Underlayment is the layer of material installed directly on the roof deck before the shingles go on. When shingles are damaged, blow off in a storm, or fail prematurely, underlayment is what prevents water from entering the structure. Most homeowners never think about it — it is invisible under the finished roof — but it plays a significant role in the long-term performance of the roofing system. Understanding underlayment is part of the broader picture of how long your roof will last — inferior underlayment choices shorten the effective lifespan of even premium shingles.
Felt Underlayment (15# and 30#)
Traditional roofing felt — made of organic or fiberglass mat saturated with asphalt — is the baseline product. 15-pound felt is lighter and thinner; 30-pound felt is heavier and more tear-resistant. Both have been used successfully for decades.
Felt has real limitations. It is susceptible to tearing in wind during installation, absorbs moisture and wrinkles when exposed to rain before shingles are installed, and is relatively fragile underfoot. In most applications today, synthetic underlayments have replaced felt as the standard product.
Synthetic Underlayment
Synthetic underlayments — typically woven or nonwoven polypropylene or polyester — are now the dominant product. They are lighter, stronger, and more tear-resistant than felt. They can be exposed to rain for longer periods without degrading, they do not wrinkle, and they provide better slip resistance for workers on the roof during installation.
Most manufacturers of synthetic underlayment warrant the product for 30 years or more. Premium products are also more vapor-open than felt, allowing any moisture that does get in to escape rather than being trapped. The cost premium over felt is modest — $10–$20 per square — and the performance advantages are clear.
Self-Adhering Membrane (Ice and Water Shield)
In cold climates, building codes require a self-adhering modified bitumen membrane — commonly sold under brand names like Grace Ice and Water Shield — in the valleys and at the eaves (typically the first 24 inches inside the heated wall line, plus the depth of the eave overhang). This "ice and water" protection is a response to ice dam risk: when ice dams force water under shingles, the self-adhering membrane provides a watertight seal that prevents the water from entering the structure.
In warm climates, ice and water is not required everywhere, but many contractors use it at all valleys, around penetrations, and at the eaves regardless — it is excellent insurance against the most common leak locations.
What to Ask Your Contractor
When comparing quotes, ask what underlayment product each contractor is specifying. "Standard underlayment" means different things to different contractors. Get the specific product name — GAF Deck-Armor, Owens Corning RhinoRoof, Grace Ice and Water — so you can compare apples to apples.
Also ask about ice and water shield coverage: where will it be installed, and how many feet up from the eaves? In northern climates with frequent ice dam risk, some contractors extend ice and water significantly higher than code minimum, which provides meaningfully better protection. Good underlayment specification works in tandem with good flashing — see our roof flashing guide for how these two components protect the same vulnerable transition points.