Do you want to know what is Below Shingles on a Roof?

When you look at a roof, what you notice first is its shape and the material — most often asphalt shingles — that covers it. Look a little closer and you might spot a vent or a pipe boot, but beyond that, it's really impossible to see what the roof is actually made of. Unless you've watched a roof being torn off and replaced, you may never realize how many layers work together underneath those shingles.

After years of tearing off and rebuilding roofs across Palm Coast, Flagler, and Volusia County, we've noticed that most of the questions homeowners ask during an inspection or a replacement — why a stain appeared when the shingles look fine, or why a quote includes materials they can't see — come down to a gap in understanding what's actually under the surface.

A shingle roof isn't a single material laid over your house — it's a system. Several roofing layers stack up between the trusses and the shingles you see from the street, each one handling a different job: shaping the roof, supporting it, regulating temperature, and keeping water out. Knowing what goes under shingles on a roof helps you understand how your roof actually functions, what can go wrong with it, and how to talk to a contractor when something needs attention.

Roofing Layers: How a Shingle Roof Is Built From the Deck Up

From the attic to the outside world, a typical asphalt shingle roof is built from these roofing system components, in this order:

• The frame (trusses or rafters)

• Insulation

• The roof deck (sheathing)

• A secondary water barrier that seals the roof deck 

• Underlayment across the full roof

• Starter strip, drip edge, flashing, vents, and finally the shingles themselves

Each layer depends on the one below it, which is why a problem low in the stack — a soft spot in the deck, a gap in the underlayment — usually shows up later as a shingle or leak problem higher up. Here's what each layer actually does.

The Roof Frame — Trusses and Rafters

The frame is what gives a roof its shape and defines its boundaries; it's also the structural support for every layer above it. In most modern homes, the frame is a series of wood trusses built off-site to the specifications of the home's blueprint. Some roofs are still framed on-site with rafters cut to size and assembled in place.

The frame has to be designed with the finished roof in mind. Heavier roofing materials — clay or concrete tile, slate — require additional reinforcement in the frame to carry their weight, so this is one of the first decisions made long before shingles ever enter the picture.

Insulation — the Energy-Efficiency Layer

Insulation regulates a home's internal temperature and reduces how hard your HVAC system has to work through seasonal swings. In a finished attic, insulation sits between the rafters; in an unfinished attic, it's more commonly laid across the attic floor.

The type and depth of insulation matter more than most homeowners realize. ENERGY STAR notes that most attics should be insulated to about R-38, which typically works out to 10–14 inches of material depending on the type used (ENERGY STAR: Adding Attic Insulation). Insulation isn't waterproofing — it's a separate layer with a separate job — but it's very much part of what's under your shingles from the inside.

The Roof Deck — Sheathing

The roof deck is nailed directly on top of the frame and creates the actual surface everything else attaches to. It's usually plywood or an engineered wood product like oriented strand board (OSB). Holes are cut into the deck wherever roof vents will eventually be installed.

The deck is also the layer most responsible for how well a roof holds up in high wind — which matters a great deal in a hurricane-prone region like Florida. The U.S. Department of Energy's Building America Solution Center recommends that roof deck sheathing be sealed and properly attached before any covering goes on, since even small breaches at the deck level can lead to much larger damage during a storm (DOE Building America Solution Center: Roof Deck Sheathing and Sealing for Sloped Roofs). The Insurance Institute for Business & Home Safety recommends similar deck-attachment standards — minimum sheathing thickness and closely spaced ring-shank nails — as part of its FORTIFIED Home high-wind construction guidance (IBHS FORTIFIED: Hurricanes and High Winds Overview). A sound, properly sealed deck is the foundation every other roofing layer depends on, and it's worth asking any contractor how they plan to handle it before a new roof ever goes on.

Secondary Water Barrier — Sealing the Roof Deck 

In Florida, this layer isn't optional. The current Florida Building Code (8th Edition) requires a secondary water barrier on asphalt shingle roofs, including when an existing roof is replaced. The code allows three ways to provide it: cover the entire deck with self-adhered, peel-and-stick underlayment; apply self-adhering strips at least 3¾ inches wide over every joint in the deck, then cover the whole roof with approved underlayment; or install two layers of approved underlayment in a specified overlapping pattern, fastened with cap nails. Valleys can also be lined with 36-inch-wide peel-and-stick membrane. In colder states, the same kind of membrane runs along the eaves to stop ice dams, which isn't a concern in Florida's climate. 

Asphalt Shingle Underlayment — the Last Barrier Before the Shingles

Underlayment goes down next, across the entire roof deck, directly beneath the shingles themselves. It's the roofing system's last line of defense if wind-driven rain or a damaged shingle lets water past the surface layer — which is exactly why asphalt shingle underlayment specifications get so much attention from code bodies and manufacturers alike.

There are three common types:

Asphalt-Saturated Felt. Until 15–20 years ago, this was the industry standard. Often called "tar paper" or "felt paper," it's a base material — wood, cellulose, polyester, or fiberglass — saturated with asphalt for water resistance, then nailed to the deck.

Synthetic Underlayment. The most widely used type today. It's made from woven or spun-bonded polypropylene or polyethylene, which makes it lighter and more tear-resistant than felt, but it's still water-resistant rather than waterproof, and it's mechanically fastened. 

Rubberized Asphalt (Peel-and-Seal). The most expensive option and the one gaining the most ground in storm-prone areas. A higher concentration of rubber and asphalt polymers gives it real strength, and its self-adhesive backing creates a fully waterproof seal on the deck with no nailing required.

Underlayment choice isn't arbitrary — it's governed by roof slope, climate, and code. The National Roofing Contractors Association's technical guidance notes that current building codes require enhanced underlayment materials and application methods for asphalt shingle roofs in low-slope or high-wind conditions, which is why a qualified contractor's underlayment recommendation should match your specific roof rather than a one-size-fits-all default (NRCA / Professional Roofing: Understanding Underlayment).

From the field: On older roofs across Palm Coast and Volusia County, we still tear off plenty of single-layer felt that did its job for two decades. That setup doesn't meet today's Florida code on a replacement, so every roof we replace gets a code-compliant secondary water barrier, and we re-nail the deck to current standards where the code requires it. Two roofs can look identical from the street and perform very differently in wind-driven rain, depending on what's under the shingles. 

Starter Strip, Drip Edge, Flashing & Shingles — What You Actually See

Once underlayment is in place, the visible parts of a shingle roof go on: the starter strip and drip edge along the eaves, flashing around chimneys, vents, and other penetrations, and finally the shingles themselves, installed across the full roof surface. This is the only layer most homeowners ever see — but as the list above shows, it's the last of several roofing system components working together, not the roof on its own.

Roofing System Components at a Glance

Why What's Under Your Shingles Matters for Homeowners

Most roof problems don't start with the shingles — they start with something underneath them. A leak that shows up as a stain on your ceiling might trace back to underlayment that's worn out, a deck seam that was never properly sealed, or flashing that's pulled away from a vent pipe. Knowing what goes under shingles on a roof gives you a better vocabulary for talking with a contractor, understanding an inspection report, or evaluating whether a repair quote is actually addressing the source of a problem rather than just the surface.

It also matters at replacement time. When a roof is torn off, the frame, deck, and insulation are typically still in place. In Florida, a replacement then means inspecting and repairing the deck, re-nailing it where the code requires, and installing a secondary water barrier, underlayment, new drip edge and flashing, and finally the shingles. Understanding that distinction helps explain why two roof replacement quotes for the "same" job can differ: they may not be assuming the same condition, or the same underlayment, underneath.

Key Takeaway for Homeowners

A shingle roof is a system, not a single surface — and the layers you never see do most of the work of keeping your home dry and comfortable. Understanding what's under your shingles, from the frame down to the underlayment, makes it easier to spot a real problem early, ask the right questions during an inspection, and know what you're actually paying for on a replacement.

If you have questions about roofing layers or underlayment, or you need any roof work done in the Palm Coast, Flagler, or Volusia area, give Florida's Best Roofing, Inc. a call and schedule a free estimate.

FAQ

What goes under shingles on a roof? 

From the bottom up: the frame, insulation, the roof deck (plywood or OSB), a secondary water barrier that seals the deck, and underlayment across the whole roof. The shingles are the final, visible layer on top of all of it.

How many layers does a shingle roof have? 

Most asphalt shingle roofs have six main roofing layers: the frame, insulation, the deck, a secondary water barrier, underlayment, and the shingle covering itself (including starter strip, drip edge, and flashing). 

What's the difference between felt and synthetic underlayment? 

Asphalt-saturated felt is the traditional, nailed-down underlayment made from a base material soaked in asphalt. Synthetic underlayment is made from polypropylene or polyethylene and resists tearing better than felt, though both are water-resistant rather than fully waterproof — unlike self-adhering rubberized asphalt underlayment. 

Does my roof need a sealed deck? 

In Florida, yes. The current Florida Building Code requires a secondary water barrier on asphalt shingle roofs, including when an existing roof is replaced. It can be full peel-and-stick underlayment, sealed deck seams under standard underlayment, or a two-layer underlayment system. Ice-dam protection at the eaves is a cold-climate measure Florida homes don't need. 

How do I know if my underlayment needs replacing? 

Underlayment isn't visible without removing shingles, so age, visible leaks, sagging or soft spots in the roof deck, and the shingles' own remaining service life are usually the best indicators. A roofing contractor can assess this during an inspection or at the start of a replacement.