Roof Underlayment and Secondary Water Barriers in Fort Walton Beach, FL

A&M Construction

2 days ago

4 min read

Roof underlayment is the water-resistant layer installed directly on your roof deck, underneath the shingles. On the Northwest Florida coast it does the job most homeowners assume the shingles are doing: when wind lifts or tears off the covering during a storm, the underlayment and the sealed deck beneath it are what keep rain out of your attic. It is the least visible part of a roof and one of the most important, and Okaloosa County documents it on every reroof for exactly that reason.

Shingles shed water. Underlayment stops it.

A shingle roof is a shedding system, not a sealed one. Water runs down the shingle courses and off the edge. That works beautifully in ordinary rain. It works less well when a Gulf storm drives rain sideways under the shingle tabs, or when 90 mph gusts peel a section of covering off entirely.

The numbers are sobering. IBHS testing found that for every inch of rain that hits an exposed roof deck, the equivalent of nine bathtubs of water can pour into a house. The same research found that sealing the seams of the roof deck can reduce that water intrusion by up to 95 percent. That is the whole argument for taking this layer seriously, laid out in the IBHS FORTIFIED Roof standard.

The three layers between the weather and your ceiling

When we tear off a roof in Fort Walton Beach or Mary Esther, we are rebuilding a system, not just re-covering it:

The deck. Plywood or OSB sheathing, refastened to the current nailing pattern. Soft or rotten sheathing gets replaced. Nothing above this layer works if the deck is compromised.
The secondary water barrier. The seams where sheets of decking meet are sealed, commonly with a self-adhered flashing tape or a fully adhered membrane, so water cannot run straight through the gaps.
The underlayment. A code-approved felt or synthetic sheet across the whole deck, lapped correctly and fastened correctly, plus peel-and-stick membrane in the valleys and around penetrations where water concentrates.

Only then do the drip edge, starter strip, and shingles go on. If a contractor's estimate does not describe these layers, ask what is going over your deck and how the seams are being handled. It is a fair question and a good roofer will have a clear answer.

Felt or synthetic underlayment?

Traditional asphalt-saturated felt has been used for a century and still performs when installed correctly. Synthetic underlayments are lighter, tear-resistant, and hold up better if the roof sits exposed for a day or two mid-project, which is a real consideration during our summer afternoon storms.

Either can be right. What matters more is that the product carries a current Florida Product Approval for the wind speed at your address, that it is part of an approved assembly with your shingle, and that it is fastened to specification. Mixing a premium shingle with an underlayment that is not part of the approved system is how warranties quietly stop applying.

Why Okaloosa County photographs this layer

Here is the part most homeowners never hear about. Before a final roofing inspection can be called, Okaloosa County asks for photographs of the roof, marked with the permit number or address, showing the nailing pattern of the decking and the secondary water barrier. Once the shingles are on, nobody can verify any of that, so it has to be captured while it is open.

In other words, the county has built its inspection process around the assumption that this hidden layer is what saves houses. You can see the current permitting and inspection requirements on the Okaloosa County permitting page, and we cover the whole process in our post on roof permits in Fort Walton Beach.

As a licensed Florida roofing contractor, license CCC1326077, we handle that documentation on every job. You can confirm any contractor's license through the state's DBPR license verification system before work starts.

Frequently asked questions

What is a secondary water barrier?

It is a water-resistant layer applied to the roof deck, typically sealing the seams between sheets of sheathing, that keeps rain out if the shingles come off. It is separate from, and works together with, the underlayment.

Can underlayment be reused during a reroof?

No. On a full tear-off the old underlayment comes off with the shingles and new material goes down on a clean, inspected deck. Anyone proposing to leave the old layer in place is cutting a corner you will pay for later.

How long does roof underlayment last?

It is designed to last as long as the roof covering above it, since it is protected from sun and weather. Left exposed, felt in particular degrades quickly, which is why a roof should never sit dried-in for weeks.

Does better underlayment lower my insurance?

Possibly. Florida offers premium discounts tied to specific hurricane loss mitigation features, and a secondary water barrier is one of the items a wind mitigation inspection looks at. Confirm the details with your insurance agent.

How do I know my roof actually has one?

Check your permit records and the inspection photos from when the roof was installed. If the roof was replaced without a permit, that documentation does not exist, which is one more reason unpermitted work causes problems later.

Want to know what is under your shingles?

A roof inspection plus a look in the attic tells us a great deal about how your roof was built and how much life it has left. If it turns out you need a new roof installation, we will show you exactly what is going on your deck and why. Family owned in Mary Esther since 1994.

Contact A&M Construction or call 850-803-0650.