Roof underlayment is a water-resistant material installed directly over the roof sheathing, before the final roofing material, serving as a critical secondary barrier against water intrusion if wind-driven rain, ice damming, or minor shingle damage allows moisture to get past the primary roof covering.
This layer isn’t typically visible once a roof is complete, yet it plays an outsized role in a roof’s overall water resistance, particularly in regions prone to severe wind or ice dam formation.
Underlayment options range from traditional asphalt-saturated felt paper to more modern synthetic materials, each offering different performance characteristics for durability, water resistance, and ease of installation.
What Types of Roof Underlayment Are There?
- Asphalt-saturated felt, the traditional option
- Synthetic underlayment, offering improved durability and water resistance
- Self-adhering rubberized asphalt for high-risk areas like valleys and eaves
- Specialized products for specific climate or roofing material needs
Why Is Roof Underlayment Important?
Even a well-installed shingle roof can experience minor water intrusion during severe wind-driven rain or ice dam conditions, and underlayment provides the critical backup protection that keeps this moisture from reaching the sheathing and interior spaces beneath. This secondary defense is precisely why code requires underlayment beneath virtually every type of steep-slope roofing material, regardless of the primary covering’s own water resistance.
In colder climates, code often requires self-adhering ice and water shield underlayment specifically at eaves and valleys, addressing the elevated risk of ice dam-related water backup that standard felt or synthetic underlayment alone might not adequately resist. Choosing the right underlayment for the specific climate and roof design is worth discussing with an experienced contractor.
Roof Underlayment vs. Roof Sheathing
Underlayment is a water-resistant material layer installed on top of the sheathing. Sheathing is the rigid structural panel substrate beneath it, and the two work together, with underlayment providing moisture protection for the structural layer below.