The underlayment is the layer beneath the visible roof material. It is what actually keeps water out of your house. The tile, the slate, the metal panel — those are weather shields. The underlayment is the membrane.
The specification tells you everything about the roofer.
What the code requires
The 2026 Florida Building Code requires, at minimum, a #30 felt or equivalent synthetic underlayment under field material, and a self-adhered membrane in HVHZ. The state-wide minimum is meaningfully lower than what we specify on any project, premium or otherwise.
What we specify
For all our projects, regardless of price tier:
- Self-adhered membrane over the entire deck. Not just the eaves and valleys; the entire deck. Adds about 30-40% to the underlayment line item over code-minimum and approximately doubles the storm-resistance of the assembly.
- High-temp rated product if the field material is dark or metal. Standard self-adhered membranes have failure modes at the surface temperatures dark metal roofs reach.
- Two-layer system under slate. Self-adhered as the base; a #30 felt or synthetic sheet over it as the slip layer that allows the slate to be set without scuffing the membrane.
What budget roofers specify
- #15 felt (older, lighter weight) where the code permits and inspectors do not flag.
- Synthetic underlayment with self-adhered only at eaves and valleys, code-minimum for HVHZ.
- Generic re-roof underlayment with no documented product approval beyond meeting the code minimum.
The cost differential between minimum-code and what we specify is in the range of $1-2 per square foot. On a 4,000-square-foot project, that is $4,000-$8,000. It is also the difference between a 15-year storm-resistant roof and a 30-year one.
What to ask
Three questions on any roof bid:
What is the underlayment product and its FPA/NOA number? A specific answer is required. "Self-adhered" is not a product; "Polyglass Polystick TU Plus, FL-15571" is.
Is it installed over the entire deck or only at vulnerable areas? "Vulnerable areas" is the cheap answer.
What is the temperature rating of the product relative to the field material being installed above it? This is the question that distinguishes a roofer who has thought about the assembly from one who has not.
A separate note on synthetic vs. felt
Synthetic underlayments are now the default everywhere outside of slate work. Synthetic is lighter, less moisture-sensitive during install, and has a longer exposed-tolerance than felt (which means it can sit under blue-sky for weeks during a long install without degrading).
Felt — specifically #30 organic felt — is still preferred under slate because it provides a slightly more forgiving surface to set the stone against. Slate work uses both: self-adhered as the waterproofing, #30 felt as the slip layer above.
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