.
Related terms
Decking · Ice & water shield · Starter strip · Roofing square