Underlayment is a secondary water-control layer installed over the roof deck and below shingles, tile or many metal systems. It works with flashing and the roof covering; it does not replace them. Florida requirements recognize multiple compliant underlayment approaches, so the proposal should identify the actual product and installation method for the roof system.
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Where underlayment sits in the roof
Underlayment is installed over the prepared roof deck and below the finished roof covering. It helps manage water that gets beneath the visible surface, but it must connect properly with flashing, edges, valleys and penetrations.
Synthetic vs self-adhered underlayment
Synthetic underlayment
Synthetic products are mechanically attached sheets designed to create a water-resistant layer beneath the roof covering. Product standards, fastening and overlap requirements depend on the approved system and installation instructions.
Self-adhered underlayment
Self-adhered membranes bond to a prepared deck and can provide a continuous layer when installed correctly. They have their own substrate, temperature, overlap and compatibility requirements.
The current 8th Edition (2023) Florida Building Code, effective December 31, 2023, recognizes multiple compliant underlayment methods and product standards, with later supplements published by the Florida Building Commission. That is why a reroof proposal should identify the actual underlayment product, approval and installation method instead of using a vague phrase such as “new felt.”
How underlayment changes by roof covering
Tile roofs
Tile sheds most surface water, but water can still reach the waterproofing layers below through joints and details. Underlayment condition is therefore important during tile roof replacement.
Shingle roofs
Shingle systems combine the roof covering with underlayment, starter materials, flashings and ventilation details. Slope and manufacturer instructions affect the approved assembly.
Metal roofs
Many residential metal systems use underlayment over the deck as part of the assembly. The correct product depends on panel type, slope, deck, temperatures and manufacturer requirements.
Deck preparation matters
Underlayment performs only as well as the surface below it allows. Loose debris, damaged decking, protruding fasteners or unsuitable substrate conditions can interfere with attachment and waterproofing.
If deck condition is uncertain, see our roof inspection information before finalizing a replacement scope.
Flashing, valleys and penetrations
Underlayment is not a substitute for flashing. Valleys, walls, roof edges, vents, skylights and other penetrations need details that direct water into the drainage path rather than behind the roof assembly.
Signs the water-control system needs review
Recurring leaks, staining after wind-driven rain, water entry around valleys or penetrations, and moisture found during tear-off can all justify a closer look at the underlayment and surrounding details. These symptoms do not prove that underlayment alone is the cause.
What homeowners should ask in a proposal
- What underlayment product is specified?
- How will the deck be prepared?
- How will seams, edges and transitions be handled?
- How does the underlayment connect to flashing?
- Is the product approved for the roof covering and slope?
- What happens if damaged decking is found?
Frequently Asked Questions
Does underlayment stop every roof leak?
No. Water can still enter through failed flashing, penetrations, edges or installation defects. Underlayment is one layer in the roof system.
Is self-adhered underlayment always required in Florida?
Do not assume one method applies to every roof. Current code, roof covering, slope, approved products and jurisdiction-specific requirements should be verified for the project.
Can old underlayment stay under a new roof?
That depends on the reroof method, existing condition, approved assembly and current requirements. It should be evaluated for the specific project.
Why does underlayment matter under tile if tile is durable?
Because the tile surface is not the only water-control layer. Underlayment and flashing below the tile remain important parts of the assembly.
Sources and References
- Florida Building Commission — Code Resources — current 8th Edition (2023) code resources, supplements and roofing references
- Owens Corning — Shingle Installation Instructions — manufacturer example showing how underlayment approach changes with slope and product instructions
Roofing Content Research & Local Information. We review homeowner questions, official local resources and roofing-system information, while keeping property-specific diagnosis with the service provider who inspects the roof.
View Author ProfileComparing reroof proposals?
Ask each provider to identify the underlayment product, deck preparation and flashing approach so you can compare complete roof systems rather than brand names alone.
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