When you look at a finished roof, all you see are the shingles. But underneath those shingles is a layer of material that does just as much work keeping water out of your house — sometimes more. That layer is the underlayment, and it is the part of a roof replacement that most homeowners never ask about, never see, and never think about until it fails.
Here is what underlayment is, why it matters, the different types Charlotte roofers use, and what you should be asking for when you get quotes for a new roof.
What Roof Underlayment Actually Does
Underlayment is a sheet material that gets installed directly on top of the plywood roof decking, before the shingles go on. It has two jobs:
- Secondary water barrier. Shingles are the first line of defense, but they aren't waterproof on their own. They overlap and shed water through gravity, but wind-driven rain, ice dams, and blown-off shingles can push water past them. The underlayment catches whatever gets through and routes it to the drip edge instead of into your attic.
- Temporary weather protection. During installation, there is always a period when the decking is exposed — between tear-off of the old roof and installation of new shingles. Underlayment protects the decking from rain during that gap. In Charlotte, where afternoon thunderstorms pop up with little warning between April and October, this temporary protection matters a lot.
Without proper underlayment, water that gets past the shingles goes straight into the plywood decking. Wet plywood rots. Rotting plywood loses structural strength. And by the time you notice a water stain on your ceiling, the decking damage can be extensive.
The Three Types of Roof Underlayment
1. Asphalt-Saturated Felt (Tar Paper)
Felt paper has been used under roofs for over a century. It comes in two weights — 15-pound felt and 30-pound felt — and it is exactly what it sounds like: thick paper saturated with asphalt to make it water-resistant.
15-pound felt is the thinner, cheaper option. It tears easily, wrinkles when it gets wet, and can deteriorate if left exposed to sun for more than a few days. Most roofers around the Charlotte area no longer use 15-pound felt for full roof installations because it just does not hold up well enough, especially in our humidity.
30-pound felt is thicker, more durable, and more tear-resistant. It costs more than 15-pound but performs significantly better. For homeowners on a tight budget who want a step up from the bare minimum, 30-pound felt is a reasonable choice. It works. It has a proven track record going back decades.
The downsides of felt: it absorbs moisture and wrinkles, it tears during installation (especially in wind), and it deteriorates faster than synthetic underlayment in Charlotte's heat and humidity. A roll of 30-pound felt costs about $25-$35. For a typical Charlotte home, felt underlayment adds $300-$600 to the total roof replacement cost.
2. Synthetic Underlayment
Synthetic underlayment is made from polypropylene or polyethylene — basically a woven plastic fabric. It is the most popular choice for new roof installations in the Charlotte market today, and for good reason.
Advantages over felt:
- Much stronger. Synthetic underlayment resists tearing far better than felt. Roofing crews can walk on it without ripping it, and it handles wind during installation without blowing away.
- Does not absorb water. Unlike felt, synthetic does not soak up moisture and wrinkle. It lays flat, stays flat, and does not deteriorate from exposure to Charlotte's humidity.
- Lighter. A roll of synthetic covers more area and weighs less than felt, which makes installation faster and keeps labor costs down.
- UV-resistant. Synthetic can be left exposed to sunlight for weeks without degrading, which gives roofers more flexibility on timing. Felt starts breaking down after a few days of direct sun.
- Slip-resistant surface. Most synthetic products have a textured surface that gives roofers better footing. This is a safety issue that actually matters.
The cost difference is small. Synthetic underlayment runs $400-$800 for a typical Charlotte home — only $100-$200 more than felt. For that difference, you get a dramatically better product. Major brands include GAF FeltBuster, Owens Corning ProArmor, and CertainTeed DiamondDeck.
Most Charlotte roofers have switched to synthetic as their standard underlayment. If a contractor quotes you felt paper without mentioning synthetic, ask why. If the answer is "to save money," the savings are not worth it.
3. Rubberized Asphalt (Self-Adhering / Ice and Water Shield)
This is the premium option — a sticky, rubberized membrane that peels and sticks directly to the roof deck. Unlike felt and synthetic, which are attached with staples or cap nails, rubberized underlayment self-seals around nail penetrations. When a shingle nail punches through it, the rubberized material squeezes around the nail shaft and seals the hole.
This is the stuff you want in the high-risk areas of your roof — valleys, around chimneys, at the eave line, around skylights, and anywhere water tends to pool or flow in volume. It is expensive ($1-$3 per square foot vs. $0.10-$0.30 for synthetic), so most roofers do not use it as a whole-roof underlayment. Instead, they use rubberized membrane in the vulnerable areas and synthetic over the rest of the field.
NC building code requires a self-adhering membrane at the eave line in areas prone to ice damming. Charlotte is technically in a region where ice dams are uncommon, but most major shingle manufacturers require ice and water shield at the eaves and in valleys as a condition of their warranty. If your contractor is not installing it in those areas, your manufacturer warranty may not be valid.
What North Carolina Building Code Requires
NC Residential Building Code requires underlayment on all asphalt shingle roofs. The minimum is a single layer of ASTM D226 Type I (15-pound felt) or ASTM D4869 Type I. Synthetic underlayment that meets ASTM D226 equivalent performance is also accepted.
In practice, the code sets a floor — the bare minimum. Most Charlotte roofing contractors exceed code requirements because the cost difference between the minimum and a better product is small compared to the price of the overall job. If a contractor is quoting you the bare code minimum on underlayment, they are cutting corners.
How Charlotte's Climate Affects Underlayment Choice
Charlotte's weather is hard on roofs. Here is why that matters for underlayment:
- Humidity. Charlotte averages 70-80% relative humidity in summer. Felt paper absorbs that moisture and can degrade faster than in drier climates. Synthetic doesn't absorb moisture at all.
- Heat. Summer temperatures regularly hit 95°F+, and attic temperatures can exceed 140°F. Underlayment bakes under those shingles all summer. Synthetic handles heat better and lasts longer than felt under extreme temperatures.
- Wind-driven rain. Charlotte gets 43 inches of rain per year, much of it coming in heavy storms with strong winds. Wind pushes water sideways and uphill under shingles. A quality underlayment with properly lapped seams catches that wind-driven moisture before it reaches the decking.
- Temperature swings. Charlotte can swing from the low 30s to the mid-60s in a single winter week. Those temperature changes cause expansion and contraction that stresses underlayment seams and fastener penetrations.
What to Ask Your Roofer About Underlayment
When you get quotes for a roof replacement, ask these specific questions about underlayment:
- What type and brand of underlayment are you using? Get the specific product name, not just "felt" or "synthetic." The quote should specify.
- Where are you putting ice and water shield? At minimum, it should go at the eaves (first 3 feet), in all valleys, around chimneys, around skylights, and around any roof-to-wall transitions.
- Is the underlayment included in the warranty? Manufacturer system warranties from GAF, Owens Corning, and CertainTeed typically require specific underlayment products from their brand. If your contractor mixes brands, the system warranty may not apply.
- Can I see the underlayment before shingles go on? A good contractor will either invite you to look or take photos showing the underlayment installation. This is your one chance to verify what went down before it is covered forever.
The Bottom Line on Underlayment
The underlayment under your shingles is the last line of defense between the weather and the wood structure of your house. Spending an extra $200-$400 for synthetic over felt — or making sure ice and water shield goes in the valleys and at the eaves — is some of the cheapest insurance you can buy on a roof that costs $10,000+.
Don't let a contractor skip it, skimp on it, or use the cheapest option to pad their profit margin. Underlayment isn't the place to save money on a roof replacement.