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Underlayment

Synthetic Underlayment vs Felt: Which Goes Under a Florida Shingle Roof?

Both are allowed under Florida's code if they meet the listed ASTM standards. Synthetic is made of woven plastic and can sit exposed longer, while felt is asphalt-soaked and should be covered quickly. Neither one alone counts as a sealed roof deck on the state wind mitigation form.

By Josh Hildebrand, owner of Coastline Roofing · Florida license CCC1331076 ·

Your quote says synthetic, or felt, or maybe both, and you want to know if it matters. Here is what the code says and what the choice changes.

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The basics

What is roof underlayment, and what is the difference between felt and synthetic?

Underlayment is the water-resistant layer between the roof deck and the shingles. Felt is asphalt-soaked paper or fiberglass mat. Synthetic is a woven or spun plastic sheet.

Shingles shed most rain, but wind can push water up under them and a storm can tear some off. The underlayment is the second layer that keeps water out of the deck. It goes down first, over the plywood, and the shingles go on top.

Felt has been the standard material for a long time. Atlas Roofing says so on its own underlayment page, and adds that newer materials have advanced. Atlas's installation instructions now highly recommend synthetic underlayment that complies with ASTM D6757 for long-term performance, and peel-and-stick membrane at the eaves and valleys.

Felt and synthetic underlayment side by side
PointAsphalt feltSynthetic
What it isAsphalt-saturated felt, sold as #15 and #30Woven or spun plastic sheet. Atlas describes a woven scrim for tear resistance
Standards named in the Florida codeASTM D226 (Type II) and ASTM D4869 (Type III or IV)ASTM D6757 and ASTM D8257
Exposure before shinglesAtlas says felt is not intended for prolonged exposure and shingles should go on the same dayAtlas says its Summit synthetic can serve as the dry-in layer for up to 6 months
HandlingAtlas warns about wetting and wrinklingAtlas says it lies flat and resists tearing
Counts as a sealed roof deck by itselfNo. A single layer does not qualifyNo. A single layer does not qualify
Used in the code's two-layer methodYes, in the types listed aboveYes, ASTM D8257 is listed

Want to know what is under your shingles?

Coastline can inspect your roof and send a written photo report. If a replacement is coming, ask us what underlayment method we would use and why. Coastline Roofing works all over Florida (license CCC1331076).

Call (941) 896-7793 or text your town and a photo.

Florida code

What does the Florida Building Code require for underlayment?

The code lists the approved standards and three ways to install shingle underlayment. Every approved material must carry a label showing its standard and type.

Section R905.1.1 of the Florida Building Code, Residential (2023) says underlayment must meet one of these standards: ASTM D226, D1970, D4869, D6757 or D8257. The label has to show the standard and any type. For asphalt shingles, Section R905.1.1.1 gives three methods:

  • Full self-adhering membrane. A self-adhering polymer-modified bitumen sheet, ASTM D1970, covers the whole deck.
  • Tape on the seams. A strip at least 3 3/4 inches wide of ASTM D1970 membrane or AAMA 711 Level 3 flashing tape goes over every joint in the deck. An approved underlayment then covers the whole roof.
  • Two layers. Two layers of ASTM D226 Type II, ASTM D4869 Type III or IV, or ASTM D8257. The first course is a half-width strip at the eaves, and the second is a full sheet from the eave.

The code also sets the laps and nails for the two-layer method: 6-inch end laps offset 6 feet, fasteners at most 12 inches apart between side laps and 6 inches apart at the end and side laps, and caps at least 1 inch across. Metal caps are required where the ultimate design speed is 170 mph or more.

On a roof replacement, the existing building code adds a secondary water barrier requirement. When a covering is removed and replaced on a wood deck, Section 706.7.2 says a secondary water barrier must be installed to Section R905.1.1 of the residential code or the building code's equivalent. Roofs below 2:12 with a continuous system, and clay or concrete tile installed to code, are exempt. For the wider chapter, see our guide to the Florida Building Code for roofing.

Types

What kinds of underlayment can go under shingles?

These are the materials that come up in the code and in the maker instructions we read.

#15 felt

Atlas lists its #15 felt to ASTM D4869 Type I or Type III. Atlas also sells it under its own name, Atlas #15 Spec Felt.

#30 felt

Atlas lists #30 felt to ASTM D4869 Type II or Type IV. Types III and IV are the felt types named in Florida's two-layer method.

Synthetic

Atlas names Gorilla Guard, Everfelt and Summit as products it highly recommends, and says the ASTM D6757 standard applies. Its Summit line is described as woven scrim with 10 squares to the roll.

Self-adhering membrane

Peel-and-stick underlayment to ASTM D1970. Atlas recommends its WeatherMaster for valleys and eaves, and says on slopes between 2:12 and 4:12 one layer of it can cover the deck.

Seam tape

A strip of D1970 membrane or AAMA 711 Level 3 tape over the deck joints, with an approved underlayment over the roof. This is one of the code's three methods.

Two layers

Two layers of felt or synthetic. The state wind mitigation form counts a double layer with no tape as a sealed roof deck, and a single layer does not count.

Insurance

Does synthetic underlayment count as a sealed roof deck?

Not by itself. The state form says standard underlayment or hot-mopped felts do not qualify as secondary water resistance. A sealed roof deck needs one of the listed methods.

This is general information, not legal advice. Your insurer decides what a feature is worth.

The wind mitigation form, OIR-B1-1802 (Rev. 04/26), asks whether the roof has a sealed roof deck, also called secondary water resistance. It says standard underlayment or hot-mopped felts do not qualify. It then lists what does: a fully adhered polymer-modified bitumen underlayment to ASTM D1970, tape over the deck seams with felt or synthetic, a double layer of ASTM D226 Type II, ASTM D4869 Type III or IV or ASTM D8257 underlayment with no tape, or spray foam installed to the form's rules.

So the choice is not felt against synthetic. It is which method sits under the shingles. A roofer who puts down one layer of the best synthetic on the market has not made a sealed roof deck. A roofer who puts down two layers of felt, or tape plus felt, may have. Our guide to the roof questions on your wind mitigation form explains each answer. A licensed inspector signs the form. Coastline's license cannot sign it, but we can coordinate a licensed inspector.

Install

How should underlayment be installed on a Florida roof?

Flat, unwrinkled, lapped to code and covered by shingles as soon as the maker allows.

  1. Start with a clean deck

    Atlas's instructions call for a clean, bare, flat deck with gaps under 1/4 inch.

  2. Lay it flat

    Atlas says underlayment must be applied flat and unwrinkled.

  3. Use peel-and-stick where water collects

    Atlas recommends self-adhering membrane for critical areas such as valleys and eaves.

  4. Respect the slope

    Atlas calls for a double layer of approved underlayment on slopes between 2:12 and 4:12, and one layer on slopes from 4:12 up to 21:12.

  5. Cover it on schedule

    Felt should be covered with shingles the same day. Atlas says its Summit synthetic can sit as a dry-in layer for up to 6 months. Our guide on how long underlayment can stay exposed covers storm season delays.

  6. Fasten to the code

    Follow the cap, spacing and lap rules above, and the maker's instructions where they are stricter.

Atlas says shingles must be installed to local code or the maker's requirements, whichever is stricter. For the next layer up, see our guide on drip edge in Florida. For tile roofs, underlayment is a different job, covered in tile roof underlayment replacement.

Coastline's default shingle is Atlas Pinnacle Pristine. We are not a certified installer for Atlas or any brand. To see how a replacement job goes, read what to expect during a roof replacement and our shingle roofing page.

Reviews

What homeowners say about working with Coastline

★★★★★
"From beginning to end the roof was done in 2 days. One night the crew stayed until 8:00. Each evening before leaving the crew ensured the yard was cleaned. We are so happy, and our roof is beautiful!"
Sharon T.Roof replacement
★★★★★
"Russell was great to work with. He was prompt in coming out to assess our roof and very knowledgeable. ... The job was complete in 2 days once work began."
Bryan S.Roof replacement
★★★★★
"Josh was especially amazing. He was professional, patient, and took the time to explain everything clearly so I felt comfortable and informed throughout the entire process. You can tell he genuinely cares about his customers and takes pride in his work."
Madison T.Roofing customer
Keep reading

Related guides

Shingle Roofing

Shingle roof repair and replacement across Manatee County and the Gulf Coast.

See our shingle roofing →

Wind Mitigation Roof Questions

What a sealed roof deck is and how the state form asks about it.

See the roof questions →

What to Expect in a Roof Replacement

The steps of a replacement from tear-off to final inspection.

Read the guide →

Also see shingle roof cost in Florida and roof deck damage.

FAQ

More questions

Is synthetic underlayment better than felt?

It can sit exposed longer and resists tearing, according to Atlas's own instructions and product page, but the code allows both. Which one fits depends on the roof, the slope, the schedule and the method your roofer chooses. Ask for the product, its ASTM standard and the method.

How long can underlayment stay uncovered?

Atlas says felt is not for prolonged exposure and shingles should go on the same day, and its Summit synthetic can serve as a dry-in layer for up to 6 months. Other makers set their own limits, so check the product's data sheet.

Do I need two layers of underlayment?

It depends on the method. The code allows one full layer of self-adhering membrane, tape over seams plus an approved underlayment, or two layers of felt or synthetic. Atlas also calls for a double layer on slopes from 2:12 to 4:12.

Does my underlayment lower my insurance?

Only if it creates a sealed roof deck by one of the methods on the state form, and then your insurer decides the credit. A single layer of felt or synthetic does not qualify.

Is peel-and-stick underlayment the same as synthetic?

No. Peel-and-stick is a self-adhering polymer-modified bitumen membrane, ASTM D1970. Synthetic is a woven plastic sheet that is fastened down. Roofers often use both on the same roof, with peel-and-stick at the eaves and valleys.

Sources

  1. Florida Building Code, Residential, 8th Edition (2023), Chapter 9, R905.1.1 and R908 (via UpCodes)
  2. Florida Building Code, Existing Building, 8th Edition (2023), Chapter 7, Section 706 (via UpCodes)
  3. Florida OIR: Uniform Mitigation Verification Inspection Form OIR-B1-1802 (Rev. 04/26), question 8
  4. Atlas Roofing: Pinnacle Pristine general installation instructions, Underlayment section
  5. Atlas Roofing: Underlayments
  6. Atlas Roofing: Pinnacle Impact, Sun and Pristine three-part specification
  7. Pacific Northwest National Laboratory, Building America Solution Center: Asphalt Shingle Roofs
  8. IBHS: Roof 101, asphalt shingles

Find out what is under your shingles

Coastline will inspect the roof, photograph the deck and edges where we can, and give you a straight written answer on what we find.

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