Roof Underlayment Guide: Felt vs Synthetic vs Ice & Water Shield (2026 NJ)

Roof underlayment is one of those roofing components that most homeowners never think about -- and that is fine, because when it is installed correctly, you should never have to. But when it fails or when a contractor skips it to save money, you notice fast. Water in the attic. Stains on the ceiling. Rot in the decking.

We install underlayment on every roof we touch across Essex County. It is a non-negotiable part of our process. The type of underlayment we use depends on the roof section, the roofing material going on top, and what NJ building code requires for your specific roof. This guide walks through all three types, when each one is the right choice, what the code says, and what it costs.

Whether you are getting a new roof, comparing shingle types, or just trying to understand what your roofer is quoting you, this is the guide.

What Is Roof Underlayment?

Roof underlayment is a sheet material installed directly on top of the roof decking (sheathing) and underneath the shingles, tiles, or metal panels. It is a secondary moisture barrier -- meaning the shingles are the primary weather protection, and the underlayment is the backup system underneath.

Think of it this way: if your shingles are the raincoat, the underlayment is the waterproof liner inside. On a perfect day with no wind and no damage, the shingles keep all the water out and the underlayment just sits there. But when a shingle blows off in a storm, or when wind-driven rain gets pushed under the shingle edges, or when ice dams force water uphill under the shingles -- the underlayment is what prevents that water from reaching the wood decking and your attic below.

Every professionally installed roof in New Jersey has underlayment. It is required by the NJ building code (which follows the International Residential Code), by all major shingle manufacturers for warranty coverage, and by basic roofing best practices. A roof without underlayment is a roof waiting to leak.

What Underlayment Does

• Secondary water barrier -- protects the decking when wind-driven rain or ice dams push water under the shingles
• Temporary weather protection -- keeps the decking dry during construction before shingles are installed (critical in NJ where rain can delay roofing work for days)
• Smooth nailing surface -- provides a consistent surface that helps shingles lay flat and allows nails to seat properly
• Code compliance -- required by NJ building code on all sloped roofs
• Warranty requirement -- GAF, CertainTeed, and Owens Corning all require proper underlayment for their shingle warranties to be valid
• Vapor management -- helps manage moisture movement between the attic space and the roof surface, especially important in NJ's humid summers

Without underlayment, every nail hole in the decking is an unprotected penetration point. Every gust of wind that lifts a shingle edge exposes bare wood to rain. In New Jersey, where we get nor'easters, heavy thunderstorms, and ice storms every winter, that is a recipe for roof leaks and decking rot.

Three Types of Roof Underlayment Compared

There are three types of roof underlayment used in residential roofing today. Each has a different job, different cost, and different place on the roof. Here is the overview, followed by a deep dive into each.

Felt Underlayment (Tar Paper)

Felt underlayment -- commonly called tar paper -- is the traditional roof underlayment that has been used on homes for over a century. It is made from an organic or fiberglass mat that is saturated with asphalt to make it water-resistant. It comes in rolls that are 36 inches wide and cover approximately 400 square feet per roll.

Felt is still used today on some roofing projects, primarily where budget is the top priority. It does the job, but it has significant limitations compared to modern synthetic underlayment -- especially in a climate like New Jersey's.

15 lb vs 30 lb Felt: What the Numbers Mean

The "15 lb" and "30 lb" designations originally referred to the weight per 100 square feet (one roofing square). Modern manufacturing has changed the actual weights, but the names stuck. Here is what matters:

15 lb felt is the thinner version. It was once the standard on most residential roofs, but it has largely fallen out of favor. It tears easily during installation, especially on windy days. It wrinkles when it gets wet and can buckle under the shingles, creating visible bumps on the finished roof. It offers minimal water protection if left exposed.

30 lb felt is about twice as thick and significantly more durable. It holds up better to foot traffic during installation, resists tearing in moderate wind, and provides better temporary weather protection. If a homeowner insists on felt over synthetic (usually for cost reasons), 30 lb is the only option we would use.

That said, the price difference between 30 lb felt and synthetic underlayment is small when you account for the larger coverage area per roll of synthetic. In most cases, upgrading from felt to synthetic adds only $100 to $250 to a full roof project -- a small investment for significantly better protection.

Pros and Cons of Felt Underlayment

Synthetic Underlayment

Synthetic underlayment is the modern standard for residential roofing. It is made from woven polypropylene or polyethylene -- essentially an engineered plastic fabric that is water-resistant, extremely tear-resistant, lightweight, and lays perfectly flat on the decking.

Synthetic underlayment has almost entirely replaced felt on quality roofing installations in the last 15 years. It costs more per roll, but each roll covers about 2.5 times more area than felt. It installs faster because it lays flatter, comes in wider and longer rolls, and does not wrinkle or buckle. For NJ roofs that face heavy rain, snow, ice, and humidity, synthetic underlayment is the clear better choice.

Popular Synthetic Underlayment Brands

The three major shingle manufacturers each make their own synthetic underlayment products. Using the same brand of underlayment and shingles can simplify warranty coverage (system warranties). Here are the most common brands we see and use in NJ:

All three perform similarly in real-world use. The choice usually comes down to which shingle brand is being used on the project -- matching the underlayment brand to the shingle brand gives the homeowner the strongest possible warranty coverage.

Pros and Cons of Synthetic Underlayment

Ice and Water Shield (Self-Adhering Membrane)

Ice and water shield is the premium underlayment product -- and the only one that is truly waterproof, not just water-resistant. It is a rubberized asphalt membrane with a self-adhering (peel-and-stick) backing that bonds directly to the roof decking, creating a watertight seal.

The key feature that makes ice and water shield different from felt and synthetic: it self-seals around nail penetrations. When a roofing nail is driven through it, the rubberized asphalt squeezes around the nail shaft and creates a waterproof seal. Felt and synthetic underlayment do not do this -- every nail hole in felt or synthetic is a potential water entry point if water reaches that layer.

This self-sealing property is why ice and water shield is required in the most vulnerable areas of the roof: eaves (where ice dams form), valleys (where water concentrates), and around penetrations like chimneys, skylights, and vent pipes -- everywhere water is most likely to get under the shingles.

Where Ice and Water Shield Goes on the Roof

• Eaves -- along the bottom edge of the roof, extending at least 24 inches past the interior wall line (NJ code minimum). Most roofers install 3 to 6 feet up from the eaves for better ice dam protection
• Valleys -- the full length of every roof valley, extending at least 18 inches on each side of the valley centerline
• Around penetrations -- vent pipes, chimneys, skylights, and any other object that passes through the roof surface
• Low-slope transitions -- where a steep roof section meets a low-slope area (common on NJ homes with additions or dormers)
• Sidewall intersections -- where the roof meets a vertical wall, such as dormers or second-story walls

On some premium installations, homeowners choose to install ice and water shield over the entire roof deckinstead of just the required areas. This provides the highest level of protection possible but adds significantly to the cost. Full-deck ice and water shield is most common on high-value homes, steep or complex roofs, and in coastal areas with extreme wind exposure.

Pros and Cons of Ice and Water Shield

Despite the higher cost, ice and water shield is non-negotiable in the areas where NJ code requires it. We have seen what happens when contractors skip it -- water damage from a single ice dam event can cost more than underlayment for the entire roof. It is insurance you pay for once and never think about again.

NJ Building Code Requirements for Roof Underlayment

New Jersey follows the International Residential Code (IRC) for residential construction, with state-specific amendments. Here is what the code requires for roof underlayment in NJ.

Specific Code Requirements

What This Means for Your NJ Roof

In practical terms, every NJ roof replacement in 2026 should include:

• Ice and water shield at all eaves (minimum 24 inches past the interior wall line, ideally 3-6 feet up from the eave edge)
• Ice and water shield in all valleys (full length, 18+ inches each side of centerline)
• Ice and water shield around all penetrations (chimneys, skylights, vent pipes)
• Felt or synthetic underlayment over the remaining roof field

When we pull a roofing permit in any Essex County municipality, the inspector will check for proper underlayment installation. Skipping ice and water shield or installing it incorrectly can result in a failed inspection, which means tearing off shingles to fix it -- an expensive mistake.

Roof Underlayment Cost Comparison (2026 NJ Prices)

Here is what each type of underlayment costs and what it looks like on a real NJ roofing project. These are material costs -- labor for underlayment installation is included in the overall roof replacement cost.

Real-World Example: Average NJ Home (2,000 sq ft roof area)

Here is what underlayment typically costs on an average Essex County home with 2,000 square feet of roof area, a hip roof with two valleys, one chimney, one skylight, and standard eave overhang:

For comparison, using felt instead of synthetic for the field area would save roughly $50 to $100 in materials -- a minimal savings on a project that typically costs $8,000 to $15,000+ total. The ice and water shield cost is the same regardless because it is required by code. This is why we recommend synthetic for the field -- the small extra cost buys significantly better performance and longevity.

When Each Type of Underlayment Is Used

Different parts of the roof need different types of underlayment. Here is when each type is the right choice.

Use Ice and Water Shield When:

• At the eaves -- required by NJ code. Protects against ice dam water backup
• In all valleys -- where water concentrates during rain and snowmelt
• Around all roof penetrations -- chimneys, skylights, vent pipes, satellite mounts
• At low-slope transitions -- where steep sections meet shallow-pitch areas
• At sidewall intersections -- where the roof meets a vertical wall
• On entire deck (premium option) -- for maximum protection on high-value homes or complex roof designs

Use Synthetic Underlayment When:

• Main roof field -- everywhere that is not covered by ice and water shield
• Under asphalt shingles -- the standard for modern architectural shingle installations
• Under metal roofing -- high-temperature synthetic recommended for standing seam and other metal roof installations
• Under cedar shakes/shingles -- breathable synthetic underlayment specifically designed for wood roofing
• Under slate -- heavy-duty synthetic for slate roofing applications
• When weather delays are likely -- synthetic handles rain exposure for weeks without degrading

Use Felt Underlayment When:

• Budget is the absolute priority -- when the homeowner needs the lowest possible material cost
• Small projects -- sheds, garages, covered porches where the stakes are lower
• Under certain cedar installations -- some cedar shake manufacturers specifically call for felt because of its breathability
• Temporary protection -- as a short-term cover during construction (though synthetic is better for this too)

Does Underlayment Quality Actually Matter?

Yes. Absolutely yes.

Underlayment is the last line of defense between the weather and your roof decking. When a shingle blows off in a nor'easter -- and they do, even on well-installed roofs -- the underlayment is what prevents water from reaching the wood. When an ice dam pushes water uphill under the shingle tabs, the underlayment (specifically, the ice and water shield at the eaves) is what stops it from soaking the decking and dripping into your attic.

We have torn off roofs where the contractor used the cheapest possible felt underlayment -- or in the worst cases, no underlayment at all in certain sections. The decking underneath was rotted, stained, and covered in mold. The homeowner had no idea because the shingles on top looked fine from the ground. The damage was happening silently underneath for years.

Contrast that with roofs where quality synthetic underlayment and proper ice and water shield were installed: even after 20+ years, the decking is clean and solid. The underlayment did its job every time water got past the shingles, and the homeowner never knew it happened -- which is exactly the point.

What Cheap Underlayment Costs You Long-Term

• Felt wrinkles and buckles can telegraph through shingles, creating visible bumps on the roof surface that look unprofessional
• Water absorption leads to faster deterioration under the shingles where you cannot see it
• Shorter lifespan means the underlayment may fail before the shingles do, leaving you unprotected for years
• Poor temporary protection during construction means any rain delay risks water damage to the decking
• Manufacturer warranty implications -- using non-recommended underlayment can limit warranty coverage on your shingles

NJ's climate is not forgiving. We get heavy thunderstorms in summer, nor'easters in winter, ice storms, high winds, and everything in between. The underlayment is the one component that protects the wood structure of your roof when all other defenses are breached. Spending an extra $100 on better underlayment is one of the smartest investments in a roofing project.

How Long Does Roof Underlayment Last?

Roof underlayment is designed to last as long as the roof system it is part of -- typically 25 to 50 years depending on the roofing material. It is not a standalone product that wears out on its own schedule. When properly installed under shingles, it is protected from UV exposure and direct weather, which is what causes degradation.

Lifespan by Type

What Shortens Underlayment Life

The main enemies of underlayment longevity are heat and moisture -- the same things that damage roof decking:

• Poor attic ventilation -- trapped heat and moisture accelerate degradation of all underlayment types, especially felt. Proper roof ventilation is the single best way to extend underlayment life
• Excessive attic heat -- dark shingles on south-facing roofs without adequate ventilation can create attic temperatures exceeding 150 F, which bakes the underlayment from above
• Repeated water exposure -- from ice dams, small leaks, or condensation. Felt absorbs water and degrades; synthetic and ice and water shield are more resistant
• UV exposure during construction -- if underlayment is left exposed too long before shingles go on, UV breaks down the material. Synthetic handles this far better than felt

The takeaway: proper attic ventilation and insulation are the best ways to ensure your underlayment lasts as long as your shingles. Without adequate ventilation, even the best underlayment will degrade prematurely. With it, synthetic underlayment and ice and water shield will comfortably outlast a 25 to 30 year shingle roof.

How R&E Roofing Handles Underlayment in Essex County

Underlayment is one of the things that separates a quality roof installation from a hack job. Here is our standard approach on every project.

We itemize underlayment in every roof replacement quote so homeowners can see exactly what they are getting. If another contractor's quote does not mention underlayment type or lumps it under "materials," ask them to specify -- because it matters. Knowing the right questions to ask can save you thousands in hidden problems.

Frequently Asked Questions

R&E Roofing serves homeowners across Essex County, NJ. We handle roofing, siding, gutters, and storm damage restoration. Licensed, insured, and registered as a New Jersey Home Improvement Contractor.