Anatomy of a Roofing System

What fails in a storm usually isn’t the shingle in the middle of the roof — it’s an edge, a transition, a fastener, or a layer underneath that was skipped or rushed. Understanding the order the layers go on makes it obvious why sequence matters. Built bottom to top:

1. Roof deck — the structural wood everything fastens to
2. Underlayment — the secondary water barrier over the whole deck
3. Leak barrier — self-sealing membrane at the vulnerable spots
4. Drip edge — metal that throws water clear of the wood edges
5. Starter strip — the sealed first course that grabs the field shingles
6. Field shingles — the visible weather layer
7. Hip & ridge cap — the finished peaks that cover the ridge vent

Running through all of it: flashing at every place the roof meets something else, and balanced ventilation moving air from the eaves to the ridge so the whole assembly stays dry and cool.

The deck is the structural wood base — plywood or OSB sheeting nailed across the rafters or trusses — that every other layer fastens to. It carries the load and gives the nails something to bite. If the deck is soft, delaminated, or rotted, nothing above it holds, which is why a real inspection looks at decking condition before anything else.

Underlayment is the secondary water barrier rolled over the entire deck, beneath the shingles. If a shingle lifts or a piece of hail cracks the surface, this is the layer that keeps water off the wood. There are two kinds, and the difference is real:

Felt is the traditional asphalt-saturated paper (the “15-pound” and “30-pound” rolls). It works, but it’s heavier, tears easily, wrinkles when it gets wet, and degrades faster in sun. Synthetic underlayment is a woven polymer (polypropylene) sheet — far lighter, dramatically higher tear strength, better UV and water resistance, and safer to walk on. Synthetic is the modern default on a quality install; both shed water, but they are not equivalent in durability.

A leak barrier is a self-adhering, waterproof membrane installed at the spots most likely to leak — the eaves, the valleys, and around chimneys, vent pipes, and skylights. Because it’s self-sealing, it grips around every nail that’s driven through it. Its two jobs are stopping ice dams (water backing up under shingles as snow melts and refreezes at the cold eave) and resisting wind-driven rain forced uphill in a storm.

Drip edge is a bent metal strip along the roof’s edges. It exists because water doesn’t fall straight off a roof — surface tension pulls it backward, around the edge, onto the wooden fascia and the ends of the deck, where it sits and rots the wood. The drip edge’s outward kick breaks that surface tension and throws the water clear.

Here’s the detail most people never notice: at the eaves, the underlayment goes over the drip edge; at the rakes (the gable ends), the underlayment goes under it. That’s not a preference — it’s the code, and there’s a reason for each. At the eave, water sheds straight down-slope, so running the underlayment over the metal makes sure any water that gets under the shingles lands on the drip edge and off the roof, never behind it. At the rake, the edge faces sideways into wind-driven rain, so the drip edge goes on top to support the shingle edge and block water from being pushed underneath.

The starter strip is a sealed first course installed along the eaves and the rakes, with a factory-applied adhesive bead positioned to grab the bottom edge of the first row of shingles. It’s what keeps wind from catching that first row and unzipping the roof from the edge in. A common shortcut — cutting up field shingles to use as starter — puts the sealant in the wrong place and measurably lowers the roof’s wind resistance at its most exposed line.

The field shingles are the visible weather layer — the part that takes the sun, rain, wind, and hail. Architectural (also called dimensional or laminate) shingles are built on a fiberglass mat, coated in asphalt and mineral granules, and layered for thickness. They outlast and out-resist the old flat 3-tab shingle, carry wind ratings up to roughly 130 mph, and typically hold a Class A fire rating, the highest. The granules aren’t just color — they shield the asphalt from UV, which is what ages a roof.

In hail country, this is the rating that matters most. UL 2218 grades a shingle’s impact resistance from Class 1 to Class 4 by dropping a steel ball of a set size from a set height and checking whether the shingle cracks through the back. Bigger ball, greater height, more energy:

Every class requires two strikes in the same spot with no crack through the backing — not just one. Class 4 is the top rating and the one most often tied to insurance discounts, though that varies by carrier and by state. Manufacturers reach it either with a polymer-modified asphalt or with a mesh/scrim reinforcement on the back of the shingle. (A parallel test, FM 4473, uses launched ice balls instead of steel.)

A roof has to move air, or it bakes the shingles from underneath and traps moisture in the attic — both of which shorten its life. The principle is balance: intake low (soffit or eave vents) and exhaust high (ridge or hip vents), in roughly equal amounts, so air flows continuously up and out by convection.

The sizing rule of thumb is 1 square foot of net free vent area for every 150 square feet of attic floor (the 1:150 rule), which can be cut to 1:300 when there’s a vapor barrier and the venting is balanced — split about half intake, half exhaust. Two rules get broken constantly: don’t mix exhaust types (a ridge vent combined with power, turbine, or gable vents short-circuits the airflow), and a ridge vent with no soffit intake is nearly useless — with nowhere to draw fresh air, it pulls conditioned air straight out of the house instead.

Flashing is the metal at every place the roof meets something else — walls, chimneys, skylights, valleys, and vent pipes. This is where leaks actually start. Most roof leaks aren’t through the open shingle field; they’re at flashing that was reused from the old roof, sealed with a bead of caulk instead of formed and lapped correctly, or skipped entirely. Caulk is not flashing — it’s a temporary patch on top of one.

The ridge cap is the row of finishing shingles installed over the peaks and hips. It covers the ridge vent and the cut edges of the field shingles where two roof planes meet — a wind-exposed line that gets its own purpose-made, pre-formed cap rather than folded field shingles.

FORTIFIED is a higher building standard from the Insurance Institute for Business & Home Safety (IBHS), the insurance industry’s research nonprofit. It targets the three ways roofs fail in severe storms, in plain terms: nail it down, seal it up, lock it in.

That means a sealed roof deck — the sheathing seams are taped or covered so that even if the wind tears the shingles off, water still can’t get into the house (this single step prevents most storm-related interior losses); ring-shank nails in an enhanced pattern that nearly double how hard the deck is to peel off the structure; and locked-in edges — a wider drip edge and a fully-adhered starter at the eaves and rakes where wind gets its grip. A home doesn’t just claim FORTIFIED; an independent, certified FORTIFIED evaluator verifies the work, which is why photos during installation matter.

Once you see a roof as a system, an inspection stops being “are the shingles okay” and becomes a record of the parts that actually decide whether the roof holds: the deck condition, the flashing at every transition, the edge details, the ventilation balance, and whether the storm-rated components are what they’re claimed to be. A documented record of what’s genuinely on your roof — before any contractor conversation — protects your position whether you’re buying, selling, filing a claim, or simply planning ahead.