Key Takeaways:

1. Underlayment is your roof’s first defense – It’s the hidden waterproofing layer that protects your home when shingles fail.
2. Synthetic is best for Illinois weather – Unlike old-school felt paper, it won’t tear in harsh weather conditions and lasts decades.
3. Cheap underlayment costs more later – Skipping quality leads to roof deck rot, mold, and even structural damage from leaks.
4. Watch for warning signs – Stains, musty smells, or curling shingles mean your underlayment might be failing.
5. Always replace it with a new roof – Reusing old underlayment voids warranties and risks leaks.

When most people think about a new roof, they picture asphalt shingles, concrete tiles, or maybe flat roofs if they’ve got a modern-style home. But there’s a hidden layer that does most of the heavy lifting—roofing underlayment.

This unsung hero sits between your roof deck and the outer materials, working 24/7 to block wind-driven rain, ice, and other weather attacks. And in Illinois, where storms swing from summer downpours to winter blizzards, good underlayment isn’t just nice to have—it’s essential.

So, what exactly is underlayment, why does it matter, and how do you pick the right kind? Let’s peel back the layers (pun intended).

Underlayment 101: Your Roof’s Raincoat

Imagine wearing a winter coat with holes in it. No matter how thick the outer fabric is, you’ll still get soaked. That’s what happens when a roof lacks proper underlayment.

This waterproofing layer serves three big jobs:

1. Blocks leak – If shingles blow off or crack, underlayment is the backup shield.
2. Protects the roof deck – Prevents wood from rotting when moisture sneaks in.
3. Adds insulation – Helps regulate attic temps, which can lower energy bills.

Fun fact: Building codes in Illinois actually require underlayment, but not all types are equal.

Types of Roofing Underlayment (And Which Is Best for Illinois)

Not all underlayment is the same. Here are the main types you’ll see:

1. Felt Paper (The Old-School Choice)

• Made of soaked paper or fiberglass.
• Cheap but tears easily.
• Still used but fading in popularity.

2. Synthetic Underlayment (The Modern Upgrade)

• Plastic-based, lighter, and way stronger.
• Grips better to the roof deck (safer for workers).
• Resists mold and lasts longer.

3. Rubberized Asphalt (For Flat Roofs or Harsh Climates)

• Self-adhesive and ultra-waterproof.
• Common for flat roofs or low-slope setups.
• Pricier but worth it for commercial buildings.

Our pick? For most Illinois homes, synthetic underlayment gives the best bang for your buck. It handles harsh weather conditions better than felt and won’t crack in freezing temps.

Why Illinois Homes Need Tough Underlayment

Illinois weather is basically a nonstop stress test for roofs. Here’s how underlayment saves the day:

• Winter ice dams – Melting snow refreezes at roof edges, pushing water under shingles. Good underlayment stops it from seeping in.
• Summer storms – Heavy rain plus wind can drive water sideways under shingles.
• Temperature swings – Materials expand and contract, which cheap underlayment can’t handle.

Skimp here, and you’ll pay later. Poor underlayment leads to roof deck rot, mold, and—worst case—structural damage from long-term leaks.

How to Spot Underlayment Problems

Think your underlayment might be failing? Watch for:

• Water stains on ceilings – Especially after storms.
• Musty attic smells – mean moisture is getting trapped.
• Shingles curling or buckling – Often a sign of trapped moisture underneath.

If you’re getting a roof replacement, ask your contractor what underlayment they’re using. Many budget crews cut corners here.

Underlayment FAQs

How long does underlayment last?

Synthetic lasts 20+ years. Felt needs replacing sooner (10–15 years).

Can you lay new shingles over old underlayment?

Not recommended. Always replace underlayment during a roofing project.

Does underlayment affect roof warranties?

Yes! Some shingle makers void warranties if you use the wrong type.

Need Help Choosing Underlayment?

At Advanced Roofing Inc., we’ve seen how the right roofing underlayment Illinois homes need can add decades to a roof’s life. Whether you’ve got asphalt shingles, wood shingles, or a flat roof, we’ll match you with the best layer of protection for your budget.

Don’t let a hidden layer cause huge headaches later. Contact us today for a free inspection—we’ll make sure your roof is armored against whatever Illinois weather throws at it.

Your roof works hard. Give it the backup it deserves!