Synthetic Roof Underlayment vs Felt Paper: Complete Guide
Roof underlayment is the moisture barrier installed between your roof deck (the plywood) and your shingles or other roofing material. It is invisible once the roof is finished, but it is arguably the most important layer for preventing water damage. The two main options are synthetic underlayment ($0.15 to $0.25 per square foot) and traditional felt paper ($0.05 to $0.10 per square foot). Synthetic costs more but outperforms felt in water resistance, durability, tear strength, and heat tolerance.
In Myrtle Beach and coastal South Carolina, this is not a close call. The combination of hurricane-force winds, heavy rain, extreme heat, and strict Wind Zone III building codes makes synthetic underlayment the standard. Felt paper cannot reliably meet coastal code requirements and degrades faster in our climate. Every roof we install at WeatherShield uses synthetic underlayment as a baseline — we will not compromise on the layer that sits between your home and water damage.
This guide explains exactly what roof underlayment does, how synthetic and felt compare in every category that matters, what each costs, why coastal building codes effectively require synthetic, and how the two materials differ in real-world installation.
Want to understand every layer of a roof? See our complete guide to roof layers for a top-to-bottom breakdown of every component. Need a repair or replacement? Visit our roof replacement services.
What Is Roof Underlayment?
Roof underlayment is a thin, sheet-like material applied directly over the roof deck (typically plywood or OSB sheathing) before shingles or other roofing material is installed on top. It serves as a secondary water barrier: if wind-driven rain gets under the shingles, or if a shingle is cracked or missing, the underlayment prevents water from reaching the wood deck and entering your home.
Every modern building code requires underlayment beneath shingles. It is not optional. The question is which type gives you the best protection for your specific climate and roofing situation.
There are actually three categories of underlayment:
- Felt paper (asphalt-saturated felt) — The traditional option. Available in 15-pound and 30-pound weights. Made from organic felt or fiberglass mat saturated with asphalt. Used for over 100 years.
- Synthetic underlayment — The modern standard. Made from woven or spun polypropylene or polyethylene. Lighter, stronger, and more water resistant than felt. Most products have been on the market since the early 2000s.
- Self-adhering ice and water shield — A rubberized asphalt membrane that sticks directly to the deck. The most expensive option, used in specific high-risk areas like eaves, valleys, and around penetrations. Required in coastal SC along the first 3 feet of the roof edge at minimum.
This guide focuses on the synthetic versus felt decision, which applies to the majority of the roof surface. Ice and water shield is used in addition to (not instead of) the primary underlayment in critical areas.
Synthetic Underlayment vs Felt Paper: Head-to-Head
| Feature | Synthetic Underlayment | Felt Paper (15 or 30 lb) |
|---|---|---|
| Cost per sq ft | $0.15 – $0.25 | $0.05 – $0.10 |
| Cost for 2,000 sq ft roof | $300 – $500 | $100 – $200 |
| Lifespan | 25 – 50 years | 15 – 20 years |
| Water resistance | Excellent (sheds water completely) | Fair (absorbs water, wrinkles when wet) |
| Tear strength | Very high (5–10x stronger than felt) | Low (tears easily during installation) |
| Weight per roll | 20 – 30 lbs (covers 1,000 sq ft) | 50 – 65 lbs (covers 200–400 sq ft) |
| UV exposure tolerance | 30 – 180 days (varies by product) | 24 – 72 hours max |
| Heat tolerance | Up to 260°F+ | Softens at 160°F+ (asphalt melts) |
| Slip resistance | High (textured surface, even when wet) | Low when wet (hazardous for workers) |
| Coverage per roll | 1,000 sq ft typical | 200 – 400 sq ft typical |
| Wrinkle resistance | Lies flat, stays smooth | Wrinkles when wet, can telegraph through shingles |
Pros and Cons of Synthetic Underlayment
Pros
- Water resistant — sheds rain completely without absorbing moisture
- 5 to 10 times the tear strength of felt — will not rip during installation
- Lighter and covers more area per roll — faster installation, lower labor cost
- Tolerates UV exposure for weeks or months if shingle installation is delayed
- Heat resistant up to 260 degrees or more — will not melt on hot summer roofs
- Non-slip surface keeps workers safer, even when wet
- Lies flat without wrinkling — smoother substrate for shingle installation
- Lasts 25 to 50 years, matching the lifespan of the roofing above it
Cons
- Costs 2 to 3 times more than felt paper per square foot
- Not a vapor barrier — does not replace the need for proper attic ventilation
- Some products can be slippery when covered with frost or ice (less common in Myrtle Beach)
Pros and Cons of Felt Paper
Pros
- Lowest upfront cost of any underlayment option
- Universally available at every building supply store
- 100+ year track record — every roofer knows how to install it
- Provides some vapor permeability, allowing minor moisture to escape
Cons
- Absorbs water — wrinkles and deteriorates when wet
- Tears easily during installation, especially in wind
- Degrades rapidly in UV exposure (24 to 72 hours max)
- Asphalt coating softens and melts on hot roofs (160 degrees and above)
- Heavy and covers less area per roll — slower to install
- Slippery when wet — worker safety hazard
- Shorter lifespan (15 to 20 years) — may fail before the roofing material above it
- Cannot reliably meet coastal SC wind zone requirements
Cost Comparison: Synthetic vs Felt Underlayment
The cost difference between synthetic and felt underlayment is real but modest in the context of a full roofing project:
| Underlayment Type | Per Sq Ft | 2,000 Sq Ft Roof | % of Total Roof Cost |
|---|---|---|---|
| 15 lb felt paper | $0.05 – $0.07 | $100 – $140 | 0.5% – 1% |
| 30 lb felt paper | $0.07 – $0.10 | $140 – $200 | 1% – 1.5% |
| Standard synthetic | $0.15 – $0.20 | $300 – $400 | 2% – 3% |
| Premium synthetic | $0.20 – $0.25 | $400 – $500 | 2.5% – 4% |
On a $12,000 roof replacement, the difference between felt and synthetic underlayment is roughly $200 to $300 — about 2% of the total project cost. That 2% buys you a secondary water barrier that lasts twice as long, survives rain delays without damage, and meets coastal building code requirements. Most contractors include synthetic underlayment in their standard estimates because the performance difference is too significant to compromise on.
Why Synthetic Underlayment Is Required in Coastal South Carolina
Myrtle Beach sits in Wind Zone III under the South Carolina building code. This designation comes with specific roofing requirements that felt paper struggles to meet:
Wind-Driven Rain Protection
In a hurricane, wind-driven rain can penetrate beneath shingles at angles that normal rain never reaches. The underlayment is the last line of defense before water hits your roof deck. Synthetic underlayment sheds this water completely. Felt paper absorbs it, allowing it to seep through to the deck beneath. In a storm producing 6 to 12 inches of rain over several hours (not unusual for a tropical system), the difference between water shedding and water absorption is the difference between a dry home and significant interior damage.
Heat Resistance for Coastal Climate
Roof surface temperatures in Myrtle Beach regularly exceed 160 degrees Fahrenheit during summer months, even on lighter colored roofs. At these temperatures, the asphalt in felt paper softens and can stick to the underside of shingles. This creates problems during future repairs or replacement because the felt tears apart when shingles are removed. Synthetic underlayment tolerates temperatures up to 260 degrees or more without softening, deforming, or bonding to the shingles above it.
UV Exposure During Installation
Roofing projects in Myrtle Beach routinely face weather delays. Afternoon thunderstorms from May through September can stop work mid-project, leaving exposed underlayment for days or weeks before shingle installation resumes. Felt paper degrades in UV exposure within 24 to 72 hours, becoming brittle and losing its water resistance. Synthetic underlayment is rated for 30 to 180 days of UV exposure depending on the product, keeping your roof deck protected regardless of weather delays.
Tear Resistance in High Winds
During installation, underlayment is temporarily exposed to wind. Felt paper tears easily in wind gusts, requiring constant patching and re-securing. In coastal areas where consistent breezes and sudden gusts are normal, this slows installation and compromises coverage. Synthetic underlayment has 5 to 10 times the tear strength of felt and resists wind during the installation process without ripping or peeling up.
Installation Differences
Beyond material performance, synthetic underlayment is significantly easier and faster to install:
- Roll coverage: A single roll of synthetic underlayment covers approximately 1,000 square feet. A roll of 30-pound felt covers only 200 square feet. This means fewer trips up the ladder and fewer overlapping seams that could leak.
- Roll weight: A synthetic roll weighs 20 to 30 pounds. A felt roll weighs 50 to 65 pounds. Lighter rolls are easier to handle on a steep roof and reduce worker fatigue.
- Walkability: Synthetic underlayment has a textured, non-slip surface that stays grippy even when wet. Felt paper becomes dangerously slippery when wet, creating a real safety hazard for the installation crew.
- Flatness: Synthetic underlayment lies flat and smooth against the roof deck. Felt paper tends to wrinkle, especially in humid conditions. Wrinkles in felt can telegraph through thin shingles, creating visible ridges on the finished roof.
- Labor time: The combination of larger coverage per roll, lighter weight, and better handling means synthetic underlayment installs in roughly half the time of felt. This labor savings partially offsets the higher material cost.
How Long Does Each Underlayment Type Last?
Felt Paper: 15 to 20 Years
Felt paper degrades over time as the organic fibers dry out and the asphalt coating becomes brittle. In Myrtle Beach's hot, humid climate, this process accelerates. After 15 to 20 years, felt paper has typically lost most of its water resistance and tensile strength. This is a problem because asphalt shingles in coastal SC last 15 to 25 years — meaning the underlayment may fail at the same time or even before the shingles need replacement. When both fail together, water damage to the roof deck is nearly guaranteed.
Synthetic: 25 to 50 Years
Synthetic underlayment is engineered to last as long as the roofing material above it. Premium products from GAF, Owens Corning, and CertainTeed carry warranties that match or exceed their shingle warranties. The polypropylene or polyethylene polymers do not absorb moisture, do not dry out, and do not become brittle with age. In a hot coastal climate, synthetic underlayment is still performing well at 25 to 30 years while felt paper installed at the same time would have failed years earlier.
This lifespan difference has a practical consequence: if you install felt paper under a 30-year architectural shingle, the felt may fail 10 years before the shingles. When the underlayment fails, any water that gets past the shingles goes straight to the deck. Synthetic underlayment eliminates this risk by outlasting most roofing materials.
Frequently Asked Questions
Is synthetic underlayment better than felt paper?
Yes, synthetic underlayment is better than felt paper in virtually every performance category. Synthetic is lighter, stronger, more water resistant, lasts longer, and does not tear or wrinkle during installation. The only advantage felt paper has is lower upfront cost. In coastal South Carolina, synthetic underlayment is essentially required because building codes in Wind Zone III mandate water-resistant underlayment that felt paper cannot reliably provide.
How much does synthetic underlayment cost compared to felt?
Synthetic underlayment costs $0.15 to $0.25 per square foot, while felt paper costs $0.05 to $0.10 per square foot. On a 2,000 square foot roof, that is $300 to $500 for synthetic versus $100 to $200 for felt. The $200 to $300 difference is a small fraction of total roofing cost and buys significantly better protection.
How long does synthetic roof underlayment last?
Synthetic roof underlayment lasts 25 to 50 years, depending on the specific product and UV exposure. Premium synthetic underlayment is designed to last as long as the roofing material above it. Felt paper typically lasts 15 to 20 years before it dries out, cracks, and loses its water resistance. In Myrtle Beach's heat and humidity, felt paper degrades even faster.
Is synthetic underlayment required by code in South Carolina?
South Carolina building code does not specifically mandate synthetic over felt by name. However, in Wind Zone III (which includes Myrtle Beach), the code requires underlayment that can withstand specific wind uplift and water penetration tests. Premium synthetic underlayment consistently meets these requirements, while standard 15-pound felt paper often does not. Most building inspectors in coastal SC will require synthetic underlayment for new roofing installations.
Can you install shingles over synthetic underlayment?
Yes. Synthetic underlayment is compatible with asphalt shingles, composite shingles, metal roofing, and tile roofing. All major shingle manufacturers recommend or require synthetic underlayment for their warranty to remain valid. It lies flatter and smoother than felt, which actually makes shingle installation easier and the finished roof look better.
What happens if underlayment gets wet before shingles are installed?
This is where synthetic underlayment dramatically outperforms felt. Synthetic underlayment sheds water and can be exposed to rain for weeks or months without degrading. Felt paper absorbs water, wrinkles, and can tear once wet. In Myrtle Beach, where afternoon thunderstorms can interrupt a multi-day roofing project, synthetic underlayment protects your roof deck even if shingle installation is delayed by weather.
Get a Quality Roof Installation in Myrtle Beach
At WeatherShield Roofing, every installation uses premium synthetic underlayment as the baseline — because the layer you cannot see is the one protecting your home from water damage. We never cut corners on underlayment, flashing, or fasteners, even when they are invisible under the finished roof.
We are GAF Certified contractors (SC License #124773) with 82 five-star Google reviews, serving Myrtle Beach homeowners since 2022. Every detail of every installation meets or exceeds Wind Zone III code requirements.