Key Takeaways
- Underlayment sits between the roof deck and the shingles. It’s the secondary water barrier that goes under shingles, metal, or tile, and it protects the deck when moisture gets past the finished roof covering. Once the roof is on, you never see it again.
- Felt is water-resistant, not waterproof. Asphalt-saturated felt holds off water infiltration well, but it wrinkles once it gets wet, and wrinkled felt shields the deck poorly. It comes in 15-pound and 30-pound weights, with 30-pound used on steep-pitch roofs, and it can last up to 30 years when it’s installed right.
- Synthetic underlayment is polymer-based and hard to tear. It’s made from polyethylene and polypropylene, so it stands up to tearing and UV exposure through a long Texas tear-off. It also puts less weight on older roof structures, and it protects better than felt when a storm strips shingles.
- Synthetic acts as a vapor barrier, so ventilation matters. On a roof that doesn’t vent well, synthetic can be the wrong call. There’s also no manufacturing standard governing what goes into synthetic underlayment, so quality varies a lot from product to product.
- Building codes and shingle warranties can decide this for you. Some local codes still require felt. Some shingle manufacturers require synthetic to keep warranty coverage valid, and using the wrong one can void it. Check both before you pick on price.
Roof underlayment is an important layer of the roofing system, installed between the roof deck and the shingles, metal, tile, or other primary roof covering. It provides a secondary layer of protection for the roof deck if moisture gets beneath the finished roofing material.
Two of the most common options are traditional asphalt-saturated felt and synthetic roof underlayment. Both can be appropriate roofing materials, but they differ in weight, tear resistance, moisture resistance, installation, cost, and product-specific performance.
In this guide, we’ll compare synthetic vs. felt roof underlayment, explain the pros and cons of each, and discuss what Dallas-Fort Worth homeowners should consider when choosing an underlayment for a new or replacement roof.
What Is Roofing Underlayment?
Simply defined, roof underlayment is the layer of your roofing system that is situated between the sheathing and the outermost layer of your roof. When you look at your roof, you won’t be able to see your underlayment level because it’ll be covered by your shingles or other roofing material. It’s important to note that roof underlayment provides a secondary moisture barrier to help keep your home dry from the elements. It goes a long way in preserving your overall roof’s lifespan and enhancing its performance.
Why Is Roofing Underlayment Important?
Your primary roofing material is designed to handle most of the weather your roof receives, but it is not the only layer protecting the roof deck. Underlayment provides an additional layer beneath shingles, metal, tile, or other roofing materials that can help protect the deck from moisture that reaches beneath the finished roof covering.
Underlayment can also help create a smooth surface for installation and provide temporary protection to the roof deck during the roofing process. The performance requirements depend on the roofing system and the specific underlayment being installed.
Underlayment selection may also affect manufacturer requirements, roofing-system approvals, and warranty eligibility. Homeowners should follow the specifications for the roofing material being installed rather than assuming that every type of underlayment is interchangeable.
Felt Underlayment
Felt is the traditional option: a mat of organic fiber or fiberglass saturated with asphalt. It comes in two common weights, usually called 15-pound and 30-pound felt. The heavier sheet resists tearing better and is the usual choice on steeper roofs, or anywhere the underlayment will be walked on for a while before the shingles go down.
Felt is water resistant, not waterproof. It sheds water well enough to protect the deck during a normal installation, and it has been used under shingles for generations. Its weaknesses show up during the job. It tears more easily than synthetic, it is heavier to carry onto a roof, and it wrinkles when it gets wet. Those wrinkles can telegraph through the shingles if they are not dealt with before the roof covering goes on. Felt also should not sit exposed to sun and rain for long, so it suits jobs where the roof will be dried in and shingled quickly.
What keeps felt in use is cost. The material is cheaper per roll, and on a simple roof with a short installation window it still does the job it was designed for.
What Is Synthetic Roof Underlayment?
Synthetic roof underlayment is a woven or spun sheet of polypropylene or polyethylene, usually coated on one or both faces, that goes over the roof deck in place of asphalt felt. It is lighter than felt, much harder to tear, and it stays flat when it gets wet. People sometimes call it synthetic felt, although it contains no felt at all.
Those properties matter most while the roof is being built. A synthetic sheet can be walked on with less risk of tearing, it covers more roof per roll so there are fewer laps, and most products are rated to sit exposed to sunlight for weeks or months before the shingles go on. The exposure limit varies by product and is printed on the manufacturer’s data sheet, which is the number worth checking if a job might stall.
The tradeoff is vapor. Most synthetic underlayments let very little moisture pass through them, so the attic below needs proper ventilation or condensation can collect under the deck. Breathable synthetic products exist for roofs where that is a concern. Quality also varies more than it does with felt, so the brand, the product name, and its code evaluation report are all worth asking about.
Is Roof Underlayment Waterproof?
Neither felt nor standard synthetic underlayment is fully waterproof. Both are water resistant, which means they shed water that runs across them but can let it through at fasteners, at laps, and anywhere water sits against them. Synthetic handles water better than felt because it does not absorb moisture or wrinkle, but it is still a secondary barrier rather than a seal.
The waterproof layer on a roof is a self-adhered membrane, often called ice and water shield. It bonds to the deck and seals around the nails that pass through it, which is why it goes where water concentrates or moves slowly: valleys, eaves, low-slope sections, and the areas around chimneys, skylights, and wall transitions. A well-built roof usually has both, with membrane at the vulnerable areas and felt or synthetic across the rest of the deck.
How to Choose Between Synthetic and Felt
Start with the shingle warranty. Many manufacturers offer their best coverage only when the whole roof is built from their own components, underlayment included, and a substitution can reduce what the warranty covers. That one detail settles the question on a lot of roofs before price enters into it.
Then look at the schedule. If there is any chance the underlayment will sit exposed for more than a few days, whether from weather or a material delay, synthetic is the safer choice. Felt that has spent a week in the sun and rain is not the same product it was when it went down.
Budget comes last, and the gap is smaller than it looks. Synthetic costs more per roll, but underlayment is a small share of a full roof replacement once labor, shingles, flashing, and tear-off are counted, so the difference rarely moves the total much. Felt still makes sense on a simple roof that will be dried in and shingled within a day or two by a crew that installs it regularly.
Choosing Underlayment For Dallas Weather
Underlayment is your secondary water barrier, and choices should reflect steep heat cycles, wind-driven rain, and hail events common across DFW. Synthetic sheets resist wrinkling and UV better during long Texas tear-offs, while modern felts remain a budget-friendly option for simple roofs with quick dry-in. Attachment pattern, edge metal integration, and valley treatment matter as much as material grade.
Pair underlayment selection with your primary roofing. Architectural shingles benefit from high-grip surfaces and ice-and-water shields in valleys. See options on asphalt shingles. Metal systems need high-temp, smooth underlayments under clips and seams to reduce abrasion and heat. Explore standing seam metal details. Live in a storm corridor? Reinforce eaves and rakes and schedule a post-install check after major weather with our storm damage team.
Synthetic vs. Felt Underlayment at a Glance
| Feature | Synthetic | Felt |
|---|---|---|
| Material | Woven or spun polypropylene or polyethylene | Organic or fiberglass mat saturated with asphalt |
| Tear resistance | High | Lower, and it tears more easily during installation |
| When it gets wet | Stays flat | Absorbs moisture and wrinkles |
| Safe exposure before roofing | Weeks to months, per the product data sheet | Short, so it should be covered quickly |
| Weight | Light, with more coverage per roll | Heavier for the same area |
| Vapor permeability | Low for most products, though breathable versions exist | Higher |
| Material cost | Higher per roll | Lower per roll |
| Waterproof? | No, water resistant | No, water resistant |
Whichever sheet goes across the deck, the valleys, eaves, and penetrations still need self-adhered membrane. If you are pricing a replacement and want to know what will go under your shingles, ask for the underlayment by brand and product name. A contractor should be able to answer that without looking it up.
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