The three main types of underlayment
| Type | What it is | Notes |
|---|---|---|
| Felt (#15 or #30) | Asphalt-saturated felt paper (ASTM D226). #30 is the heavier grade. | The traditional option. Nailed down. |
| Synthetic | A woven or spun polymer sheet (ASTM D8257). | ARMA says it is lighter than felt and covers more roof per roll. Nailed down. |
| Self-adhered (“peel and stick”) | Rubberized asphalt with a sticky back (ASTM D1970). | Seals around nail shanks. ARMA notes it blocks vapor, so the attic needs good ventilation. |
What a “sealed roof deck” is
Roof sheathing is made of 4-by-8 sheets with gaps between them. If the shingles and underlayment blow off, rain goes straight through those gaps. A sealed roof deck, also called secondary water resistance (SWR), closes them. The FORTIFIED program accepts three ways to do it:
- 4-inch self-adhered tape over every deck seam, then felt or synthetic underlayment.
- Two layers of #30 felt fastened with cap nails.
- Self-adhered underlayment over the whole deck.
In IBHS testing, an unsealed deck let in so much water that the collection tubs overflowed about 3 minutes into a 15-minute test. IBHS puts the cost of sealing a deck at about $500 and says insurer adjusters estimated repairs on unsealed homes would cost about three times more.
What Florida law and code require
- Florida Statutes s. 553.844 requires a roof replacement on a site-built single-family home to include a secondary water barrier and improved roof deck attachment.
- The 2023 Florida Building Code offers three underlayment paths for shingles: self-adhered membrane over the whole deck, a self-adhered strip over all deck joints with underlayment over it, or two layers of approved underlayment with specified laps, according to the Florida Roofing and Sheet Metal Contractors Association (FRSA).
It can lower your insurance
Question 6 on Florida’s wind mitigation form (OIR-B1-1802) asks whether the home has secondary water resistance. It counts when self-adhered underlayment is applied directly to the sheathing or foam adhesive seals the deck. Get a new wind mitigation inspection after your new roof so your insurer gets credit for it.
My Safe Florida Home has funded secondary water resistance for eligible homes.
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Frequently asked questions
What is a secondary water barrier?
A layer that keeps rain out if the shingles blow off, usually self-adhered tape over the deck seams or self-adhered underlayment over the whole deck. Florida requires one when a site-built home is reroofed.
Is synthetic underlayment better than felt?
Synthetic is lighter and covers more area per roll. What matters more in Florida is whether the deck is sealed underneath, and that the product and method meet the Florida Building Code.
Does a sealed roof deck lower insurance?
It can. The Florida wind mitigation form asks about secondary water resistance, and insurers must offer discounts for wind-mitigation features.
More on how roofs work
- How asphalt shingles are made, and why Florida sun wears them out
- Shingle wind ratings explained: what Class F and Class H really mean
- Impact-resistant shingles: what Class 4 really means
- Black streaks on your roof: what they are and how to get rid of them
- How hurricanes damage roofs, and what keeps yours on
- Metal roofing near the coast: Galvalume, aluminum and salt air
- Concrete tile roofs explained: how they are made and what fails first
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