Underlayment is the waterproof or water-resistant layer installed directly on your roof deck (plywood/OSB) before shingles, tiles, or metal panels go on top. You never see it after installation, but in Florida, it might be the most important component of your entire roof system.
Here's why: when a Category 2+ hurricane rips shingles off your roof (which happens), the underlayment is what prevents water from pouring into your home. Without proper underlayment, lost shingles equal immediate interior water damage. With self-adhering underlayment, your home stays dry even with exposed roof deck. Florida Building Code requires it for exactly this reason.
Types of Roof Underlayment
| Type | Cost/sq ft | Weight | Water Resistance | FL Code |
|---|---|---|---|---|
| 15-lb Felt | $0.15-0.25 | Light | Water-resistant (not waterproof) | Not compliant alone |
| 30-lb Felt | $0.20-0.35 | Moderate | Better water resistance | Acceptable in some areas |
| Synthetic | $0.25-0.50 | Very light | Excellent water resistance | Acceptable with proper fastening |
| Self-Adhering (Peel & Stick) | $0.50-0.75 | Moderate | Fully waterproof | Gold standard for FL |
| Rubberized Asphalt | $0.60-0.90 | Heavy | Fully waterproof | Exceeds FL requirements |
Florida Building Code Requirements
Florida takes underlayment more seriously than any other state. Here's what the code requires:
Standard Florida (Non-HVHZ)
- Minimum: ASTM D226 Type II (30-lb felt) or ASTM D4869 Type IV synthetic
- Enhanced protection: Self-adhering polymer modified bitumen at eaves, rakes, ridges, and all penetrations
- For steep-slope roofs (most residential): two layers of underlayment or one layer of self-adhering
High Velocity Hurricane Zone (HVHZ)
Pinellas County falls under Florida's enhanced wind requirements. While not technically in the "HVHZ" (which covers Miami-Dade and Broward), Pinellas County's wind speed requirements are stringent enough that most contractors install to HVHZ-level underlayment standards:
- Self-adhering modified bitumen underlayment (full deck coverage) OR
- Two layers of 30-lb felt with staggered joints and enhanced nail patterns
- All seams and laps sealed
- 6-inch minimum overlap at all horizontal seams
Synthetic vs Felt: Which Is Better for Florida?
Synthetic Underlayment Advantages
- UV resistance: Can be exposed for up to 6 months without degradation. Felt degrades in weeks. This matters if your project gets delayed.
- Tear resistance: 3-5x more tear-resistant than felt. Won't rip during installation or in wind.
- Lighter weight: Easier for crews to handle, especially on steep pitches.
- Lies flat: No wrinkles or bubbles that can telegraph through shingles.
- Slip-resistant surface: Safer for crews to walk on during installation.
Felt Underlayment Advantages
- Lower cost: 30-50% cheaper than synthetic per square foot.
- Proven track record: Decades of use in Florida with documented performance.
- Breathability: Some contractors prefer felt's ability to dry out after minor moisture exposure.
Our Recommendation for Pinellas County
Self-adhering peel-and-stick underlayment (full deck). Yes, it costs more ($0.50-0.75/sq ft vs $0.20-0.35 for felt). On a 2,000 sq ft roof, that's an extra $600-800. But it provides true secondary water resistance. If a hurricane takes your shingles, your home stays dry. That $600-800 investment can prevent $50,000+ in water damage. It's the single best insurance policy on your roof.
Underlayment and Your Insurance
Self-adhering underlayment qualifies as "secondary water resistance" (SWR) on yourwind mitigation inspection. This is one of the highest-value credits available:
- SWR credit can reduce your premium by $200-800/year
- At $200/year savings, the $600-800 upgrade pays for itself in 3-4 years
- At $800/year savings, payback is under 1 year
- The credit continues for the life of the roof (25-50 years depending on material)
When to Replace Underlayment
Underlayment should be replaced during every roof replacement. Even if the existing underlayment looks intact, it has been exposed to decades of heat cycling, moisture, and UV (through nail holes and shingle gaps). Reusing old underlayment under new shingles is false economy.
For tile roofs, underlayment often needs replacement at 20-25 years even though the tiles themselves last 50-75 years. This is the hidden cost of tile roofing that many homeowners don't anticipate. The tiles get removed, new underlayment is installed, and the same tiles go back on. Cost: $5,000-10,000 depending on tile type and roof complexity.
Questions About Your Underlayment?
During our free roof inspection, we check underlayment condition (where visible from the attic) and recommend the right underlayment system for your next roof. Available across St. Petersburg,Clearwater, and all Pinellas County.