The Role of Underlayment in Protecting Your Roof

Most homeowners never think about what is under their shingles. That is understandable because you cannot see it once the roof is finished. But the underlayment layer between your shingles and your roof deck is one of the most critical components of your roofing system. It is your roof's second line of defense against water, and in many situations, it is the only thing standing between a storm and serious damage to your home. Here is everything you need to know about it.

What Is Roof Underlayment?

Roof underlayment is a sheet material that gets installed directly on top of the roof decking before the shingles go on. Think of it as a waterproof or water-resistant barrier that protects the plywood or OSB deck beneath it. If wind-driven rain gets under a shingle, or if a shingle gets torn off in a storm, the underlayment is what keeps water from soaking into your decking and dripping into your attic.

Every properly installed residential roof has underlayment. It is not optional, and building codes require it for good reason.

The Three Main Types of Underlayment

Not all underlayment is created equal. There are three main types, and the differences between them matter, especially in a climate like Austin's.

Asphalt-Saturated Felt (Tar Paper)

This is the traditional underlayment that has been used for decades. It is made from organic or fiberglass mat that has been saturated with asphalt.

• Available in 15-pound and 30-pound weights
• The least expensive option
• Provides basic water resistance but is not waterproof
• Can wrinkle, tear, and deteriorate when exposed to UV light and heat
• Absorbs moisture over time, which can cause it to buckle

Felt underlayment gets the job done in mild climates, but in Texas heat, it has significant limitations. I have torn off roofs where the old felt underlayment was brittle, crumbling, and no longer providing any meaningful protection.

Synthetic Underlayment

Synthetic underlayment is made from woven or spun polyethylene or polypropylene. It has largely replaced felt as the go-to underlayment for quality roofing installations.

• Significantly stronger and more tear-resistant than felt
• Lighter weight, which makes it easier and faster to install
• Lays flat without wrinkling or buckling
• Does not absorb moisture
• UV-resistant and can be left exposed for weeks during installation without degrading
• Better slip resistance for crews walking on it during installation

Synthetic underlayment costs more than felt, but the performance difference is substantial. For Austin's extreme heat, intense UV, and sudden storms, synthetic is the clear winner for the main field of the roof.

Ice and Water Shield (Self-Adhering Membrane)

Ice and water shield is a premium, self-adhering underlayment that creates a completely waterproof seal. Unlike felt or synthetic, it sticks directly to the roof deck and seals around nail penetrations.

• Creates a true waterproof barrier, not just water-resistant
• Self-sealing around nails and fasteners
• Extremely durable and long-lasting
• Significantly more expensive than felt or synthetic
• Used strategically in high-risk areas rather than over the entire roof

In most Austin installations, ice and water shield is used in valleys, around chimneys, along eaves, around skylights, and anywhere else water tends to concentrate or penetrate. Using it in these critical areas provides maximum protection where it matters most.

Texas Building Code Requirements

Texas building codes specify minimum requirements for underlayment, and they have gotten stricter over the years as we have learned more about how roofs fail.

• The International Residential Code, adopted by most Texas jurisdictions, requires at least one layer of underlayment on all asphalt shingle roofs
• In areas with specific wind exposure requirements, two layers of felt or one layer of synthetic may be required
• Ice and water shield is required in valleys and at eave edges in many jurisdictions
• Manufacturer installation instructions also dictate specific underlayment requirements, and those instructions must be followed to maintain warranty coverage

The code sets the minimum. I always install to a higher standard because I have seen what happens when underlayment fails, and it is never pretty.

What I Install on Austin Homes

When I do a roof replacement in the Austin area, here is my standard underlayment approach:

1. Synthetic underlayment across the entire roof deck as the primary barrier
2. Ice and water shield in all valleys where two roof planes meet
3. Ice and water shield along the eave edges, extending at least 24 inches past the interior wall line
4. Ice and water shield around all penetrations including pipes, vents, chimneys, and skylights
5. Ice and water shield at any wall-to-roof transitions

This combination provides comprehensive protection that goes beyond code minimums. The synthetic handles the main field reliably, and the ice and water shield provides extra security in every area where water is most likely to find its way in.

Some homeowners ask if they should do ice and water shield over the entire roof. For most homes, that is overkill and adds significant cost. But for low-slope sections, complex roof designs, or homes that have had chronic leak issues, full coverage can make sense. I evaluate each roof individually and make recommendations based on the specific situation.

Signs of Underlayment Failure

Since underlayment is hidden under your shingles, you usually cannot see problems developing. But there are warning signs that your underlayment may be failing:

• Water stains on your attic ceiling or rafters, especially in areas away from obvious penetration points
• Damp or deteriorating decking visible from inside the attic
• Mold growth on the underside of the roof deck
• Shingles that appear to be buckling or not lying flat, which can indicate wrinkled or swollen underlayment beneath them
• Roof leaks that seem to move or appear in unexpected locations

If your roof is over 20 years old, there is a good chance the underlayment has degraded significantly, even if the shingles still look decent from the outside. Older felt underlayment in particular becomes brittle and loses its water resistance over time. This is one of the reasons I recommend full replacement rather than just laying new shingles over old ones. When we tear off the old roof, we can inspect the decking and install fresh, high-quality underlayment. For more about decking issues, see my post on signs your roof decking needs replacement.

Why Cutting Corners on Underlayment Is a Mistake

I have seen contractors try to save money by using the cheapest possible underlayment or by skipping ice and water shield in critical areas. This is one of the worst places to cut corners on a roof installation. Here is why:

• Underlayment is your backup when shingles fail, which they will eventually
• The cost difference between budget and quality underlayment is small compared to the total roof cost
• A failed underlayment can lead to decking damage, mold, and interior water damage that costs thousands to repair
• Skipping ice and water shield in valleys and around penetrations creates leak points that may not show up for years

When you are investing in a new roof, the underlayment is not the place to pinch pennies. It is the hidden layer that makes everything else work, and it needs to perform for the full life of your shingles.

Questions to Ask Your Roofer

When getting estimates for a new roof, ask these questions about underlayment:

• What type of underlayment do you use on the main field of the roof?
• Do you use ice and water shield in valleys, at eaves, and around penetrations?
• Does the underlayment you use meet or exceed the manufacturer's installation requirements for the shingles being installed?
• Is the underlayment included in your warranty?

Any reputable roofer should be able to answer these questions clearly and confidently. If they seem vague or dismissive about underlayment, that tells you something about the quality of their work.

If you are planning a roof replacement and want to make sure every layer is done right, give us a call at Alta Roofing at (737) 260-7765. We use premium underlayment on every job because your roof's long-term performance depends on it.