The shingles or tiles are what you see, but the layer underneath them, the secondary water barrier, is what keeps water out if a storm strips part of the roof away. In Florida that layer has its own rules: the building code sets three ways to build it on a shingle or metal roof, a re-roof on an older house must include it, and the state’s wind mitigation form gives credit for one particular version. This guide explains the secondary water barrier and roof underlayment in Florida, including peel and stick membranes, and what to ask for when you re-roof.
What is roof underlayment?
Underlayment is the water-resistant layer installed over the roof deck, under the shingles, metal or tile. It sheds water that gets past the roof covering and protects the deck while the roof is being built. Florida’s building code sets the materials, the laps and the fastening for each type of roof covering.
Section R905.1.1 of the Florida Building Code, Residential, 2023 (8th edition) says that underlayment for roofs of slope 2:12 and greater must conform to the standards listed in the chapter, and that materials required to comply with standards such as ASTM D226, D1970, D4869, D6757 or D8257 must bear a label showing that compliance.
In plain terms, those standards cover the main families of underlayment:
| Standard | What it covers |
|---|---|
| ASTM D226 and D4869 | Asphalt-saturated felt underlayments |
| ASTM D1970 | Self-adhering polymer-modified bitumen underlayment, the peel and stick type |
| ASTM D8257 | Mechanically attached synthetic underlayment |
| ASTM D6757 | Inorganic underlayment for steep-slope roofing |
For concrete and clay tile, section R905.1.1.2 points to the code’s separate tile provisions, so tile roofs follow their own underlayment rules.
What is a secondary water barrier?
A secondary water barrier is underlayment built to keep water out of the house if the roof covering is lost, by sealing the deck or its joints rather than only shedding water. Florida’s code builds it into the underlayment options for shingle and metal roofs, and requires one when an older house is re-roofed.
The Florida Building Code, Existing Building is explicit. Section 706.7.2 of the Existing Building code says that when a roof covering is removed and replaced on an existing wood-decked house, a secondary water barrier must be installed in accordance with section R905.1.1 of the residential code or the matching sections of the building code.
It sets two exceptions:
- Low-slope roofs. Slopes under 2:12 with a continuous roof system are deemed to comply.
- Tile roofs. Clay and concrete tile systems installed as the building code requires are deemed to comply.
Florida law asked for this. Section 553.844 of the statutes, on windstorm loss mitigation, directs the Florida Building Commission to adopt standards for secondary water barriers on roofs, and says the criteria may not be limited to one method or material.
What are the code’s underlayment options for shingle roofs?
Three. For asphalt shingles and metal roofs, section R905.1.1.1 allows a full deck covering of self-adhering ASTM D1970 underlayment; or self-adhering strips at least 3 3/4 inches wide over every deck joint, with approved underlayment over the whole roof; or two layers of felt or synthetic underlayment laid with set laps and fastening.
The three methods, side by side:
| Method | What goes down | Key details |
|---|---|---|
| 1. Full self-adhered | ASTM D1970 self-adhering underlayment over the entire deck | Installed to both the underlayment and roof covering manufacturers’ instructions; not permitted for wood shingles or shakes |
| 2. Taped joints | 3 3/4-inch self-adhering strips over all deck joints, then approved underlayment over the entire roof | Strips are ASTM D1970 membrane or AAMA 711 Level 3 flashing tape |
| 3. Two layers | Two layers of ASTM D226 Type II, D4869 Type III or IV, or D8257 underlayment | Half-sheet starter, then laps of half a sheet plus 1 or 2 inches, 6-inch end laps offset by 6 feet |
Each method is a secondary water barrier under the code. What differs is how the deck is protected if the shingles go: a fully adhered membrane, sealed joints under a second layer, or a doubled layer of mechanically fastened underlayment.
Is peel and stick roof underlayment good?
It is one of the code’s three approved methods, and the one Florida’s wind mitigation form credits as secondary water resistance. Self-adhering ASTM D1970 underlayment bonds to the deck, so if shingles blow off, the deck stays sealed. It must be installed to both the underlayment and the roof covering manufacturers’ instructions.
That last condition is in the code itself. Section R905.1.1.1 requires a full self-adhered deck covering to be installed in accordance with both manufacturers’ instructions for the deck material, roof ventilation configuration and climate exposure. In practice that means the underlayment and the shingles or metal panels must be compatible, and the attic ventilation must suit the system.
Points to weigh:
- Coverage. The whole deck is sealed, not only the joints.
- Insurance. A self-adhered deck can meet the wind mitigation form’s definition of secondary water resistance.
- Compatibility. Both manufacturers’ instructions must allow the combination.
- Not for wood shakes. The code does not permit this method under wood shingles or shakes.
- Future re-roofs. The code allows a later re-roof to keep a verified existing self-adhered layer, with new underlayment over it.
Does peel and stick roof underlayment need to be nailed?
A full self-adhered underlayment bonds to the deck rather than relying on nails, and is installed to the manufacturers’ instructions. The other two methods are fastened: in the taped-joint and two-layer methods, the underlayment over the deck is attached with corrosion-resistant ring or deformed shank nails with caps at set spacing.
The fastening rules for those methods in section R905.1.1.1 are specific:
| Detail | Requirement |
|---|---|
| Spacing | Maximum 12 inches on centre between side laps, measured horizontally and vertically |
| Laps | One row at end and side laps, fastened 6 inches on centre |
| Nails | Annular ring or deformed shank nails |
| Caps | Metal or plastic caps at least 1 inch in diameter |
| Metal caps | Required where the ultimate design wind speed is 170 mph or more |
| Penetration | Long enough to go through the sheathing, or at least 3/4 inch into it |
Cap nails hold the underlayment flat and in place under wind, before and after the roof covering goes on. A self-adhered membrane does that job by bonding to the deck.
Do you need underlayment over peel and stick on a roof?
It depends on the method. With the full self-adhered method, the self-adhering membrane covers the whole deck. With the taped-joint method, the code requires approved underlayment over the entire roof on top of the self-adhering strips. On a later re-roof, a verified existing self-adhered layer gets new underlayment over it.
That re-roof rule is worth knowing. The code says an existing self-adhering modified bitumen underlayment previously installed over the deck can stay where it can be confirmed or verified, along with any deck re-nailing required by section 706.7.1 of the Existing Building code. An approved underlayment is then applied over the entire roof on top of it.
The roof covering manufacturer may also have its own requirements over a self-adhered membrane, which is another reason the code ties the method to both manufacturers’ instructions.
How does underlayment affect the wind mitigation form?
Question 6 of Florida’s OIR-B1-1802 form asks about secondary water resistance. Self-adhering polymer-modified bitumen underlayment applied directly to the sheathing, or a foam adhesive barrier, qualifies. Standard underlayments or hot-mopped felts do not, and the answer can affect the wind mitigation credits an insurer applies.
The OIR-B1-1802 form describes secondary water resistance, also called a sealed roof deck, as a supplemental means to protect the house from water intrusion if the roof covering is lost. Its three answers are: secondary water resistance present, none, or unknown.
Section 627.0629 of the Florida Statutes requires insurers’ rate filings to include discounts, credits or other rate differentials for construction techniques shown to reduce windstorm loss. Which credit a sealed roof deck brings depends on the insurer and the policy, so ask your agent.
Because the form grades what is applied directly to the sheathing, the method matters. Ask the contractor which of the code’s methods they plan to use, and whether it will meet the form’s definition.
What should you ask for when you re-roof?
Ask which of the code’s three underlayment methods the quote includes, the product names and their ASTM standards, and whether the method will meet the wind mitigation form’s secondary water resistance definition. Ask too about deck re-nailing, which the Existing Building code requires on older houses during a re-roof.
A checklist for the quote:
- Method. Full self-adhered, taped joints with underlayment over, or two layers.
- Products. Names, ASTM standard and product approval.
- Compatibility. Confirmation that the underlayment and roof covering manufacturers both allow the combination.
- Fastening. Cap nail type and spacing for mechanically attached layers.
- Deck re-nailing. To section 706.7.1 of the Existing Building code.
- Paperwork. Photos and invoices of the underlayment before the covering goes on, for a future wind mitigation inspection.
The form requires documentation for each feature it verifies, so photographs taken while the deck is exposed are worth asking for.
Does a tile roof need a secondary water barrier?
Tile roofs follow their own underlayment rules. The residential code sends concrete and clay tile underlayment to its tile provisions, and the Existing Building code treats tile systems installed as the building code requires as meeting the re-roof secondary water barrier requirement.
Tile underlayment is covered in more depth in its own guide, because the tile manual’s details for laps, valleys and eaves are extensive. The principle is the same: under tile, the underlayment is the layer that keeps water out, and the tile above it mainly protects that layer from the sun and weather.
When comparing tile quotes, ask for the underlayment system by name and whether it is approved for the tile being installed.
How do you know what underlayment is on your roof now?
Check the permit records and any wind mitigation form you already have. Question 6 on the form records whether secondary water resistance was found. If the house has no form, or the answer was unknown, the underlayment can be confirmed when the roof is next opened up.
Residential Roofing Orlando can explain the underlayment options for your roof covering and include the method, products and photographs in a re-roof quote. See our roof installation service or call (407) 305-0865.
Frequently asked questions
These are short answers to questions homeowners search about roof underlayment and peel and stick membranes. Each points back to a fuller section above. Roof decks are steep and slippery when underlayment is exposed, so leave installation to a roofing contractor.
Is peel and stick underlayment good for a roof?
It is one of the Florida code’s three approved methods for shingle and metal roofs, and meets the wind mitigation form’s definition of secondary water resistance when applied directly to the sheathing. It must be installed to both the underlayment and roof covering manufacturers’ instructions, and is not permitted under wood shakes.
Does peel and stick roof underlayment need to be nailed?
A fully self-adhered membrane bonds to the deck and is installed to the manufacturers’ instructions. In the code’s taped-joint and two-layer methods, the underlayment over the deck is fastened with corrosion-resistant ring or deformed shank cap nails at set spacing.
What is a secondary water barrier?
An underlayment built to keep water out if the roof covering is lost, by sealing the deck or its joints. Florida’s Existing Building code requires one when an older wood-decked house is re-roofed, with exceptions for low-slope continuous systems and code-compliant tile roofs.
Does a sealed roof deck lower insurance in Florida?
It can affect credits. Florida law requires insurers’ rate filings to include discounts or credits for features that reduce windstorm loss, and the wind mitigation form records secondary water resistance. Which credit applies depends on your insurer and policy, so ask your agent.
