Roof Layers Explained: The 5 Parts of a Roof System

A shingle roof has five main parts, built from the bottom up: the deck, the underlayment, the metal flashing, the ventilation and the shingles. Roofers also use "layers" to mean how many old roofs are stacked on a house, which is why we call these five the parts of a roof system. The shingles are the only part you see from the street. The other four do most of the work of keeping water out, and they're where most leaks start.

By Van Naik, owner · Updated September 21, 2026

A roof is complicated. Buying one should be simple. Here's each component of a healthy roof in plain English, what it does, and what to look for in a quote.

The 5 parts of a roof at a glance

Part 1: the roof deck

The deck, also called sheathing, is the solid wood surface sitting on your rafters. On most homes it's plywood. On older homes it's often plank boards. Everything else gets nailed into it.

Shingles need a solid deck under them. Wood shake, tile and metal roofs are often built on skip sheathing, which is boards spaced a few inches apart. If you're switching to shingles from one of those, we install new plywood over those boards to give the shingles a solid surface to nail into. It's billed the same way as dry rot: labor and materials at our actual cost.

You won't see the deck until the old roof comes off. After 37 years tearing off Bay Area roofs, we expect most houses to have at least some dry rot. It usually shows up near valleys, along the lower edges above the eaves, and anywhere the attic couldn't breathe.

The fix is simple: cut out the soft wood and put in new wood that will hold a nail. We replace every board the deck needs to be solid, and we leave the sound wood alone so you're not paying for work your roof doesn't need. Many cities also have an inspector check the deck before the new roof goes on. If the inspector wants more replaced, we replace it at the same rate. What matters most is how it's billed. Dry rot repair is charged at our actual cost: $300 per person per day plus workers' comp and payroll tax, with lumber at what we paid for it. Because it's billed at cost, we're not looking for reasons to find it. That rate is printed in your proposal before you sign.

Part 2: underlayment

Underlayment is a water-resistant sheet rolled across the whole deck before any shingles go down. It's the backup layer: when wind-driven rain, ice or a damaged shingle lets water past the top, underlayment keeps it off the wood.

It's also what keeps your house dry during the job. Once the old roof is off, we get underlayment down fast, and we watch the forecast the whole time. If your city needs to inspect the bare deck and the inspection is days away, we dry the roof in anyway, then pull the underlayment back for the inspector and lay it down again. Your house never sits open waiting on a schedule.

The right kind depends on how steep your roof is, what your city's code requires, and what the shingle manufacturer needs for its warranty.

Roofers describe steepness as slope, written as two numbers. A 5/12 roof rises 5 inches for every 12 inches it runs across. The higher the first number, the steeper the roof and the faster water runs off.

• Standard and steep roofs (4/12 and up) get a synthetic underlayment such as ABC ProGuard (currently made by Owens Corning), CertainTeed DiamondDeck or Owens Corning ProArmor. Synthetics don't rot or wrinkle the way old felt paper did.
• Low-slope sections at 2/12 or 3/12 drain slowly, so water can back up under the shingle edges. Code calls for a double layer of underlayment there. We go further and use 100% peel-and-stick, which bonds to the deck and seals around every nail that goes through it.
• High-heat peel-and-stick, such as Owens Corning Titanium PSU30, is an optional upgrade that also qualifies for Owens Corning's top extended warranties.

Every city writes its own rules on top of that. Some require peel-and-stick ice and water shield in the valleys, for example. We build to the code of the city your house is in.

Part 3: metal flashing

Flashing is the metal at the spots where a roof is most likely to leak: the edges, where the roof runs into a wall, around the chimney, and around every pipe coming through.

If you only care about one part, make it this one. Leaks almost always start at the metal.

Every piece of metal on our jobs is new: drip edge, step flashing and pipe boots, painted to match your roof. Here's why that matters. When an old roof is torn off, the metal gets pried up with it. Edge metal bends and loses the tension that held it tight against the roof. Step flashing along your walls comes out full of nail holes from the last install. Rubber pipe boots have usually been cracking in the sun for years.

A fresh coat of paint makes old metal look new. It still has the bends, the nail holes and the cracks. Manufacturers know a new shingle roof can't outlast worn metal underneath it, which is why reusing metal can cost you their best warranty. When you compare quotes, ask each roofer one question: is the metal new, or reused and repainted?

When a chimney or skylight is wide enough to trap water behind it, we have a local sheet metal shop custom-fabricate a "saddle" that splits the water around it.

Valleys, where two slopes meet, are the one place we don't default to metal. Metal valleys are a good option. They're manufacturer-approved, and they frame the lines of a roof nicely. They're also the simplest valley to install: lay the metal down, run the shingles up to it and cut them. The metal does the work.

We use a California cut valley, where the roofers do the work. The shingles on one slope run straight through the valley and partway up the other side. Then the shingles on the other slope are laid over the top of them and trimmed in a straight line along the valley. Water running down the valley flows over a continuous sheet of overlapping shingles. Getting that overlap and cut line right is the craft, and the manufacturers approve this method too.

Part 4: ventilation

Your attic needs to breathe. Cool air comes in low, through vents at the eaves. Warm, damp air goes out high, through vents near the peak.

When that airflow stops, two things happen. In summer an attic can pass 150 degrees and bake your shingles from underneath, which makes them curl and shed their granules early. In winter, moisture from showers, cooking and laundry rises into the attic, hits the cold plywood and condenses there. Year after year, that's how a deck gets dry rot without ever having a leak.

New ventilation is part of every roof we install. Intake and exhaust work as a pair, and most homes we see already have intake vents at the eaves but not enough exhaust up top. So we build the exhaust side into every bid: one low-profile O'Hagin vent for about every 300 square feet of roof. Paired with the intake you already have, that brings your attic into balance with building code. We want your deck in good shape 20 years from now, and that starts with fixing the airflow today.

Ventilation is easy to miss on a quote, so it's worth asking two questions. How many exhaust vents are going on, and are they in the base price? The answers tell you whether your attic is getting what it needs.

Balanced airflow also protects your warranty. Manufacturers require it to honor their extended warranties, so a roof with poor ventilation can put your coverage at risk.

Part 5: shingles

Shingles are the part you pick and the one your neighbors notice. Our bids start with Owens Corning Duration or CertainTeed Landmark. Both are architectural shingles, the thicker, dimensional style that's become the standard on Bay Area homes.

From there you can step up:

• Thicker architectural shingles: Owens Corning Duration Max or CertainTeed Landmark PRO, with extra layers for more depth and a bolder shadow line.
• A wood shake look: Owens Corning Woodcrest or CertainTeed Presidential Shake, which look like hand-split cedar without the fire risk or upkeep of real wood.
• The heaviest shake style: Owens Corning Woodmoor or CertainTeed Presidential TL, the thickest and most dramatic option in either line.

Every line comes in Cool Roof versions, and color doesn't change the price. Any color in the line you choose costs the same.

Every one of these lines comes with a lifetime limited manufacturer warranty, which is prorated after the first 10 years. If you want stronger coverage, we can register your roof for an Owens Corning extended warranty that covers your materials for 50 years, non-prorated, for a small added cost. It's available through Owens Corning Preferred Contractors like us, and it's one of the most affordable ways to get that level of protection.

On top of the manufacturer's coverage, our own workmanship warranty covers the installation for 10 years.

Which part of a roof fails first?

On older Bay Area roofs, leaks tend to start in the same few places, and the shingles are rarely one of them:

• Pipe boots. The rubber gasket around each plumbing pipe dries out in the sun and cracks, usually within 10 to 12 years.
• Valleys and the back side of chimneys. Leaves and debris pile up there and hold water against the roof.
• Metal flashing. Old metal full of nail holes from the original install is a leak waiting to happen, and it's the part nobody ever looks at.
• The deck. Years of poor ventilation soften the wood from the inside until the nails lose their grip and shingles start to lift.

That's why a roof can look fine from the driveway and still leak.

What should a roofing quote include?

A good quote names all five parts. If yours only lists the shingle, ask about the rest. Look for:

• Full tear-off down to the deck, and how dry rot or new plywood will be billed if the deck needs it
• The underlayment type, matched to your roof's slope and your city's code
• New metal: drip edge, step flashing and pipe boots, and whether any of it is reused and repainted
• How the valleys will be done
• The ventilation plan, with vent type and count, and whether the vents are in the base price
• Shingle brand, product line, and both warranties: the manufacturer's and the roofer's own
• How the permit is handled

On that last one, we pull and schedule the permit and bill it at exactly what the city charges, because the fee changes by city and job size. The only other things that can move our number are what we find once the old roof is off: dry rot, a hidden second roof, or spaced boards that need new plywood. Each is billed at the rates printed in your proposal before you sign.

One more question for any roofer: can I talk to a customer whose job had a change order? That answer tells you more than the price.

Common questions

What are the layers of a roof?

A shingle roof is built from five parts: the deck, the underlayment, the metal flashing, the ventilation and the shingles. Roofers also use "layers" to mean how many old roofs are stacked on a house, and every one of those comes off in a full tear-off.

How long does a roof last in the Bay Area?

Most architectural asphalt shingle roofs last about 20 to 30 years. Attic ventilation and sun exposure make the biggest difference. Many insurance companies now look closely at roofs older than 15 to 25 years, so it's worth knowing when yours went on.

Do I need new flashing when I get a new roof?

Yes. Metal bends when the old roof comes off and won't seal the same way again, and a coat of paint doesn't change that. Reusing it can also disqualify you from the manufacturer's best warranty.

Can you put a new roof over the old one?

Code sometimes allows it, but we don't recommend it. It hides the deck, so any dry rot stays trapped under your new roof. It adds weight. And insurers are paying closer attention to roofs with more than one layer.

What is a cool roof?

A cool roof uses shingles made to reflect more of the sun's heat. That keeps your attic cooler, and because the shingles don't heat up as much, they may also go through less of the daily expand-and-contract cycle that wears roofing down over time. When enough homes in a neighborhood have them, they lower what's called the urban heat island effect, so the whole area stays a little cooler. A cool roof is one way to meet California's Title 24 energy code, but it isn't the only way. Many homes meet it with good attic insulation and ventilation instead. Both Owens Corning and CertainTeed offer Cool Roof versions of every line we install.

How do I know if my roof needs replacing?

Look for curling or missing shingles, stains on your ceilings, or daylight showing in the attic. Also look closely at the shingle surface. The granules on top shield the asphalt from the sun, and a few washing into the gutters is normal as a roof ages. Once enough have worn away, the sun breaks down the bare asphalt underneath until the fiberglass mat starts to show through. That's the sign the shingles are done. Cracks across the shingles that look like a road map are another sign they've reached the end of their life.

Or have someone look for you. If it doesn't need replacing, a good roofer will say so.

A roof is complicated. Buying one should be simple.

The five parts are our job to get right. Your side should be simple: one price for everything your roof needs, the permit passed through at what the city charges, the same crew from tear-off to cleanup, and daily photos so you can see what's happening on your house.

Not sure what your roof needs? Call us at (925) 381-9162. If you don't need a new roof, we'll be the first to tell you.

About the author. Van Naik owns Affordable Roofing Systems, a family-owned roofer based in Contra Costa County and serving the Bay Area since 1989. CSLB #1145218 (C-39). Owens Corning Preferred Contractor and CertainTeed Master Craftsman.