Synthetic Underlayment vs Felt: Which Goes Under Your Roof?

When you compare synthetic underlayment vs felt, you are comparing two versions of the layer that sits between the wood deck and your shingles. It is the part of the roof system most homeowners never see and most estimates describe in a single line. Here is the plain version. Felt is the older, familiar product: a paper-like sheet saturated with asphalt. Synthetic underlayment is a newer product made from polymer materials. Both are used under shingles, both can work well when installed correctly, and both have trade-offs. Which one makes sense for your roof depends on the manufacturer's installation rules for the shingle you chose, local code, the weather during the job, and how your roofer works.

There is one more point the crew makes loudly, and we will return to it below: whichever new underlayment goes on, the old underlayment should come off.

This page does not list prices. Product details and code requirements vary and change, so everything here should be checked against the manufacturer's current instructions and your local requirements.

Key takeaways

• Underlayment is a secondary water barrier under the shingles. The shingles shed most water; the underlayment is the backup layer.
• Felt and synthetic are two common types. Synthetics are generally described as lighter, more tear-resistant and more stable when wet, while felt is the long-used traditional option. Details vary by product.
• Neither is a substitute for good deck preparation, flashing and installation.
• Our crews' practice is to remove old underlayment during tear-off, so the new roof goes onto a clean, inspected deck. This is our practice, and it is worth asking any roofer about theirs.
• Ask for the underlayment type and product name to appear on your estimate, and confirm that it meets the shingle manufacturer's requirements for your warranty.

What is roofing underlayment, and why does it matter?

On a typical asphalt shingle roof, the layers from bottom to top are: the framing, the deck (sheets of plywood or oriented strand board, or in some older homes, spaced boards), the underlayment, any specialty membranes in vulnerable spots, the flashing details, and then the shingles.

Shingles are designed to shed water. But wind can drive rain under shingles, a shingle can crack or lift, a nail can back out, and ice or debris can disrupt the normal flow of water. The underlayment is there so that when water does get past the shingle layer, it has another barrier to meet before it reaches the deck. It also protects the deck during construction, between the time the old roof comes off and the new shingles go on.

That second job matters in Oklahoma, where weather changes quickly. A deck that is covered with underlayment and then hit by an unexpected shower is in a better position than a bare deck, although crews also keep tarps ready when rain interrupts a job.

What are the main types of roofing underlayment?

There are a few families of products you will see on estimates.

• Felt (asphalt-saturated felt paper). The traditional roofing underlayment. It comes in rolls in different weights, and it has been used for a very long time.
• Synthetic underlayment. A polymer-based sheet, usually woven or non-woven, sold in rolls. Manufacturers market different grades with different features, including surface texture for traction and different UV exposure tolerances.
• Self-adhering membranes. A sticky-backed waterproof membrane used in specific areas such as eaves, valleys and around penetrations, instead of across the whole roof. Many roofs use a combination: a self-adhering product in the vulnerable spots and felt or synthetic across the rest.

This page focuses on the first two, because they are the usual choice for the main field of the roof.

How do synthetic underlayment and felt compare?

The table describes general tendencies. Individual products vary, and every statement should be checked against the manufacturer's current data sheet.

"Usually lower" and "usually higher" describe the general relationship, not a price. We do not publish prices here.

What do the manufacturers require?

Shingle manufacturers publish installation instructions, and those instructions say what underlayment is required or acceptable under their shingles, and in which locations. Some warranty programs are tied to using a specific kind of system. The rules differ between manufacturers and can differ between product lines, so the practical rule is simple: the underlayment on your estimate should be one the shingle manufacturer allows for your shingle, and your roofer should be able to show you where that appears in the manufacturer's instructions.

Local building code also sets requirements for underlayment on roofs. Codes vary by jurisdiction and by the year they were adopted. Your roofer and your local building department can tell you what applies where you live.

Why do our crews pull the old felt off the deck?

This is the strongest opinion in the field notes from the owner of Total Roof Protection OKC, and it is worth explaining. He has posted, in no uncertain terms, that the old felt should be pulled off the roof during a replacement. That is our crews' practice, and it is not a universal rule, so treat it as a practice and not as a code requirement. Here is the reasoning behind it.

You cannot inspect what you cannot see. The point of tearing off a roof is to see the deck. If old underlayment is left in place, the roofer cannot fully evaluate the wood underneath. Soft spots, old moisture damage, and gaps in the boards can hide beneath it.

The old layer carries the old roof's history. After many years of heat cycles, nails and repairs, old felt can be brittle, torn and full of holes from fasteners. The owner describes seeing hundreds of holes in old decking when a roof comes off. Putting new material over an old layer that already has holes does not give the new roof a clean starting surface.

A clean deck gives a flat, consistent base. Layered materials can leave ridges and uneven areas that telegraph through to the new shingles.

Manufacturer instructions are the final word. Our crews follow the shingle manufacturer's instructions for the product being installed. Confirm what the manufacturer's installation instructions say about what must be on the deck beneath their shingles, and ask your roofer to show you.

Because this is our practice, ask any contractor you are considering how they handle old underlayment and why. A clear answer that names the manufacturer's instructions is a good sign.

What about the deck itself?

Underlayment sits on the deck, so the deck matters. Our crews see several deck types in Oklahoma homes, including spaced boards on older houses, plywood and oriented strand board. The owner notes that plywood has been so expensive that crews often use OSB, and that spaced decking on older homes often has to be redone or overlaid with sheet material before shingles go on. Redecking is a common reason an estimate changes, and it is worth reading about before you sign. In either case, the underlayment is only as good as the surface under it.

Is synthetic underlayment vs felt the right call for your home?

A reasonable way to think about it:

1. Start with the shingle manufacturer's instructions. What do they require or allow under the shingle you chose? That narrows the options.
2. Ask what your roofer recommends and why. A good answer refers to the roof, the shingle line and the conditions, not just habit.
3. Think about job conditions. If a roof may sit open for a while because of weather or material delays, ask how the roofer plans to protect it and how long the chosen product can sit exposed according to the manufacturer.
4. Consider the vulnerable spots. Eaves, valleys and penetrations often get a different, self-adhering product. Ask what goes where.
5. Weigh budget honestly. If one type costs more, ask for the difference on the estimate so you can decide with the facts.
6. Make sure the type and product name are on the estimate. "Underlayment" alone is not enough to compare bids.

If you are still choosing shingles, read our comparison of GAF Timberline HDZ, Owens Corning Duration and Tamko Titan XT and our guide to Class 3 and Class 4 shingles.

What mistakes do homeowners make with underlayment?

• Accepting a one-word line on the estimate. "Underlayment" tells you nothing. Ask for the type and the product.
• Assuming synthetic is always better or felt is always worse. Product quality varies within each family. What matters is the specific product, the manufacturer's rules and the installation.
• Not asking what happens to the old layer. Ask whether the old material is fully removed and the deck inspected.
• Ignoring weather plans. Ask how the roof will be protected if rain arrives mid-job.
• Forgetting the vulnerable areas. Eaves, valleys and penetrations deserve their own answer.
• Taking a blog post as final. Check the manufacturer's current instructions, including this page.

What should you ask your roofer about underlayment?

• What underlayment type and brand are you using, and where does it appear on the estimate?
• Does it meet the shingle manufacturer's requirements for my warranty?
• Do you remove the old underlayment completely, and do you inspect the deck?
• What goes at the eaves, valleys and around penetrations?
• What is your plan if rain arrives while the deck is exposed?
• How do you handle decking that turns out to be damaged?

Frequently asked questions

Is synthetic underlayment really better than felt? Synthetics are generally described as lighter, more tear-resistant and more stable when wet, and many people prefer them. But quality varies by product in both families, and the shingle manufacturer's instructions come first. Ask your roofer to explain the choice for your roof.

Is felt paper still used on roofs? Yes. It is the traditional option and remains in use. Whether it is appropriate on your roof depends on the shingle manufacturer's instructions and local requirements.

Can new underlayment go over old felt? Our crews do not do that; we remove the old underlayment during tear-off so the deck can be inspected. That is our practice. For what a manufacturer requires, read the installation instructions for the shingle in question.

How long can underlayment stay exposed before shingles go on? It varies by product, and manufacturers specify limits. Ask your roofer which product is going on and what the manufacturer's exposure guidance is, especially if weather may delay the job.

Could my underlayment choice affect my roof warranty? It can. Some warranty programs depend on using particular components or systems. Read the warranty document and ask your roofer to confirm the underlayment meets it.

What goes at the eaves and valleys? Many roofs use a self-adhering membrane in those areas. Ask your roofer to point to where each product goes on your roof.

What happens if it rains while my roof is open? A good crew protects the roof with tarps and plans around the forecast, and will tell you what they do if weather interrupts the work. Ask before you sign.

Does underlayment matter for hail? It is a different part of the system. Impact resistance is mostly about the shingle and the deck. The underlayment is the water barrier below. Both parts matter to the whole roof.

Want to see what is under your shingles?

We can walk through the whole roof system with you, from deck to shingle, and put the underlayment type, product and accessories on an itemized written estimate so you can compare. An inspection tells you about the roof. It does not predict or replace your insurer's decision, and we will not tell you otherwise. Call Total Roof Protection OKC at 405-415-5841.

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