Roof underlayment is the water-resistant layer installed on the roof deck before shingles go down. The three main roof underlayment types are asphalt-saturated felt, synthetic, and rubberized ice and water shield. Felt is the cheapest and shortest-lived. Synthetic lasts longer and resists moisture better. Ice and water shield is required in New Jersey at the eaves. It is the only one of the three that’s fully waterproof on its own.
What Does Roof Underlayment Actually Do?
Underlayment is a protective layer between the roof deck and the roof covering. Whether that’s asphalt shingles, metal panels, or another roofing material. It’s not decorative, and it’s not optional. If a shingle lifts in high winds, cracks from age, or loses a tab in a storm, the underlayment is what keeps water out of the house until the roof covering is repaired.
Most roofing problems that show up as ceiling stains or attic moisture didn’t start with a failed shingle. They started with an underlayment that was the wrong type, installed incorrectly, or past its functional life. Getting this layer right matters more than its low visibility suggests.
Asphalt-Saturated Felt Underlayment
Felt, sometimes called tar paper, is the traditional underlayment most older New Jersey roofs were built with. It’s made from a paper or fiberglass mat saturated with asphalt, and it comes in two weights: No. 15 and No. 30. The heavier 30-pound felt resists tearing better and holds up longer than the 15-pound version.
Felt underlayment typically lasts 12 to 20 years under normal conditions. It’s the least expensive option per square foot, which is why it still shows up on budget-conscious projects and some lower-cost re-roofs. The tradeoffs are real, though. Felt absorbs moisture instead of shedding it, which means it can wrinkle, buckle, or tear if it’s wet before the shingles go on. It also has a limited window of UV exposure, generally somewhere between 30 and 90 days, before sun exposure starts degrading it. On a job where weather delays push installation back, that timeline matters.
Synthetic Roofing Underlayment
Synthetic underlayment is made from woven or spun polypropylene or polyethylene rather than asphalt-saturated paper. It’s become the standard choice on most roofing projects today, and for good reason: it’s lighter, faster to install, more tear-resistant, and offers better slip resistance for the crew working on the roof.
Synthetic underlayment typically lasts 20 to 30 years, with some premium products rated up to 50 years, depending on the manufacturer. It can also handle far more UV exposure before installation than felt, often staying intact for up to six months if a project gets delayed. The tradeoff is cost. Synthetic underlayment generally runs two to three times more than felt per square foot. For a full roof replacement on a typical New Jersey home, that difference adds up, but it’s usually worth it given how much longer synthetic outlasts felt and how much better it performs in wind-driven rain.
Rubberized Asphalt (Ice and Water Shield)
Rubberized asphalt underlayment, known in the trade as ice and water shield, is different from the other two types in one important way: it’s self-adhering. The underside has an adhesive backing that bonds directly to the roof deck and seals tightly around nail penetrations, creating a fully waterproof seal rather than just a water-resistant one.
This is the underlayment New Jersey code requires at the eaves. and it’s also the right call for valleys, around chimneys, and anywhere water tends to concentrate or pool. It’s the only one of the three types that can stop a leak caused by an ice dam. Since it seals around the fasteners that would otherwise let water through once ice forces water backward under the shingles.
What New Jersey Code Requires
New Jersey’s residential building code is based on the International Residential Code. Adopted and amended through the NJ Uniform Construction Code. Under IRC Section R905.1.2, as adopted in New Jersey’s Residential Subcode, any area with a history of ice forming along the eaves and causing a backup of water must have an ice barrier installed. That barrier has to be either two layers of underlayment cemented together. Or a single layer of self-adhering rubberized asphalt, and it must extend from the lowest edge of the roof. A point at least 24 inches inside the exterior wall line.
New Jersey’s winters qualify. The code’s trigger point is an average January temperature of 25°F or lower, and that threshold is met across Morris, Essex, Union, Somerset, and Bergen Counties. In practice, this means ice and water shield at the eaves isn’t a premium upgrade in New Jersey. It’s a code requirement, and skipping it isn’t a corner worth cutting.
Low-slope sections of roof carry a separate requirement. For asphalt shingles on a roof pitch between 2:12 and 4:12, the code calls for either two full layers of heavier underlayment. A single layer of ice and water shield covering the entire deck, not just the eaves.
Ice dams aren’t the only way water finds a gap. Wind-driven rain during a nor’easter can push moisture sideways under shingles that would otherwise hold up fine in a normal storm. If a roof has already taken storm damage, the underlayment underneath is worth checking even if the shingles look intact from the ground.
Underlayment by Roofing Material
The right underlayment also depends on what’s going on top of it. Asphalt shingles, metal roofing, and other roof coverings each have manufacturer specifications that pair with specific underlayment types, and using the wrong combination can void the roofing system’s warranty even if the installation itself is sound.
| Underlayment | Lifespan | Best Suited For |
| Felt (No. 15 / No. 30) | 12-20 years | Budget-conscious projects, mild climates |
| Synthetic | 20-30 years, up to 50 for premium | Most asphalt shingle and metal roofing projects |
| Rubberized ice and water shield | Matches roof covering above it | Eaves, valleys, chimneys, low-slope sections |
GAF and other manufacturers publish specific underlayment requirements for each shingle line. Following those specs isn’t just about meeting code. It’s what keeps the manufacturer’s warranty intact if something goes wrong down the line. AHC installs from GAF’s full shingle lineup, and underlayment compatibility is part of how each system is specced, not an afterthought.
How Underlayment Gets Installed
Proper installation follows a bottom-up sequence so each course overlaps the one below it, directing water down and off the roof rather than letting it work backward under a seam. A few details separate a correct installation from one that fails early:
- The roof deck must be clean, dry, and structurally sound before any underlayment goes down
- Felt and synthetic underlayment both require cap nails, not standard roofing nails, to prevent tearing around the fastener
- Ice and water shield goes down first at the eaves and valleys, with the broader underlayment layered over and around it according to manufacturer instructions
- Multiple layers of underlayment may be required depending on roof pitch and local code
This is also where it’s worth being honest about something most homeowners don’t see: a rushed underlayment installation is invisible until it fails. By the time a leak shows up, the shingles, the labor, and the inspection are long done. A roof inspection won’t always catch a bad underlayment job before problems start, but it’s the fastest way to find out once something’s gone wrong. Asking a contractor specifically how they handle underlayment, not just what shingle brand they install, is a fair question and a useful one.
Conclusion
Underlayment is the part of a roofing system nobody sees, and almost everybody underestimates. Felt, synthetic, and ice and water shield each have a place. But in New Jersey, the ice barrier requirement at the eaves isn’t optional, and the underlayment choice for the rest of the roof should match. Both the roofing material going on top and the realistic exposure the roof deck will face during installation.
If your roof is due for a replacement, the underlayment decision happens early in the process, usually before you’ve even picked a shingle color.
FAQs
What roofing underlayment is best?
Synthetic underlayment offers the best overall balance of durability, moisture resistance, and lifespan for most New Jersey roofs. Ice and water shield is required at the eaves regardless of which underlayment covers the rest of the roof.
What is the best time of year to replace shingles?
Spring and fall offer the most reliable installation temperatures in New Jersey. Extreme summer heat and winter cold can both affect how well shingles seal and how underlayment performs during installation.
Which roof underlayment is waterproof?
Rubberized asphalt, or ice and water shield, is the only fully waterproof underlayment type. Felt and synthetic materials are water-resistant, meaning they shed water effectively but aren’t designed as a complete waterproof barrier on their own.
What is the typical underlayment for a roof in New Jersey?
Most New Jersey roofs use synthetic underlayment across the main roof field. With rubberized ice and water shield required at the eaves and recommended in valleys and around chimneys.
Can felt and synthetic underlayment be mixed on the same roof?
Yes, and it’s common. Many roofs use ice and water shield at the eaves and valleys. With a synthetic or felt underlayment covering the remaining roof deck.
Does underlayment affect my shingle warranty?
It can. Manufacturers often specify approved underlayment types for their shingle warranties to apply. Using an unapproved underlayment can void coverage even if the shingles themselves are installed correctly.