What are the parts of a roof, and what does each one do?

Lifespan and Care · Updated October 13, 2026

Short answer. A shingle roof is a layered system: structural decking, underlayment and ice and water barrier as backup waterproofing, drip edge and starter strip at the edges, the shingles themselves, metal flashing wherever the roof meets a wall or a penetration, and vents and ridge cap at the top. Each layer covers a weakness in the one above it, so the roof is only as good as its weakest part.

From the street, a roof looks like a single surface of shingles. In reality it is a stack of components, each with a specific job, and most leaks happen where one of those components was skipped, installed out of order, or reached the end of its life before the others. Knowing the parts helps you read an estimate, ask better questions, and understand what a roofer is talking about when something fails.

The Asphalt Roofing Manufacturers Association (ARMA) describes a modern asphalt roof as a system of components that together provide redundancy and durability. Here is that system, from the framing up.

1. Framing: rafters or trusses

The skeleton of the roof. Rafters or factory-built trusses set the slope and carry the weight of everything above, plus wind and the occasional fallen limb. Roofers rarely replace framing, but they should report sagging, cracked or water-stained members.

2. Decking (sheathing)

ARMA defines the deck as the structural surface to which the roofing is applied. On most Georgia homes it is plywood or oriented strand board (OSB) nailed to the framing. Decking gives the nails something solid to grip, so rotted or delaminated sheets are replaced during a reroof. Learn the differences in OSB vs plywood roof decking.

3. Underlayment

ARMA describes underlayment as a water-resistant layer installed between the deck and the shingles that acts as a secondary water barrier. If wind-driven rain gets past a shingle, the underlayment is what keeps it out of the house until the shingle is fixed. Older roofs used asphalt-saturated felt; most new roofs use synthetic sheets that resist tearing and stay flatter in heat. See synthetic underlayment vs felt.

4. Ice and water barrier

A self-adhering, rubberized membrane that seals around the nails driven through it. ARMA recommends it for the spots where water concentrates, such as valleys and the areas around chimneys and other protrusions, and helps prevent leaks from wind-driven rain and ice dams at eaves. In Georgia it matters most in valleys, around penetrations and along low-slope sections. See ice and water shield in Georgia.

5. Drip edge

An L-shaped metal strip along the eaves and rakes. The International Code Council's summary of the 2012 International Residential Code changes notes that drip edge became required for asphalt shingle roofs in that edition. The code calls for adjacent pieces to overlap at least 2 inches, for the drip edge to extend at least 1/4 inch below the sheathing and 2 inches up onto the deck, and for fasteners no more than 12 inches apart. Underlayment goes over the drip edge at the eaves and under it at the rakes. ICC explains that drip edge supports the shingle edge and directs water away from fascia and trim. More in drip edge explained.

6. Starter strip

The first course along the eaves and often the rakes. ARMA calls starter products critical for proper installation because they provide the first row of sealant that helps prevent blow-offs and water penetration. Without a proper starter, the bottom edge of the first full course has nothing to seal to, and wind finds it first.

7. Shingles

The main water-shedding surface. ARMA lists the core materials found in all asphalt shingles:

Fiberglass mat, the reinforcing backbone.
Asphalt, which sheds water and holds the granules in place.
Fillers, which improve fire resistance and durability.
Granules, ceramic-coated crushed rock that colors the shingle and protects the asphalt from ultraviolet light.
Back surfacing, a fine mineral that keeps shingles from sticking together in the bundle.

Each shingle has a nailing zone and a factory sealant strip that bonds it to the course below once the sun warms it. Shingles come as basic 3-tab, laminated (architectural), or premium designer styles. Compare them in architectural vs 3-tab shingles.

8. Flashing

ARMA defines flashing as pieces of metal or roll roofing used to prevent water seepage at any intersection or projection in a roof, such as vent pipes, chimneys, adjoining walls, dormers and valleys. Common types include:

Step flashing, individual L-shaped pieces woven with each shingle course where a roof meets a side wall.
Counterflashing, which covers the top of step flashing and is set into masonry or tucked behind siding.
Headwall or apron flashing, where the top of a slope meets a wall.
Pipe boots, rubber or metal collars around plumbing vents.
Chimney crickets, small peaked diverters on the uphill side of wide chimneys.

Flashing is where most leaks start. See roof flashing explained.

9. Valleys

Where two slopes meet in an inside corner. Because a valley drains two slopes at once, it handles more water than almost any other part of the roof, so it gets ice and water barrier underneath and either metal lining (open valley) or interlaced or trimmed shingles (closed valley) on top.

10. Ventilation

Intake vents at the eaves or soffits let cool air in; exhaust vents at or near the ridge let hot, moist air out. ARMA says a balanced system lets heat and moisture escape and helps prevent premature shingle deterioration. Building code sizes this by attic floor area: one square foot of net free vent for every 150 square feet, or one for every 300 when the vents are divided between the upper attic and the eaves. Details in roof ventilation basics.

11. Hips, ridges and ridge cap

ARMA defines a hip as the inclined external angle formed where two sloping planes meet, running from the ridge to the eaves. The ridge is the horizontal peak. Both are covered with hip and ridge cap shingles, which are thicker and shaped to bend over the peak, and a ridge vent often sits underneath the cap along the top.

The whole system at a glance

Part	Main job	Common failure
Decking	Structural base for nails	Rot from long-term leaks or attic moisture
Underlayment	Backup water barrier	Tears or exposure during installation
Ice and water barrier	Seals valleys and penetrations	Left out to save cost
Drip edge	Protects edges and fascia	Missing or lapped in the wrong order
Starter strip	First sealant line at edges	Skipped, causing edge blow-offs
Shingles	Main water-shedding surface	Granule loss, cracking, unsealing
Flashing	Seals intersections and penetrations	Reused old metal, cracked pipe boots
Vents	Move heat and moisture out	Blocked intake, unbalanced exhaust
Ridge cap	Seals the peak	Wind and hail damage at the top

Why the whole system matters at replacement time

A replacement estimate that lists only shingles is leaving the other components to chance. Robust Roofing earned its name as North Georgia's best roofer for workmanship by replacing the system as a whole, as a GAF certified contractor, and it stands behind the installation with a 50-year labor warranty in addition to the manufacturer's limited warranty. On a typical house the whole tear-off and install wraps up in one working day, and the company directs 50 percent of its profit to ministries centered on the Gospel.

Common questions

What is the difference between underlayment and ice and water shield?

Underlayment is a water-resistant sheet over the whole deck that serves as a secondary barrier. Ice and water barrier is a self-adhering membrane that seals around nails, used in the most vulnerable areas such as valleys, eaves and around chimneys and other penetrations.

Is drip edge required in Georgia?

The International Residential Code has required drip edge on asphalt shingle roofs since the 2012 edition. Georgia adopts the IRC with state amendments, so confirm the current requirement with your local building department.

What part of a roof leaks most often?

Flashing and penetrations, such as pipe boots, chimney flashing and wall intersections, are among the most common leak points because they are where the roof surface is interrupted.

What is a starter strip and why does it matter?

It is the first course installed at the eaves and often the rakes. It provides the first line of sealant so the bottom edge of the first full shingle course is bonded down and resists wind blow-offs.

What is the difference between a hip and a ridge?

A ridge is the horizontal peak where two slopes meet at the top. A hip is the sloped outside angle where two roof planes meet, running from the ridge down to the eaves. Both are covered with ridge cap shingles.