When you picture your roof, you probably imagine shingles, maybe flashing, and gutters. These are all parts of your roof, yes, but there’s one more very important thing hiding underneath it all. It’s called roof underlayment, and though it’s out of sight, it plays a huge role in keeping your home or building protected from the elements.
Whether you’re replacing your roof, dealing with a leak, or just curious about how your roofing system works, we’re here to break it all down. Let’s take a look at roof underlayment.
What Is Roof Underlayment?
Roof underlayment is a water-resistant or waterproof barrier installed between your roof’s deck (the base layer of plywood or OSB) and the final roof covering (like shingles or metal). Think of it as your roof’s secret bodyguard: a layer that steps in to defend your home if the outer covering is ever compromised.
What It Looks Like and Where It Goes
It’s usually rolled out in large sheets and secured with nails, staples, or peel-and-stick adhesive. Depending on the material, it may look like thick paper, durable fabric, or rubber-like membrane. It goes directly over the roof deck and under all your visible roofing material.
Why Roof Underlayment Matters
Shingles and tiles are tough, but they’re not waterproof on their own. They’re your roof’s first line of defense, but there have to be other lines too. The underlayment is the last one, and is especially important in areas like Western Washington, where heavy rain is a regular guest.
What It Helps Protect Against:
- Water damage from leaks or blown-off shingles
- Ice dams, which can force water under shingles
- Wind-driven rain and snow
- General moisture intrusion into your attic or insulation
When it’s working well, underlayment helps your whole roofing system last longer.
How Roof Underlayment Works
It creates a barrier that stops moisture from seeping through your roof if the top layer is damaged. It improves temperature control by helping manage heat and humidity, and on top of all that, the seals around nails and other fasteners help block water entry. Though it should be on every roof, this moisture barrier is particularly important under shingles, which can shift or crack over time.
Types of Roof Underlayment
There are three main types of underlayment, and each has pros and cons depending on your roof type, budget, and weather exposure.
Asphalt-Saturated Felt (Felt Underlayment)
Often called felt paper, asphalt-saturated felt is made from the base of an organic or fiberglass mat soaked in asphalt, and is available in 15lb or 30lb weights.
Pros: Inexpensive, widely used, decent tear resistance
Cons: Heavier, can wrinkle with moisture, not as durable as synthetics
Synthetic Underlayment
Synthetic underlayment is made from lightweight polymers (usually polypropylene or polyethylene), and is tough and tear-resistant, even in strong winds.
Pros: Excellent durability, lightweight, lays flat
Cons: Can be more expensive, not always compatible with every roofing material
Rubberized Asphalt
Rubberized asphalt, also known as peel-and-stick underlayment, has a sticky back and a rubberized layer for better waterproofing.
Pros: Self-seals around fasteners, ideal for vulnerable areas
Cons: Costs more, best used in specific areas like roof valleys or eaves
When Underlayment Matters Most
You always want underlayment under your roof, but there are certain situations where it becomes even more important. For example, roof replacements, especially on older roofs where decking can be exposed, are an instance where your underlayment is going to come in clutch. Emergency repairs after storms are also a great time to have roof underlayment, as well as in high-moisture areas like Washington and low slope roofs.
Do You Really Need Roof Underlayment?
In short? Yes.
Without underlayment, you’re putting your entire roofing system at risk. Even minor leaks can lead to mold, rot, insulation damage, or warped ceilings. Plus, many building codes require it, and some manufacturers void warranties if it’s not installed.
How Long Does Roof Underlayment Last?
That depends on the type:
- Felt underlayment: 10–20 years
- Synthetic underlayment: 25–40 years
- Rubberized asphalt: 25+ years, with proper installation
If your roof is leaking or you’re installing new shingles, it’s a good idea to check the underlayment while you’re at it.
How Many Layers of Underlayment Do You Need?
For most homes, a single layer is the effective standard. But if your roof has a steep slope or you’re using certain types of shingles, two layers might be recommended. We can help determine what your specific roofing project needs.
What to Expect when Replacing Roof Underlayment
If your underlayment is failing, replacing the underlayment might involve removing the outer roof covering. It’s often done during full roof replacements, but it can be done selectively in some cases. What you should know is that the cost depends on the type of underlayment and how much roofing material needs to be removed. The timeline of the project is usually around 1-2 days, and we strongly recommend hiring a pro to prevent water intrusion or code issues.
Closing Up
Underlayment might not be the star of the show, but it’s working quietly in the background to protect your roof, attic, and entire home from water damage and leaks. If you’re planning a roofing project, dealing with leaks, or just want peace of mind, now’s the time to think about what’s beneath your shingles.
Why Trust The Roof Doctor with Your Underlayment?
At The Roof Doctor, we know Western Washington roofs. We’ve worked on thousands of them over the past 60+ years, and we understand how critical it is to protect your home or business from water damage. We’re family-owned, licensed, bonded, insured, and always on call for roofing help whether you need an inspection, a quick fix, or a full replacement.
Let’s talk about your roof.
Call now or schedule an estimate with our team. We’re happy to chat with you and we want to be here when you need us most.
