Component
What Is Roofing Underlayment?
Also called: roofing felt, roof underlay, felt paper
Underlayment is the water-resistant layer installed between the roof deck and the finished roof covering. It sheds any water that gets past the covering, protects the deck during installation, and — in self-adhered "ice and water" form — seals around fasteners where codes require extra protection.
In plain English
Your roof covering is the armor; underlayment is the rain jacket underneath it. If wind-driven rain, an ice dam, or a damaged panel lets water past the surface, the underlayment is what keeps it out of your ceiling while the water finds its way to the gutter.
How underlayment works
Three families dominate: asphalt-saturated felt (#15/#30, the traditional choice), synthetic polymer sheets (lighter, stronger, more walkable, UV-tolerant during construction), and self-adhered rubberized-asphalt membranes that bond to the deck and self-seal around fastener penetrations — required by code at eaves in ice-dam regions and used at valleys and penetrations. Under metal roofing, high-temperature-rated underlayment matters: metal coverings run hotter than asphalt, and standard membranes can soften and adhere to the panels. A lifetime roof covering deserves an underlayment specified to a comparable service life.
Why it matters for your roof
Underlayment is the cheapest insurance on the roof and impossible to upgrade later without tearing off the covering. Under a 50-year metal roof, specifying a high-temp, long-service underlayment means the layer you can't see won't be the layer that fails first.
Common problems
Wrinkled or torn underlayment telegraphs through coverings; low-temp membranes can stick to hot metal panels; felt left exposed too long degrades before the covering is installed.
Don't confuse it with
Not to be confused with vapor retarders or house wrap — wall and interior moisture layers with different jobs.
- Felt vs synthetic underlayment
- Underlayment vs ice & water shield (self-adhered membrane)
Regional & climate notes
Most relevant in: Snow-heavy climates, Rain-heavy climates.
Frequently Asked Questions
Is underlayment required under a metal roof?
Yes — both code and manufacturers require it. Under metal, use a high-temperature-rated synthetic or self-adhered membrane so it won't soften against hot panels.
Can a metal roof be installed over old shingles with new underlayment?
Often yes — many interlocking metal systems are engineered for overlay installation with an approved underlayment between, avoiding tear-off cost and landfill waste where the structure and code allow it.
How long does underlayment last?
It depends on the product class: felts are rated in decades at best, while premium synthetics and self-adhered membranes are engineered for much longer service — match the underlayment's life to the covering's.
Related terms
See how an Interlock® lifetime aluminum roof handles this — engineered, manufactured, and installed by one company.
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