Roof Underlayment Explained: Why the Layer You Can't See Matters Most

When most Charlotte homeowners think about their roof, they think about the shingles. That is what you can see, after all. But underneath those shingles is a layer that may matter even more: the underlayment. Underlayment is the material installed directly on the roof deck before the shingles go on, and it serves as the last line of defense between weather and the inside of your home.

If your shingles are damaged by a storm, blown off by wind, or cracked by hail, the underlayment is what keeps water from pouring into your attic. It is the backup plan that works when the primary plan fails, and in Charlotte's climate, that backup gets tested regularly.

What Underlayment Does

Underlayment has several jobs:

• Water barrier: It provides a secondary waterproofing layer beneath the shingles. When wind-driven rain pushes water under shingle edges during Charlotte's thunderstorms, the underlayment stops it from reaching the decking.
• Moisture management: It helps manage moisture vapor that can migrate through the roof assembly, particularly in Charlotte's humid climate where attic moisture is a constant concern.
• Protection during installation: During a roof replacement, there is a period where the old roof is removed and the new shingles are not yet installed. The underlayment protects the bare decking from any rain that might occur during this window, which is a real concern during Charlotte's pop-up afternoon thunderstorms in summer.
• Code compliance: The North Carolina building code requires at least one layer of underlayment on all residential roofs.

Types of Roof Underlayment

Felt Underlayment (Tar Paper)

Felt underlayment, commonly called tar paper, has been the standard for decades. It is made from a fiberglass or cellulose base saturated with asphalt. It comes in 15-pound and 30-pound weights, with 30-pound being thicker and more durable.

Felt is the cheapest option and does the job in many situations. But it has real drawbacks in Charlotte's climate. Felt absorbs water instead of repelling it, meaning it can become saturated and wrinkle during heavy rain. It tears easily during installation and degrades quickly in UV light, so it cannot be left exposed for long. In Charlotte, where afternoon storms can interrupt a roofing job with no warning, this is a real concern. If the crew has to stop for the day with exposed felt, a heavy rain could saturate it overnight.

Synthetic Underlayment

Synthetic underlayment has become the standard for quality roofing installations in the Charlotte market. Made from woven or spun polypropylene or polyethylene, it is stronger, lighter, and more water-resistant than felt.

The advantages for Charlotte homeowners are significant. Synthetic does not absorb water, so it handles heavy rain exposure without wrinkling or failing. It is much stronger than felt, resisting tears during installation and foot traffic from the roofing crew. It has better UV resistance, so if the project pauses for a day or two, it withstands sun exposure without breaking down. And it lies flat and smooth, providing a consistent surface for shingle installation.

Synthetic costs more than felt, typically adding $200 to $500 to a residential roof replacement depending on roof size. For the performance difference, it is worth every dollar in a climate like Charlotte's.

Ice and Water Shield

Ice and water shield is a self-adhering membrane that provides the highest level of waterproofing. Unlike felt and synthetic, which are held in place by fasteners, ice and water shield bonds directly to the roof deck. When a nail goes through it, the membrane seals around the nail shaft, creating a watertight seal at every fastener point.

In the Charlotte area, ice and water shield is typically applied at eaves, valleys, around penetrations, and along wall-to-roof transitions rather than across the entire roof. The NC building code requires it at eaves and valleys. Many experienced local roofers apply it beyond the minimums because Charlotte's wind-driven rain and occasional ice events warrant the extra protection. Read our detailed post on ice and water shield for more about where and why it is used.

What the NC Building Code Requires

The North Carolina Residential Code requires at least one layer of underlayment over the entire roof deck for asphalt shingle roofs. The minimum acceptable product is ASTM D226 Type I (15-pound felt) or equivalent. Synthetic underlayments meeting equivalent performance standards are also accepted.

For low-slope sections (2:12 to 4:12 pitch), the code requires either two layers of underlayment or a single layer of self-adhering membrane. Low-slope areas are more vulnerable to water infiltration because water moves across the surface slowly.

Ice and water shield is required at eaves, extending from the edge to at least 24 inches past the interior wall line, and in valleys. This requirement applies throughout the Charlotte metro, from Charlotte to Mooresville to Tega Cay.

How Charlotte's Weather Makes Underlayment Critical

Thunderstorms

Charlotte's severe thunderstorm season runs from March through September. These storms produce high winds, heavy rain, and hail. When storm damage compromises shingles, the underlayment is what protects your home until repairs happen. Quality synthetic underlayment or ice and water shield in critical areas gives you a much better safety margin than old felt paper.

Humidity

Charlotte's high humidity creates conditions where moisture can build up in the roof assembly. Proper attic ventilation is the primary defense, but breathable synthetic underlayment allows some moisture vapor to pass through while blocking liquid water. This helps keep the deck dry from both sides, which is particularly important in older Charlotte homes where ventilation may not be up to current standards.

Temperature Extremes

Charlotte experiences wide temperature swings. Summer roof surface temperatures exceed 150 degrees on dark shingles. Winter lows occasionally drop into the single digits. These thermal cycles cause materials to expand and contract repeatedly. Synthetic underlayments handle this better than felt because they do not become brittle in cold or soft in extreme heat.

What to Ask Your Contractor

When getting estimates for a roof replacement, ask about underlayment. Good questions:

• What type of underlayment do you use? If they say 15-pound felt across the board, that could indicate corner-cutting.
• Where will ice and water shield go? It should be at all eaves, valleys, and around penetrations at minimum.
• What brand and product? This lets you look up the specs yourself.
• Is underlayment included in the estimate? It should always be part of a roof replacement estimate, never an add-on.

The contractor's answers tell you a lot about their approach to quality. Someone who uses premium underlayment and applies ice and water shield generously is building a roof that will protect your home. Someone using the cheapest felt paper and only applying ice and water shield where strictly required by code is building to the minimum.

Get Your Roof Done Right with Peak Roofing

At Peak Roofing, we use quality synthetic underlayment and apply ice and water shield at all critical areas on every roof we install. Our team has over 10 years of experience roofing homes across the Charlotte area, from Waxhaw and Weddington to Cornelius and Lake Wylie. We do not cut corners on the parts you cannot see, because those are the parts that matter most when the next storm hits.

Call us at (704) 313-9341 or contact us online to schedule a free roof inspection and get a detailed estimate for your project. We will walk you through every layer of your new roof so you know exactly what you are getting.