You are about to spend thousands of dollars on a new roof. You have researched shingle brands, compared colors, and maybe even picked out a contractor. But there is a critical layer of protection hiding beneath those shingles that most homeowners never think about – and it can make or break your roof’s performance for the next 25 years. That layer is your roof underlayment, and choosing the wrong type for Central Illinois weather can cost you far more than the price difference between the two options.

Synthetic underlayment being installed on a residential roof before shingle application. This secondary moisture barrier is one of the most important components of any roofing system.

What Is Roof Underlayment and Why Does It Matter?

Roof underlayment is a secondary moisture barrier installed directly on your roof deck – the plywood or OSB sheathing – before any shingles, tiles, or metal panels go on top. Think of it as a waterproof shield between your home’s structural decking and the visible roofing material above.

Here is why it matters more than most homeowners realize. Your shingles are your roof’s first line of defense against rain, snow, and ice. But shingles are not watertight on their own. Wind-driven rain pushes water sideways and upward beneath shingle edges. Ice dams force meltwater under shingle tabs during freeze-thaw cycles. Hail impacts crack and displace shingles, creating gaps. In every one of these scenarios – all extremely common in Central Illinois – your underlayment is what actually stops water from reaching the wood decking and the interior of your home.

Without underlayment, or with underlayment that has failed, water reaches the roof deck and starts rotting the wood from the top down. By the time you see a water stain on your ceiling the structural damage may already be extensive. This is why regular roof inspections are critical – and why the type of underlayment your contractor uses during a roof replacement deserves real attention.

Traditional Felt Underlayment: The Old Standard

Felt underlayment – often called tar paper or roofing felt – has been the standard underlayment material for over a century. It is made from a base of natural fibers like cellulose or fiberglass that is saturated with asphalt to provide water resistance. Felt comes in two common weights:

15-pound felt is the lighter option. It is thinner, less expensive, and easier to handle. However, it tears more easily during installation and provides less protection if left exposed to weather. Many building codes across Illinois still accept 15-pound felt as the minimum underlayment requirement for standard asphalt shingle installations.

30-pound felt is roughly twice as heavy and significantly more durable. It resists tearing better during installation, provides better water resistance, and holds up longer if the shingles above it are damaged or displaced during a storm. When felt is the chosen underlayment, 30-pound is the professional-grade option – 15-pound is increasingly considered inadequate for anything beyond budget-driven projects.

Felt has earned its long history for a reason. It works. It provides a basic moisture barrier, it is widely available, and every roofing contractor in the country knows how to install it. But felt also has well-documented limitations that become especially problematic in climates like ours here in Central Illinois.

A roof deck exposed during construction. This plywood sheathing is what underlayment protects – once the deck gets wet and starts rotting, you are looking at a far more expensive repair than just replacing shingles.

Synthetic Underlayment: The Modern Standard

Synthetic underlayment is made from woven or spun polypropylene or polyester – essentially a high-tech plastic fabric engineered specifically for roofing applications. It entered the market in the early 2000s and has rapidly become the preferred choice among professional roofing contractors nationwide. Here is why.

Strength and tear resistance. Synthetic underlayment is dramatically stronger than felt. Where a gust of wind can rip felt paper off a partially completed roof during installation, synthetic materials hold firm. Roofers can walk on synthetic underlayment without it bunching, tearing, or creating wrinkles that compromise the moisture barrier. This matters enormously on steep-slope roofs where crew members need reliable footing.

Moisture performance. Synthetic underlayment does not absorb water. Period. Felt absorbs moisture, which causes it to wrinkle, buckle, and eventually degrade. Synthetic materials shed water completely and maintain their dimensional stability regardless of how wet they get. In a state where your roof endures rain, snow, sleet, and ice within the same week, this is not a minor advantage.

UV resistance. If a roofing project is delayed or your shingles are damaged in a storm, the underlayment may be exposed to direct sunlight for days or weeks. Felt degrades rapidly under UV exposure – sometimes in as little as 24 to 48 hours of direct sun. Most synthetic products can withstand 60 to 180 days of UV exposure depending on the manufacturer, giving your roof temporary protection even without shingles in place.

Weight. A roll of synthetic underlayment covers the same area as felt but weighs significantly less. This makes it faster to carry onto the roof, easier to handle during installation, and less fatiguing for the crew – which translates to faster installation times and fewer handling errors.

Head-to-Head Comparison: Synthetic vs. Felt Underlayment

When you look at the comparison above the performance gap is clear. The only category where felt wins outright is upfront material cost per roll. But even that advantage disappears when you factor in coverage per roll – synthetic covers significantly more area per roll so the total project cost is often comparable or only marginally higher.

Why Central Illinois Weather Demands Better Underlayment

If you live anywhere in the Central Illinois corridor – from Springfield to Jacksonville and across Sangamon County and the surrounding region – your roof endures one of the most demanding weather cycles in the Midwest.

Freeze-thaw cycling. Central Illinois winters routinely swing between below-freezing nights and above-freezing days. This creates ice dam conditions where snow melts on the upper roof, runs down to the colder eaves, refreezes, and backs water up under your shingles. Your underlayment is the only thing standing between that backed-up water and your roof deck. Felt absorbs that water and wrinkles. Synthetic sheds it completely.

Wind-driven rain. Spring and fall thunderstorms regularly produce wind gusts over 50 mph across our flat Central Illinois terrain. These winds drive rain horizontally and push water upward beneath shingle edges at angles that gravity alone would never achieve. Synthetic underlayment with properly overlapped seams handles this pressure. Felt that has been wet, dried, and re-wet through multiple cycles develops gaps and compromised seams that let water through.

Summer heat. Illinois summer temperatures regularly exceed 90 degrees with intense UV exposure. Attic temperatures can reach 140 to 160 degrees. Over years of this thermal cycling felt becomes brittle and cracks. Synthetic underlayment maintains its flexibility and structural integrity through decades of extreme temperature swings.

Hail exposure. When hail displaces or cracks shingles the underlayment becomes your primary moisture barrier until repairs are completed. If your underlayment is felt that has been degrading for 10 to 15 years it may already be compromised before the hail even hits. Synthetic underlayment provides reliable backup protection even after years of service beneath the shingles.

A full roof replacement in progress. When old shingles come off, the condition of the underlayment beneath them tells the real story of how well the previous roof was protecting the home.

What Owens Corning Recommends for Their Shingle Systems

As an Owens Corning Preferred Contractor, Campbell Construction installs OC roofing systems that are engineered to work as an integrated whole – not just individual components. Owens Corning is explicit in their recommendations regarding underlayment:

For their Total Protection Roofing System – which delivers the best warranty coverage available – Owens Corning specifies their own synthetic underlayment products including ProArmor and Deck Defense. These synthetic underlayments are specifically designed to complement OC shingle products and are required components if homeowners want to qualify for the top-tier System Warranty that covers not just materials but also workmanship.

Owens Corning’s position is clear: synthetic underlayment is the appropriate choice for modern roofing installations. Their warranty structure reflects this by offering enhanced coverage when synthetic underlayment is used as part of a complete system installation. When you pair OC Duration or Duration FLEX shingles with OC synthetic underlayment installed by a Preferred Contractor, you get a warranty package that covers materials and labor – a level of protection that simply is not available with felt underlayment installations.

Campbell Construction’s Standard: Always Synthetic on New Installs

At Campbell Construction we made the decision years ago to standardize on synthetic underlayment for every new residential roof installation. This is not an upsell. It is not an optional add-on that we push to increase the ticket price. Synthetic underlayment is our baseline because it is the right product for Central Illinois roofs. Period.

Here is what that means for you as a homeowner. When we install your new roof every square foot of your roof deck gets covered with high-quality synthetic underlayment before a single shingle goes down. We install it with proper overlaps at every horizontal seam – typically six inches – and we seal all penetrations including pipe boots, vents, and chimney flashings with compatible sealing products. The result is a continuous moisture barrier across your entire roof that will perform for the full lifespan of your shingles.

We also install ice and water shield – a self-adhering membrane – at all eaves, valleys, and around penetrations as required by Illinois building code and Owens Corning specifications. The synthetic underlayment covers everything else. This combination provides the best possible protection against the freeze-thaw, wind-driven rain, and hail exposure that every roof in our service area faces.

Our commitment to synthetic underlayment is part of a broader approach to roofing that prioritizes long-term performance over cutting corners. It is the same reason we use ring-shank nails, proper starter strips, and manufacturer-specified hip and ridge caps on every project. When you invest in a roof that should last 25 to 50 years every component matters. Financing options are available to make quality roofing accessible for every budget.

When Felt Underlayment Is Still Acceptable

Despite synthetic being the superior product in nearly every measurable category, there are limited scenarios where felt underlayment is still an acceptable choice:

Budget-constrained partial repairs. If you are repairing a small section of an existing roof that already has felt underlayment it can make sense to match the existing material rather than creating a transition between two different underlayment types. This is a repair scenario – not a full replacement.

Low-slope covered structures. For covered porches, carports, or other structures with minimal weather exposure, felt can provide adequate protection at lower cost. These are not primary living spaces and the consequences of underlayment failure are far less severe.

Temporary or short-term roofing. If a structure has a known limited remaining lifespan – perhaps it is scheduled for demolition or major renovation within five to ten years – the longevity advantage of synthetic underlayment is less relevant.

For any full residential roof replacement on a home you plan to live in for the foreseeable future, synthetic underlayment is the clear choice. The performance difference is too significant and the cost difference is too small to justify felt on a primary residence in Central Illinois.

How Underlayment Affects Your Roof Warranty

This is where underlayment choice directly impacts your wallet in a way most homeowners do not realize until it is too late. Roofing manufacturers – including Owens Corning, GAF, CertainTeed, and others – tie their warranty coverage to system requirements. When you use their specified underlayment products as part of a complete system installation your warranty coverage is significantly broader.

With Owens Corning’s system, using OC synthetic underlayment installed by a Preferred Contractor like Campbell Construction qualifies your roof for warranty coverage that includes both materials and workmanship for an extended period. Use felt instead and you may still get a basic material warranty – but you lose the enhanced system warranty that covers labor and provides substantially greater protection.

When you are investing $8,000 to $15,000 or more in a new roof the warranty coverage matters. The difference between a basic material warranty and a full system warranty can represent thousands of dollars in coverage over the life of your roof. Synthetic underlayment is one of the requirements that unlocks that superior protection. This is not marketing – it is the manufacturer’s published warranty terms.

What to Ask Your Roofing Contractor About Underlayment

If you are getting estimates for a roof replacement here are the questions every homeowner should ask about underlayment – and the answers you should expect from a reputable contractor:

What type of underlayment do you use as your standard? The answer should be synthetic. If a contractor defaults to felt on a full replacement, ask why. If the answer is cost savings, understand that you are trading long-term roof performance for a marginal upfront reduction.

What brand and product line of underlayment will be installed? A professional contractor should be able to name the specific product – not just say “synthetic.” Products like Owens Corning ProArmor, Deck Defense, or comparable brands each have published specifications you can verify.

How does the underlayment choice affect my warranty? Your contractor should be able to explain exactly what warranty coverage you qualify for with the proposed underlayment and what coverage you would lose by downgrading. If they cannot answer this question clearly, they may not be installing to manufacturer specifications.

Will ice and water shield be installed at eaves, valleys, and penetrations? This is separate from the field underlayment and is required by code in Illinois. Make sure your estimate explicitly includes ice and water shield at all required locations.

At Campbell Construction we answer these questions before you even ask them. Our estimates include detailed line items for every underlayment product, specify the manufacturer and product name, and explain exactly what warranty coverage you will receive. Schedule a free roof inspection to get a transparent assessment of your roof’s condition and a detailed estimate for replacement if needed.

The Bottom Line for Central Illinois Homeowners

Roof underlayment is not glamorous. You will never see it once your shingles are installed. Your neighbors will never compliment it. But it is one of the most important decisions in your entire roofing project because it directly determines how well your roof handles the worst weather Central Illinois throws at it – and whether your warranty actually covers you when something goes wrong.

Synthetic underlayment outperforms felt in every category that matters for our climate: water resistance, tear strength, UV tolerance, durability, and installation quality. The cost difference on a typical residential project is marginal when measured against the total project investment. And the warranty implications of choosing synthetic over felt can represent thousands of dollars in additional coverage.

Campbell Construction has been protecting Central Illinois homes since 2000. We use synthetic underlayment as our standard on every new installation because it is the right product for the job. As an Owens Corning Preferred Contractor and Duro-Last Certified installer, we build roofing systems that are engineered to perform as a complete unit – not a collection of the cheapest available parts. If you are considering a roof replacement, storm damage repair, or just want to know what is currently protecting your home, we are here to help.