Roof underlayment is a water-resistant layer installed directly over the wood deck and under the shingles. It is the backup line of defense: if wind-driven rain gets past a shingle or a shingle is lost, the underlayment helps keep water from reaching the deck. Most homeowners never see it, which is part of why it gets so little attention on an estimate.
What does underlayment do?
Shingles shed water, but they do not make a perfectly sealed surface. Water can be pushed sideways under tabs by wind, and the seams between shingles are not designed to be waterproof on their own. Underlayment adds a second layer that:
- Gives a secondary barrier against water that gets under the shingles
- Protects the deck during the days between tear-off and shingle installation
- Provides a smoother surface than raw wood for the shingles to rest on
- Can separate the shingles from resin and other deck chemistry
- Is part of the system the shingle manufacturer expects, which can matter for warranty terms
None of these makes it a substitute for a sound deck or proper flashing. See what is roof flashing for how the other pieces work with it.
What kinds of underlayment are there?
Three families are common.
| Type | What it is | General notes |
|---|---|---|
| Felt (asphalt-saturated) | Paper-like sheet soaked in asphalt | Long-used and familiar; can absorb moisture and wrinkle |
| Synthetic | Sheet made from polymer materials | Often lighter and tougher to handle; behaves differently when wet |
| Self-adhered (peel-and-stick) | Rubberized asphalt membrane with an adhesive back | Often used in valleys, around penetrations and at edges where extra protection is wanted |
Each has strengths, and the right choice depends on the shingle manufacturer's instructions, local requirements, the slope and the job. The full side-by-side is in our synthetic underlayment vs felt comparison, and the pairing with ice and water barrier is covered in ice and water shield vs synthetic underlayment.
Is underlayment required?
Requirements vary by city and county; ask your building department or roofer. Shingle manufacturers also give instructions about underlayment, and the warranty may depend on following them. As a practical matter, nearly every professionally installed shingle roof has some form of underlayment, and low-slope roofs are the most sensitive. Our guide on low-slope roofs and underlayment explains why a shallow roof needs different treatment than a standard one.
Why do crews pull old felt off the deck?
Our owner has strong opinions on this, and our crews' view is that the old felt should come off during a replacement. That is our practice, not a universal rule. The reasons we give:
- Old felt is a record of the old roof, and it can be brittle, torn or full of nail holes.
- With the felt off, the crew sees the deck as it truly is, including rot and gaps.
- A new underlayment goes down on a clean, flat surface instead of on top of wrinkles and old fastener marks.
- Leaving layers behind can add weight and hide problems.
A roof can be built with a second layer of underlayment in place, and requirements differ by manufacturer, but pulling the old material is what we do. Once the felt is off, the deck can be checked and, if needed, redecked. See what is a redeck. Our owner has also posted that old decking can show hundreds of holes when the roof comes off, and those holes are easy to miss under an old layer.
Can underlayment get wet during a job?
Underlayment is not meant to replace a finished roof. It is made to cope with short exposure while the roof is being built, but how long depends on the product. If rain interrupts a job, crews tarp or dry in as needed. Ask your roofer how they handle weather and what the product's exposure limits are.
Frequently asked questions
Is underlayment the same as tar paper? Tar paper is a common name for asphalt felt, one of the underlayment types.
Can new underlayment be installed over old? It sometimes is, but our crews remove the old layer first. Manufacturer instructions and code vary, so check both.
Which is better, felt or synthetic? Neither wins in every case. Synthetics are popular for handling and moisture behavior, and felt is a long-used standard. See the comparison page for trade-offs.
Can underlayment choice affect my shingle manufacturer warranty? It can. Manufacturers describe what they expect under their shingles, so ask your roofer to follow the instructions for your product.
Do I need special underlayment in Oklahoma? Climate and slope both matter. Ask your roofer what they recommend for your roof and why.
Will insurance cover underlayment? Policies differ. Read yours and ask your agent.
Talk to us about your roof
Total Roof Protection OKC can explain what goes under your shingles and why. Call 405-415-5841 to schedule an inspection. An inspection describes your roof and does not predict an insurer's decision.