What is Roof Underlayment?
Roof underlayment is the hidden layer between your roof deck (the wood sheathing) and the visible roofing (shingles, metal, or tile). When wind lifts a shingle or a storm drives rain sideways, this layer is your roof’s backup plan—helping keep water out of your home. Think of it as the safety net that turns a “minor issue” into “no big deal.”
What Roof Underlayment Actually Does
- Creates a continuous, water-shedding layer: Underlayment sits between the wood deck and your shingles, catching wind-driven rain and snowmelt that sneak past the roof covering. It breaks capillary action and helps channel moisture safely toward the eaves instead of into your home.
- Seals high-risk details (with the right products): Self-adhered “ice & water” membranes—used at eaves, valleys, skylights, chimneys, and vents—bond to the deck and self-seal around nails, reducing leak paths where roofs are most vulnerable.
- Works as part of a layered system: Underlayment only performs at its best when it’s correctly lapped and integrated with drip edge, flashing, and balanced attic ventilation. Proper sequencing creates a continuous plane that sheds water instead of trapping it.
- Supports fire and code performance: Many asphalt-shingle roofs achieve their top fire rating as a tested assembly that includes the underlayment. Using the specified materials and coverage helps the roof meet code and manufacturer requirements.
- Helps the roof last longer: By keeping the deck drier and smoothing minor irregularities, underlayment reduces the chances of deck rot, shingle buckling, and recurring repairs—quietly extending the service life of the entire roof system.
Main Types of Roof Underlayment (and Where They’re Used)
Asphalt-Saturated Felt (“Felt Paper”)
The traditional roof underlayment option. It provides basic protection, but it can wrinkle if it gets wet and is more prone to tearing than modern synthetics. Many budget-focused projects still use it, though most reroofs now favor synthetics for durability and handling.
Synthetic Underlayments (Polypropylene/Polyethylene)
These are today’s most common choice under shingles. They’re lighter to carry, more tear-resistant, and typically allow longer short-term UV exposure if weather delays shingle installation. Always follow the product’s printed exposure limit and fastening pattern.
Self-Adhered “Ice & Water” Membranes
Peel-and-stick underlayment that self-seals around nail penetrations. These are commonly installed along eaves, in valleys, and around skylights and chimneys—anywhere leaks are most likely. In cold-weather regions with a history of ice dams, an ice barrier along the eaves is typically required.
High-Temperature (HT) Underlayments
These types of roof underlayment are formulated to resist elevated temperatures beneath metal or tile roofing. These membranes help prevent adhesive flow and premature failure in high-heat conditions. Manufacturers often call for HT-rated products under metal systems.
Climate & Roof Design Considerations
- Cold/snow climates: Use self-adhered membranes at the eaves (and often valleys and penetrations). Where ice dams are common, codes typically require an ice barrier that extends inside the exterior wall line.
- Low-slope sections (2:12 up to less than 4:12): Expect special underlayment procedures—often “double coverage” or a full self-adhered layer—because water drains more slowly.
- Coastal/high-wind zones: Consider enhanced fastening and sealed-deck methods to limit water entry if the roof covering is damaged in a storm.
- Hot/sunny climates: Under metal or tile, use a high-temperature rated underlayment as specified by the manufacturer.
Best-Practice Installation You Should Expect
- Deck prep: Replace rotten or delaminated wood and ensure a clean, dry surface.
- Correct overlaps & fastening: Follow the printed lap lines and fastener spacing on the underlayment—especially on low slopes and in high-wind areas.
- Drip-edge integration: At eaves, underlayment typically laps over the drip edge; at rakes, it typically goes under—this layering sheds water correctly.
- Critical areas: Valleys, skylights, chimneys, and roof-to-wall transitions get self-adhered membrane plus layered flashing for extra protection.
- Ventilation: Balanced intake (soffit) and exhaust (ridge/roof) ventilation helps manage moisture and temperature in the attic, supporting the whole roof system.
Common Problems & Warning Signs
- Wrinkling or buckling that telegraphs through shingles (often from wet felt or improper fastening).
- UV damage occurs if the underlayment is left exposed beyond its rated window before shingles are installed.
- Leaks at eaves/valleys/penetrations where self-adhered membrane or flashing was missing or layered incorrectly.
- Attic clues: stained decking, rusty nail tips, or musty odors—signs moisture is getting in or not venting away.
Underlayment by Roof Material
- Asphalt shingles: Synthetic underlayment across the field, with self-adhered membrane at eaves/valleys/penetrations as needed for climate and design; use special low-slope methods where required.
- Metal roofing: High-temperature rated underlayment is commonly specified under panels to handle heat buildup.
- Tile/slate: Heavier, longer-life systems often call for premium or HT underlayments and specific fastening patterns; follow the manufacturer’s system instructions.
Costs, Warranties & Insurance
Costs: Upgrading from basic felt to synthetic, and adding self-adhered membranes at high-risk areas, is a modest material increase that can prevent expensive water damage.
Manufacturer warranties: “Roofing system” warranties often require approved underlayment types and accessories. If you want enhanced coverage, ask which underlayment is required and confirm your contractor can register the warranty.
Insurance/code upgrades: After a covered storm loss, your standard policy may not automatically pay for code-required changes like ice barriers; adding “ordinance or law” coverage can help with mandated upgrades.
Maintenance & When to Replace
Underlayment isn’t typically repaired by itself—it’s addressed during a reroof. What you can do is keep gutters clear, trim overhanging limbs, watch the attic for moisture or staining, and schedule an inspection after major wind or hail events. If shingles are nearing the end of their life, plan a proactive replacement so underlayment isn’t asked to do more than it’s designed for.
How C&W Roofing, Siding & Window Co. Installs Underlayment the Right Way
- We match the underlayment to your roof and climate—synthetic across the field for most shingle roofs, self-adhered membranes at eaves/valleys/penetrations where risk is highest, and HT-rated products under metal or tile where specified.
- We follow the current code for ice barriers and integrate underlayment with drip edge, flashing, and ventilation so the whole system sheds water correctly.
- If you want additional resilience (especially in coastal or high-wind zones), we can add sealed-deck methods and provide photo documentation so you can see each step before shingles go on.
Quick FAQs
Q. Is synthetic underlayment “better” than felt? For most reroofs, yes—synthetics are lighter, more tear-resistant, and often allow longer short-term UV exposure than felt. Always follow the specific product’s data sheet.
Q. Do I need ice & water shield if my area rarely freezes? It’s usually required along eaves only in regions with a history of ice dams; otherwise, it’s used strategically in valleys and around penetrations. Local code and roof design determine placement.
Q. What should I expect on low-slope parts of my roof? Between 2:12 and less than 4:12, expect double underlayment coverage or full self-adhered coverage. Below 2:12, shingles aren’t recommended; other roof systems are used.
Q. What’s the bottom line? Your roof isn’t just shingles—it’s a system. The right underlayment, installed correctly for your climate and roof design, is what keeps small issues from becoming major headaches.
Ready to Install New Underlayment on Your Roof?
Roof underlayment is the quiet hero of a healthy roof. It sheds wind-driven rain that sneaks past shingles, buys time during storms or installation delays, and supports the whole system when it’s paired with good flashing and balanced attic ventilation. The “right” choice depends on your home: traditional felt for basic protection, modern synthetics for strength and safer exposure windows, self-adhered membranes at leak-prone areas and in snow zones, and high-temperature options under metal or tile.
If you want clear recommendations, transparent workmanship, and photo documentation of what’s under your shingles, choose C&W Roofing, Siding& Window Co. for underlayment services. To start your project, contact us today.
