Roofing underlayment is the layer that sits between the roof deck and the shingles. It adds a second line of defense against moisture and provides extra insulation for the top of the house, keeping water that slips past the shingles away from the attic and the framing below. Despite that job, it is one of the most overlooked parts of a roof.
There are three main types of roofing underlayment in use today: asphalt-saturated felt, rubberized asphalt, and synthetic. Each has different costs, exposure limits, and installation methods, and the differences between them are spelled out in this comparison of reinforced roofing underlayment types, exposure limits, and installation. Like the shingles above them, underlayment layers need replacement at semi-regular intervals based on the roof’s condition, so understanding what you are buying matters at every stage of a roof’s life.
What Roofing Underlayment Does
Underlayment is installed directly over the roof deck and under the final roof covering. While shingles shed the bulk of rainwater, wind-driven rain, ice dams, and minor shingle damage can still let water reach the deck. The underlayment catches that water and directs it down the slope before it soaks into the sheathing and the attic.
The Secondary Barrier
Think of the shingles as the primary roof and the underlayment as the backup. During installation, the underlayment also protects the bare deck from weather between the time the old roof is stripped and the new one goes on. That temporary protection window matters, because a storm that hits an open deck can ruin sheathing in a single afternoon.
The barrier role continues after the shingles are on. Asphalt shingles are nailed through the underlayment, and those nails are a permanent set of small holes in the roof plane. The underlayment is what keeps water from following the nail shank down into the wood.
Underlayment vs. Full Waterproofing
Standard underlayment is water-resistant, not fully waterproof. Nail penetrations and seams are the weak points, and water that collects at a seam or works its way around a fastener will eventually find the deck. That is why installers add self-adhering membrane at valleys, eaves, and other high-risk zones.
Where Full Waterproofing Is Required
Valleys, roof edges, and areas around chimneys and skylights get extra protection as a rule. In cold climates, a self-adhering membrane is often run several feet up the slope from the eave to defend against ice dams. Low-slope sections of the roof may skip shingles entirely and use a membrane system, and homes with those sections commonly combine underlayment with thermoset roofing systems such as EPDM rubber membranes, which are installed and detailed differently from shingle roofs.
Asphalt-Saturated Felt
Asphalt-saturated felt is the traditional underlayment, sold in rolls rated at 15 pounds or 30 pounds per square, where a square covers 100 square feet of roof. The base is an organic or fiberglass mat saturated with asphalt, and it has protected roofs for generations because it is cheap, familiar, and easy to install. Roofers know exactly how it behaves.
Fifteen-Pound vs. Thirty-Pound Felt
Fifteen-pound felt is the thinner, lighter roll and the most common choice for standard shingle roofs. Thirty-pound felt is roughly twice as heavy, more tear-resistant, and a better choice for steeper roofs and higher-traffic conditions. The heavier felt costs more per roll but covers the same area with a tougher barrier.
How Felt Is Installed
Roofers lay felt in horizontal courses starting at the eave, overlapping each upper course over the lower one by 2 to 4 inches. The sheets are fastened with roofing nails or staples, and the laps are positioned so water flows over them rather than under. Felt is water-resistant rather than waterproof, so seams and fasteners must be detailed carefully.
Felt also has limits in direct sun. Its asphalt coating softens in heat and degrades under ultraviolet light, which is why manufacturers cap its exposure time around 90 days. If a roof sits partially covered through a summer, the exposed felt can wrinkle and lose its water-shedding ability. For covered porches and patios, the full set of patio roofing materials, from polycarbonate panels to metal sheets, is worth reviewing before you choose a covering for those spaces.
Rubberized Asphalt Underlayment
Rubberized asphalt underlayment is a self-adhering membrane made from modified asphalt blended with rubber polymers. It comes with a peel-and-stick backing, so it bonds directly to the deck, and it seals around nails rather than letting them become leak points. That sealing ability makes it the closest thing to a waterproof layer available in a rolled underlayment.
Where Self-Adhering Membranes Shine
Roofers use rubberized asphalt at the spots most likely to leak: eaves in cold climates, valleys, rakes, and penetrations. The membrane is also the standard first course at the bottom of the roof, where it bridges the transition between deck and fascia.
Ice Dam Zones
In snowy regions, ice dams form when heat escapes through the attic and melts snow at the ridge, then the water refreezes at the colder eave. A self-adhering membrane run 3 to 6 feet up the slope from the eave keeps that meltwater from backing up under the shingles. Building codes in many northern jurisdictions require this detail.
Installation Considerations
Self-adhering membranes demand a clean, dry deck and temperatures warm enough for the adhesive to grip. Once down, they are hard to reposition, so layout is marked before the backing is peeled. Full-coverage rubberized asphalt roofs cost more than felt, but they buy serious leak protection on premium homes and low-slope areas. For low-slope sections that need a single-ply approach, thermoplastic roofing systems such as TPO and PVC offer another route.
Synthetic Underlayment
Synthetic underlayment is the modern option, woven or spun from polypropylene or polyethylene polymers. It weighs a fraction of felt for the same coverage, resists tearing and slipping, and survives sun exposure far longer than traditional felt. A single roll covers roughly twice the roof area of a felt roll, so crews install it faster and carry less weight up the ladder.
Why Roofers Switched
Synthetic underlayment does not absorb water the way felt can, it does not tear at the nail line as easily, and its surface gives workers better footing on steep slopes. The plastic weave also shrugs off the ultraviolet exposure that degrades felt, so a partially covered roof can wait weeks instead of days for its shingles.
Exposure Ratings and Storage
Manufacturers publish exposure limits, typically 6 to 12 months for synthetics versus about 90 days for felt. That number assumes the product is stored out of direct sun and installed with the correct laps. The UV rating is about unshaded exposure, so a synthetic underlayment left bare through a full summer still needs shingles within its rated window. The same polymer family shows up across the whole roof system, and modern synthetic roofing materials now include polymer-based shingles, panels, and accessories that pair naturally with synthetic underlayment.
How to Choose the Right Underlayment
The right underlayment depends on the roof’s slope, the covering material, the local climate, and the budget. A code official can tell you the minimum required for your area, but the minimum is rarely the best value over the life of the roof. Matching the barrier to the covering is the first rule, and climate is the second.
Match the Underlayment to the Roof Covering
Heavy coverings demand more from what sits underneath. Clay and concrete tiles hold heat and weigh far more than asphalt shingles, so they need an underlayment with a high temperature rating and enough tear strength to survive foot traffic during installation. Clay tile roofing systems require specific deck prep and flashing details that differ from shingle roofs, so read the covering manufacturer’s requirements before choosing the underlayment grade.
Cost, Climate, and Trade-Offs
Felt costs the least and is fine for a budget re-roof in a mild climate with a straightforward slope. Synthetic costs more per roll but installs faster, which can offset the material price with labor savings, and its tear resistance pays off on steep roofs. Rubberized asphalt costs the most and is reserved for high-risk zones or full coverage on premium roofs. The table below summarizes the trade-offs.
- Roof slope: steeper roofs shed water faster and can use lighter underlayment.
- Covering type: heavy tile and stone need higher-rated barriers.
- Climate: cold regions require self-adhering membrane at eaves and valleys.
- Budget: felt saves up front, synthetic saves labor, rubberized asphalt saves callbacks.
| Type | Weight per square | Relative cost | UV exposure limit | Water resistance | Best use |
|---|---|---|---|---|---|
| Asphalt-saturated felt | 15-30 lb | Lowest | About 90 days | Water-resistant | Budget re-roofs, mild climates |
| Rubberized asphalt | Self-adhering sheet | Highest | Not rated for exposure | Seals around fasteners | Eaves, valleys, ice dam zones |
| Synthetic | Lightweight roll | Moderate | 6-12 months | Water-resistant, tear-resistant | Most shingle and tile roofs |
Installation, Lifespan, and Inspection
Underlayment is replaced with the shingles, so its lifespan tracks the roof covering rather than a fixed calendar. A 25-year asphalt shingle roof usually gets new underlayment at year 25; a slate or metal roof can run far longer, which puts more years of work on the barrier beneath.
When to Replace
Any time the deck is exposed is the right time to replace the underlayment. Re-roofing over old felt is a false economy, because the aged barrier will fail long before the new shingles do. Natural stone roofs are the extreme case: slate roofing systems can last a century, so the underlayment under them has to be the best available grade, installed with the same care as the stone above it.
Signs of Underlayment Failure
- Water stains on attic rafters or the underside of the deck after heavy rain.
- Shingle edges lifting or wind damage exposing the layer beneath.
- Torn, wrinkled, or rotted felt visible at the rake or eave.
- Mold or mildew odor in the attic on warm days.
If any of those signs appear, have the roof inspected from both sides before the next storm. The underlayment is the quiet layer that does the dirty work, and a small repair at the right time is far cheaper than replacing soaked sheathing.