What Is a Roofing System?

What the term means

Most people picture a roof as the shingles they can see from the street. A roofer thinks of it as a stack of layers. The shingles are only the top one.

Under them sit sheets that catch stray water, a wood deck that holds the nails, and the framing that holds it all up. Around the edges and openings sit metal pieces called flashing. Vents let air move through the attic.

"Roofing system" is the word for that whole stack. You will see it used in three ways. Roofers use it to mean every part of the roof.

The building code uses the term roof assembly and lists the parts it covers ¹.

Shingle makers use "roofing system" to mean a set of their own branded parts, sold together under one warranty ⁴.

The three meanings overlap but are not the same. The code version does not include framing; framing is covered in Chapter 8 of the IRC. A maker's system warranty does not cover parts it did not make.

One warranty we read says it does not cover non-brand parts such as flashing, fasteners, pipe boots and wood decking ⁴. Knowing which meaning someone uses helps you read a quote or a warranty.

How the layers work together

Each layer has one main job. Water should always run from one layer down onto the next, never behind it.

Shingles stop most rain. ARMA calls underlayment "a secondary layer of protection beneath the shingles" that shields the deck from wind-driven rain ³.

Flashing handles the spots where the covering is cut or meets a wall. Vents deal with the moisture that comes from inside the house.

If one layer fails, the others are left doing a job they were not built for.

Types of roofing systems

Roofers often group systems by slope and by the covering on top. The covering decides what goes under it.

• Steep-slope shingle systems: asphalt shingles over underlayment, the most common roof on houses. See asphalt shingles.
• Metal systems: metal panels or metal shingles over underlayment. Some standing-seam systems carry their own load to the framing. See metal roofing.
• Tile and slate systems: heavy units that need framing sized for the weight.
• Wood shingle and shake systems: the code has its own sections for these coverings ¹.
• Low-slope membrane systems: single-ply or built-up roofs on flat or nearly flat roofs, where water drains slowly.
• Maker "systems": one brand's shingles plus that brand's starter, ridge cap, underlayment, ice barrier or vents, sold for a warranty ^{4 5}.

Your roof pitch narrows the list. The code sets a minimum slope for each covering, so a nearly flat roof cannot take regular shingles.

Worked example: one shingle roof, layer by layer

Here is how a full replacement on a steep asphalt shingle roof builds up, bottom to top. The rules shown come from the 2021 IRC ¹.

Seven steps, seven layers or parts. A quote that lists only step 5 leaves six layers to chance. Use our roofing calculator to size the materials, measured in roofing squares.

Where the weak spots are

The open middle of a roof rarely leaks. Trouble starts where the covering stops, turns or is cut. The code names these spots: where a roof meets a wall, where the slope changes, around roof openings, and where an eave meets a side wall ¹. Every one of them needs flashing.

The eaves are the second weak spot. In cold places, ice can build up at the edge and push water back under the shingles.

That is why the code calls for an ice barrier at the eaves where ice dams are a known risk.

The edges also carry a drip edge that sends water into the gutter.

The third weak spot is the attic. Warm, moist air from the house rises into it. If the air cannot get out, it can wet the underside of the deck. ARMA warns that without enough airflow, a roof system and deck can wear out early, especially when sticky underlayment covers the whole deck ³.

Why thinking in systems helps

• You compare quotes on all the layers, not just the shingle name.
• You catch missing items, such as no new flashing or no vent plan.
• You can check that the fire rating applies to the whole assembly, not one product ^{2 3}.
• You know which parts a maker's warranty covers and which it leaves out ⁴.
• You and the roofer use the same words as the building inspector.

Limits of the term

• "System" is not a legal grade. Any seller can call a set of products a system.
• A maker's system warranty covers its own products, not the roofer's work, unless the terms say so.
• The code definition of roof assembly does not include the framing, so framing is checked under a different chapter.
• Matching brands does not fix a poor install. The warranty we read requires the maker's own install rules to be followed ⁴.

What drives the cost of a roofing system

We do not list prices here. These are the things that move the total, layer by layer.

• Roof size in squares, and how many hips, valleys and openings cut it up
• How many old layers must come off, and how much deck needs replacing
• The covering type, from asphalt shingles to metal, tile or slate
• How much ice barrier your area and roof shape call for
• The flashing count: chimneys, skylights, walls and pipes
• Vent work, such as adding intake at the eaves or a ridge vent
• Whether the maker's system warranty needs extra branded parts or a certified roofer

See our roofing cost guide or run the roof cost calculator for your house.

How a roofing system is installed

A licensed roofer does this work. Do not go on the roof yourself.

• The roofer checks the framing and the attic vents before the job starts.
• Old coverings come off down to the deck on a full replacement ¹.
• Damaged deck boards are replaced so the nails have solid wood to hold.
• Drip edge, ice barrier and underlayment go on in the order that sheds water.
• The covering goes on per the maker's instructions, which the code requires ¹.
• New flashing goes in at walls, chimneys, valleys and openings; on a reroof the code says flashing is rebuilt per the maker's instructions ¹.
• Vents and ridge caps finish the roof, and the roofer registers any system warranty if one applies ⁴.

Keeping the system working

A roofing system lasts as long as its weakest layer. The covering wears in the sun. Flashing seals can crack. Vents can get blocked by insulation or debris. Each of those can let one layer fail early and put strain on the rest.

Simple habits help. Keep gutters clear so water leaves the eaves. Check the attic after big storms for stains or damp wood. Replace missing shingles soon, before the underlayment below sits in the sun and rain.

Have a roofer look at the flashing around chimneys and skylights from time to time, since these spots wear before the open roof does. A roof inspection looks at every layer you cannot see from the ground.

Signs one layer is letting the others down

• Ceiling stains that show up after rain, even with no missing shingles
• Leaks around a chimney, skylight or wall joint, which points to flashing
• Frost, mold or damp wood on the attic side of the deck, which points to airflow
• A sagging line on the roof, which can point to the deck or framing
• Ice dams along the eaves in winter

These signs tell you something is wrong, but not which layer. A roofer can trace the cause. See roof repair if you have an active leak.

What the code says about roof assemblies

These rules come from the 2021 International Residential Code, read as adopted in Maryland. Your state or city may use a different edition or change parts of it, so check with your building department.

Sources: [1] [2]

Why fire ratings belong to the whole assembly

A fire rating is tested on a roof built a certain way, not on a single shingle in a lab. The code says listed Class A, B or C roofing is tested to ASTM E108 or UL 790 ².

ARMA states that "underlayment is a required component in a fire-classified roof assembly" ³. So if the roofer swaps or skips the underlayment, the roof may not match the rating the shingle is sold with.

This matters most near property lines and in places that require rated roofs by law. The code calls for a rated roof where the roof edge is less than 3 feet from a lot line ². Ask your roofer which rating your roof needs, and which underlayment the shingle maker lists for it.

Matched-component system warranties

A system warranty rewards you for using one maker's parts. Here is what two makers' own pages say. Terms change, so read the current warranty for your shingle.

CertainTeed's page also says its standard wind rating rises "when the right combination of starter shingles and hip & ridge accessories are used" ⁵. GAF and Malarkey also sell system warranties; we could not load their full terms, so we do not list them.

Roofing system vs related terms

Common misconceptions

Myth The roof is the shingles.

Reality Shingles are one layer. The deck, underlayment, flashing and vents all keep water out too.

Myth A system warranty covers everything on the roof.

Reality The one we read does not cover non-brand parts such as flashing, fasteners, pipe boots or wood decking ⁴.

Myth Any shingle with a Class A label gives a Class A roof.

Reality The rating is for a tested assembly, and ARMA says underlayment is part of it ³.

Myth New shingles can always go over old ones.

Reality The code bars a recover over two or more existing coverings, or over a soaked or worn-out roof ¹.

Myth Venting is separate from the roof.

Reality Airflow under the deck keeps it dry. One system warranty requires a ventilated deck to qualify ⁴.

Local rules and climate

The code rules above come from the model code. Each state or city adopts a version of it, sometimes years old, and may change parts. Your building department decides which edition applies to your address.

Local conditions change which layers a roof needs. Wind design areas bring tighter nailing and heavier underlayment. Cold areas with ice dams bring an ice barrier at the eaves.

Some places require fire-rated roofs by law ². Our ice barrier study looks at where that rule applies. We do not list local rules town by town; ask your building department or roofer.

What to ask your roofer

• Can you list every layer by product name: underlayment, ice barrier, starter, shingle, ridge cap, vents?
• Will you tear off down to the deck, and how will you price deck repairs?
• Will all flashing be new, including at the chimney and walls?
• Does my attic have enough intake and exhaust venting?
• Which system warranty applies, what parts does it need, and will you register it?
• What fire rating does my roof need, and does the assembly you are building meet it?

Keep going

• Roof replacementWhere every layer of the system gets rebuilt
• Roofing materialsHow the coverings on top compare
• Reroofing rulesWhen a new layer can go over an old one
• Choosing a rooferHow to compare quotes line by line

Related terms

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Roof Flashing

Roof flashing is thin, water-shedding material, usually bent metal, installed at the joints of a roof where shingles alone cannot keep water out: along walls, around chimneys and pipes, in valleys and at the roof edges.

Roof Ventilation

Roof ventilation is the system of vents that lets outside air enter an attic low, near the eaves, and leave high, near the ridge, so heat and moisture do not build up under the roof deck.

Roof Sheathing

Roof sheathing is the layer of plywood, OSB or wood boards nailed across the rafters or trusses.

Roof Structure

The framing that holds a roof up: rafters or trusses, ridge board, ceiling joists and bracing. The roof deck and covering sit on top of it.

Roof Covering

The weather-shedding top layer of a roof, such as shingles, tile, metal panels or a membrane. Building codes use "roof covering" for this layer.

Ice and Water Shield

Ice and water shield is a self-adhering, rubberized asphalt membrane laid directly on the roof deck under the roof covering, where it seals around nails and blocks water that ice dams or wind-driven rain push back under the shingles.

Drip Edge

Drip edge is a strip of bent metal flashing installed along a roof's eaves and rake edges that carries water off the edge of the roof and away from the fascia and the deck beneath.

Asphalt Shingles

Asphalt shingles are roofing pieces made from a fiberglass mat coated with asphalt and topped with mineral granules, laid in overlapping rows so water runs off the roof.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

Also referenced from: Roof Warranty, Manufacturer Warranty