How a Residential Asphalt Shingle Roof Works

Your roof is a system, not a product

When most homeowners think "roof," they picture shingles. But the shingles are only the outermost layer of what is actually a multi-part assembly — a stack of components that work together to keep water out, manage temperature, resist fire and wind, and let your house breathe.

According to the Asphalt Roofing Manufacturers Association (ARMA), a properly built residential shingle roof has six core components: roof deck, underlayment, ice and water barrier, flashing, shingles, and ventilation. Remove or compromise any one and the rest are working harder than they should — usually with shorter life as the result.

Ohio code treats it the same way. The Ohio Residential Code (ORC), Chapter 9, defines a roof assembly as "the roof deck, substrate or thermal barrier, insulation, vapor retarder and roof covering" — meaning the inspector isn't just checking shingles, they're checking the whole stack.

Here's how each layer does its job.

Layer 1: The roof deck (sheathing)

The deck is the structural skin of your roof. On almost every modern Ohio home, it's plywood or oriented strand board (OSB) nailed across the rafters or trusses.

Everything above it — underlayment, ice and water shield, shingles, vents — is fastened into the deck. If the deck is rotten, soft, or delaminating, nothing above it can hold properly. This is why a quality roofer pulls the old shingles down to the deck and inspects every square foot before re-roofing. Patching over a bad deck is one of the most common shortcuts in the industry, and it always fails.

Ohio Residential Code Chapter 8 governs deck materials, fastener schedules, and span requirements. Wood structural panels must conform to DOC PS 1, DOC PS 2, or equivalent standards and be grade-stamped.

Layer 2: Ice and water barrier (the eaves and valleys)

This is a self-adhered, rubberized asphalt membrane that sticks directly to the deck. Where it goes, water cannot get through — even if the shingles fail above it.

This is not optional in Ohio. Ohio adopts the ice barrier requirement statewide through the Residential Code. The membrane must be installed along eaves and extend from the lowest edge of the roof to at least 24 inches inside the exterior wall line of the building. Most contractors also install it in valleys, around penetrations, and around skylights — those are leak-prone areas regardless of climate.

Why it matters in Ohio specifically: our freeze-thaw cycles create ice dams. Snow on the upper roof melts from attic heat, runs down, and refreezes at the cold overhang. Water then backs up under the shingles. Without ice and water shield, that water enters the home. With it, the water hits a waterproof rubber layer and runs back off.

Layer 3: Underlayment

Above the deck and ice barrier, the rest of the roof gets covered in underlayment — either traditional asphalt-saturated felt (#15 or #30) or, more commonly today, a synthetic woven product.

Underlayment is the secondary water barrier. Shingles do the bulk of the work shedding water, but wind-driven rain, hairline cracks around fasteners, and any future shingle damage will let some moisture through. Underlayment catches it.

ARMA also notes that many synthetic underlayments are slip-resistant, which improves safety for the installers walking on a steep pitch.

Layer 4: Flashing

Flashing is sheet metal (aluminum, galvanized steel, or copper) installed anywhere the roof meets something else — a wall, a chimney, a vent pipe, a skylight, a valley. These intersections are where roofs leak.

Common flashing locations on a residential roof:

• Step flashing — interlocking L-shaped pieces where the roof meets a vertical wall
• Counter-flashing — overlaps step flashing, typically tucked into a chimney mortar joint
• Valley flashing — open metal "V" in roof valleys (or closed-cut shingles, depending on style)
• Pipe boots / vent flashing — the rubber-and-metal collars around plumbing vents
• Drip edge — metal trim along eaves and rakes that directs water away from the fascia

Ohio code (ORC R903.2) requires flashing at all wall and roof intersections, at penetrations, and wherever the roof changes slope. Skipping or reusing old flashing is one of the most common causes of premature roof failure on otherwise-decent installations.

Layer 5: The shingles

This is what you see from the curb — but it's only the final waterproofing layer.

Modern asphalt shingles are typically laminated (also called "architectural" or "dimensional"), made from a fiberglass mat saturated with asphalt and surfaced with mineral granules. The granules do three jobs: they protect the asphalt from UV degradation, add fire resistance, and give the shingle its color.

Most asphalt shingle assemblies achieve a Class A fire rating — the highest available — when tested under UL 790 or ASTM E108. Class A means "effective against severe fire-test exposure." Ohio code references these same standards in ORC Section 902.

Shingles also include:

• A factory-applied adhesive strip that activates with sun heat, bonding each course to the one below to resist wind uplift
• Algae-resistant granules (usually copper-based) on AR-rated shingles — important in humid climates where black streaking is common
• A starter strip at the eaves (a separate product, not just cut-up regular shingles) that provides the first sealed course
• Hip and ridge shingles designed to bend over the peaks without cracking

Wind ratings for asphalt shingles are set by ASTM D3161 and ASTM D7158. Ohio code (ORC R905.2.4) requires shingles to be rated for the design wind speed of the location. Columbus and most of central Ohio fall in a wind zone that requires shingles rated for 90 mph or higher.

Layer 6: Ventilation

This is the one most homeowners overlook — and it's the one that quietly kills roofs.

A residential attic needs intake vents low (at the soffits or eaves) and exhaust vents high (at the ridge or near the peak). Cool outside air enters low, warms as it rises, picks up moisture from the house below, and exits high. This air "washes" the underside of the roof deck.

The Ohio Residential Code (Section R806.2) sets the minimum: 1 square foot of net free ventilating area for every 150 square feet of attic floor space, with the ventilation balanced roughly 50% intake / 50% exhaust. The code allows a reduced 1:300 ratio if a Class I or II vapor retarder is installed on the warm side of the ceiling and the upper vents are placed within 3 feet of the ridge.

Without proper ventilation:

• Summer: attic temperatures climb past 150°F, cooking the shingles from underneath and shortening their life
• Winter: warm moist air from the house condenses on the cold underside of the deck, causing rot, mold, and dripping insulation
• Year-round: ice dams form more aggressively, shingle warranties may be voided by the manufacturer, and HVAC costs go up

ARMA explicitly recommends vented attics for asphalt shingle assemblies for exactly these reasons.

How the system fails

When a roof leaks, it's almost never the shingles "wearing out" first. The usual culprits, in rough order of frequency:

1. Flashing failure — old caulk cracks, step flashing rusts through, pipe boots split from UV
2. Ventilation problems — attic moisture rots the deck from the inside out
3. Ice damming — without ice and water shield, backed-up water enters the home in winter
4. Wind damage — a few shingles lift and tear, exposing the underlayment
5. Storm/hail impact — granules knocked off, mat exposed, accelerated UV failure begins
6. Improper nailing — shingles installed too high, too low, overdriven, or under-driven from the start

Notice that most of these are installation or component failures — not the shingle itself failing. A well-built asphalt shingle assembly in central Ohio should reliably last 20–30 years depending on the shingle quality, pitch, ventilation, and exposure.

What this means for homeowners

When you're getting estimates for a new roof, the price difference between contractors usually isn't the shingle brand. It's whether they're installing a full system or just the visible top layer.

Ask any contractor:

• What underlayment are they using, and is the ice and water shield extending the full code-required distance past the wall line?
• Are they replacing all flashings, or reusing the old ones?
• Are they upgrading or correcting the ventilation, or just installing whatever was there?
• Are they tearing off to bare deck (and what's the cost if rotten sheathing is found)?
• What is the wind rating of the shingle, and what nailing pattern will they use?

A roof is the most important weather-protective system on your home. Treating it as a system — not a product — is what makes one last twice as long as another.

This article is informational and reflects code requirements in effect at time of publication. Local jurisdictions in Ohio may have additional amendments. Always verify current code with your local building department before beginning a roofing project.