The shingles on your roof get all the attention, but what sits underneath them does just as much to keep water out of your home. Roof underlayment is the hidden protective layer installed directly on the plywood before the first shingle goes down. It acts as a backup barrier when shingles crack, blow off, or let water through during heavy storms. In Maryland, where ice dams form in winter and summer storms drive rain sideways, the right protective layer can mean the difference between a roof that holds up and one that leaks within a few years.
Most homeowners never think about underlayment until they need a roof replacement. By then, your contractor is making decisions about materials you might not fully understand. This guide walks you through the three main underlayment types, explains where each one works best, and points out the installation mistakes that cause problems later. When you know what goes under your shingles, you can make better decisions about your roof repair or replacement project.
What Roof Underlayment Is And Why It Matters
Roof underlayment is a water-resistant or waterproof layer that covers your plywood deck. It sits between the wooden decking and your shingles, creating a second line of defense against moisture. If a shingle cracks, a nail pops up, or wind tears part of your material during a storm, underlayment keeps water from soaking into the wood and dripping into your attic.
Underlayment also protects your home during the construction process. When your old roofing material comes off and the new one goes on, your house is exposed to the weather. A sudden rainstorm during a roof replacement can soak your decking and cause damage if there is no underlayment in place. Good underlayment materials give your roofers time to finish the job without racing against the forecast.
The Three Main Underlayment Types At A Glance
There are three main categories of underlayment used in residential roofing. Each one is built differently, performs differently, and costs a different amount. Understanding the strengths and limits of each type helps you see why your contractor recommends one over another. The sections below break down what each underlayment material is made of, where it works best, and what tradeoffs you accept when you choose it.
Synthetic Underlayment
Made from woven or spun polypropylene or polyethylene, these products create a lightweight sheet that resists tearing and lies flat on your roof deck. It replaced felt as the standard choice for most residential roof replacement projects over the past two decades. They also come in rolls that are easier to handle than heavy felt, and they stay stable in sunlight and weather for weeks or even months if installation gets delayed.
Pros and Cons
The biggest advantages are its durability and ease of use. It does not tear as easily as felt when workers walk on it, and it does not wrinkle or buckle under shingles. It can be left exposed to the weather longer without breaking down, which matters if your project runs into delays. It also sheds water better than felt, so it does not absorb moisture and get heavy or lose its shape.
The main downside is cost. These products cost more than asphalt felt, which adds to your total roof replacement cost in Maryland. Some synthetic products are also slippery when wet, creating safety concerns for workers on steep roofs. Contractors need to plan their footing and use extra caution during installation, especially in damp conditions.
Asphalt-Saturated Felt Underlayment
Asphalt-saturated felt, often called tar paper, is the traditional underlayment material that has been used in roofing for decades. It consists of a base mat, either organic or fiberglass, that is saturated with asphalt to make it water-resistant. Felt underlayment comes in different weights, with 30-pound felt being heavier and more durable than 15-pound felt. It has a rougher texture than synthetic products and a distinctive smell from the asphalt.
Pros and Cons
Pros – Felt underlayment costs less than synthetic, making it an option for homeowners on a tight budget. It also provides good traction for workers, which reduces the risk of slipping on steep roofs. Felt is breathable, allowing some moisture vapor to escape from the deck, which can be helpful in certain climates and roof designs.
The cons are significant. It tears more easily than other products, especially when it gets wet. It absorbs moisture over time, which can cause it to wrinkle and lose its shape under shingles. Felt also has a limited exposure time. If it sits uncovered for too long, it breaks down in sunlight and loses its protective qualities. Wrinkles in felt can telegraph through shingles, creating visible bumps on your finished roof.
Rubberized Asphalt Self-Adhered Underlayment
Rubberized asphalt self-adhering products, commonly called peel-and-stick or ice and water barrier, is a fully waterproof membrane that sticks directly to your plywood deck. It has a rubberized asphalt backing protected by a release liner. You peel off the liner and press the membrane down, creating a seal that prevents water from getting through, even around nail holes. Self-adhering membranes are thicker and heavier than synthetic or felt.
Pros and Cons
The main advantage is its waterproof performance. It seals around fasteners, creating a watertight barrier that stops leaks in areas where water tends to pool or back up under shingles. It is the most effective protection against ice dams, which form along eaves in winter and force water under roofing materials. Self-adhered products also resist wind-driven rain better than other types.
The downsides are cost and installation complexity. These products are the most expensive option per square foot, which makes it impractical for full-roof coverage on most projects. It requires more labor to install correctly, and it is temperature-sensitive. If it is too cold, the adhesive does not stick well. If it is too hot, the membrane becomes difficult to handle and can stretch out of shape. Removing it later, if repairs are needed, is basically impossible.
Best for
Peel-and-stick underlayment works best as targeted protection in the most vulnerable areas, not as a blanket layer across the entire roof. It is most useful in valleys, along eaves where ice dams form, around chimneys and skylights, and on low-slope sections where water drains more slowly. In many areas, building codes require an ice-and-water barrier in these locations, and it is one of the smartest upgrades for long-term roof performance.
Where Each Type Is Used On A Real Roof
Full-Roof Coverage Choices
Standard underlayment is the layer that covers the roof deck across the entire field of the roof. It runs from the lower edges up toward the ridge, creating continuous coverage over most of the plywood. That full-surface layer helps manage moisture that gets under the shingles and gives the roof system a consistent water-shedding barrier from end to end.
Targeted Protection Strategy
Most roofs use a combination of materials rather than one type everywhere. The common approach is to install self-adhered rubberized asphalt products in vulnerable areas and cover the rest of the roof with normal underlayment. This strategy balances cost with performance, putting the most protection where it matters most. The peel-and-stick goes along the eaves, in valleys, around penetrations like vents and chimneys, and on any low-slope sections. The rest of the roof gets the normal water barrier for reliable coverage without the expense of self-adhered products everywhere.
Roof Geometry Considerations
Your roof pitch affects which products work best. Steep roofs shed water quickly, so they can often perform well with synthetic or felt as the primary underlayment and targeted peel-and-stick in key areas. Low-slope roofs, where water moves more slowly and can pool, need more aggressive protection. Building codes typically require self-adhering products on sections with a pitch below a certain threshold. If your project has a mix of slopes, your roofers will adjust the underlayment strategy to match the geometry. Understanding how roof pitch works helps you see why different areas of your roof need different protection levels.
Valleys and Flashing Integration
Valleys are one of the most critical areas for underlayment because they channel water from two slopes into a single path. Ice and water shield is the standard choice in valleys because it creates a watertight seal and prevents leaks even if pieces shift or crack. The same principle applies around chimneys, skylights, and other penetrations where flashing is used. Self-adhered products stick to the flashing and the roof deck, creating a continuous barrier that stops water from sneaking through gaps. Proper integration between underlayment and flashing is just as important as the materials themselves.
Common Mistakes Made With These Roofing Materials
Skipping Peel-And-Stick In Vulnerable Areas
One of the most damaging mistakes is leaving out self-adhering products where it is needed most. Homeowners trying to save money sometimes skip the peel-and-stick along eaves or in valleys, relying only on basic underlayment. This decision catches up with you when ice dams form in winter or wind-driven rain pushes water under your shingles during a storm. The areas that need the most protection are the ones that fail first without it. Building codes require ice and water barrier underlayment in certain locations for a reason, and ignoring those requirements leads to leaks and expensive repairs down the road.
Wrinkled Felt Under Shingles
Felt underlayment that absorbs moisture or gets installed improperly can wrinkle and buckle under your shingles. Those wrinkles create visible bumps on your finished roof, and they can trap water in low spots where it sits and works its way through over time. Wrinkled felt is usually the result of poor installation practices, like not fastening it properly or leaving it exposed to rain. Once the roofing is complete, fixing wrinkled underlayment means tearing off the top layer of material and starting over. Using synthetic underlayment instead of felt avoids this problem because it lies flat and does not absorb moisture.
Using Low-Traction Synthetic Without Safety Planning
Some products have a smooth surface that becomes slippery when wet. Contractors who do not plan for this create dangerous working conditions, especially on steep roofs. Workers can lose their footing and fall, leading to serious injuries. The solution is either choosing a product with better traction or implementing extra safety measures during installation. Homeowners should ask their contractor how they handle safety on steep roofs, because a cheap product that saves money up front is not worth the risk of a worker getting hurt on your property. This is one reason many homeowners choose professional roofers over DIY approaches.
Improper Fastening That Creates Blow-Offs
Underlayment needs to be fastened to the plywood deck with the right number of fasteners in the right pattern. Skipping fasteners or spacing them too far apart allows wind to get under the lift, which can lead to blow-offs during high winds. This is especially common with felt underlayment, which is more prone to tearing when it flaps in the wind. Proper overlap between sheets and adequate fastening along seams are critical to keeping your material in place during storms.
Bad Flashing Details Blamed On Underlayment
When you get a leak around a chimney, skylight, or valley, homeowners sometimes assume the metal or shingles failed. In many cases, the real problem is poor flashing installation. Underlayment can only do so much if the flashing is not integrated correctly or if the flashing itself is damaged or improperly sealed. A leak around a penetration does not always mean you need a different base product. It often means the flashing details need to be redone properly. A thorough roof inspection can identify whether the issue is underlayment, flashing, or both, so you fix the right problem instead of wasting money on unnecessary work.
Schedule A Roof Inspection With RoofPRO
If you are not sure what underlayment is on your home or whether it is still doing its job, we can assess it during a roof inspection. We look at your roof deck, check for signs of a leaky roof, and evaluate whether your underlayment is protecting your home the way it should. If you need a complete reroof, we walk you through the options that make sense for your roof geometry, your budget, and the weather conditions in Maryland.
RoofPRO has handled projects across Maryland for over 20 years. We know which materials hold up in our climate and where targeted protection matters most. Whether you need a large installation or repairs that requires new underlayment in a specific area, we explain your options clearly and get the work done right the first time. Understanding the complete parts of a roof system helps you make informed decisions about your investment.
Get a free estimate by contacting us today. We will schedule a time to inspect your roof, answer your questions about roofing materials, and provide a clear recommendation based on what you need.