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Every part of your roof, and how each one fails
A roof is fourteen components doing fourteen different jobs, and only one of them is the shingle. Here is the whole assembly in section, with what each part does and what happens when it stops doing it.
01 · Structure
Roof decking
- What it is
- The plywood or OSB sheathing nailed across the rafters. Everything else on the roof is fastened to it, so it has to hold a nail.
- How it fails
- Decking rots where water has been getting past the shingles, usually at the eaves, in valleys, and around chimneys. Soft decking will not hold a nail, and a nail that does not hold means shingles that lift in the next wind event.
- What ignoring it costs
- Soft decking is found during tear-off, not before, and it is replaced by the sheet. Widespread rot turns a straightforward replacement into a structural repair and can take the job from one day to several.
Every component is written out in full in the key below, whether or not scripting is available.
Two details that need their own drawing
A valley needs two roof planes and a box gutter is a different cut through the building, so neither reads properly inside the main section.
The key
01Roof decking
- What it is
- The plywood or OSB sheathing nailed across the rafters. Everything else on the roof is fastened to it, so it has to hold a nail.
- How it fails
- Decking rots where water has been getting past the shingles, usually at the eaves, in valleys, and around chimneys. Soft decking will not hold a nail, and a nail that does not hold means shingles that lift in the next wind event.
- What ignoring it costs
- Soft decking is found during tear-off, not before, and it is replaced by the sheet. Widespread rot turns a straightforward replacement into a structural repair and can take the job from one day to several.
02Ice and water shield
- What it is
- A self-adhering rubberised membrane bonded directly to the decking at the eaves, in valleys, and around penetrations. It seals around the nails driven through it.
- How it fails
- It rarely fails on its own. What fails is its absence — it is the layer most often skipped on a cheap bid because it is buried and nobody sees it.
- What ignoring it costs
- This is the layer that stops an ice dam from pushing water backwards under the shingles and into the ceiling below. In a climate that freezes and thaws repeatedly through the winter, an eave without it is the reason a roof leaks in January and is bone dry in July.
03Synthetic underlayment
- What it is
- A woven sheet rolled over the rest of the deck above the ice and water shield. It is the secondary barrier and the layer that keeps the deck dry between tear-off and shingle installation.
- How it fails
- Felt underlayment tears around fasteners, wrinkles when it gets wet, and degrades in sunlight if the roof is left open. Synthetic is dimensionally stable and does not wrinkle.
- What ignoring it costs
- When a shingle blows off, the underlayment is what stands between the storm and the deck. With a torn or absent underlayment, one missing shingle becomes a wet ceiling.
04Drip edge
- What it is
- An L-shaped metal flashing along the eaves and rakes. It carries water off the deck edge and into the gutter instead of letting it curl back under.
- How it fails
- It gets left off, installed under the ice and water shield at the eave instead of over it, or dented and rolled back by a ladder.
- What ignoring it costs
- Water that curls back behind the gutter runs down the fascia. That is how fascia boards rot, how soffits stain, and how a roof problem becomes a carpentry problem.
05Starter strip
- What it is
- A purpose-made first course at the eave with its adhesive strip positioned to bond the bottom edge of the first visible shingle course.
- How it fails
- Cut-up field shingles used as starter put the adhesive in the wrong place, which leaves the first course unbonded along its bottom edge.
- What ignoring it costs
- The eave is where wind uplift is highest. An unbonded first course is the single most common place a roof starts peeling in a straight-line wind event, and this market gets far more damaging wind than hail.
06Field shingles
- What it is
- The visible courses across the roof. Architectural shingles are laminated in two layers, which is why they read as a textured surface rather than flat tabs.
- How it fails
- Granule loss exposes the asphalt to sunlight, which then dries and cracks. Hail bruises the mat without always breaking the surface. Wind creases a shingle where it folds back, and a creased shingle is compromised even after it lies back down.
- What ignoring it costs
- Shingle damage is cumulative and quiet. The roof looks the same from the street for years while the mat underneath gets progressively more brittle, which is why the first symptom is often a leak rather than a visible defect.
07Ridge vent
- What it is
- A continuous vent along the ridge that lets warm attic air out. It only works if there is matching intake at the soffits.
- How it fails
- A ridge vent with blocked or absent soffit intake pulls air from the house instead of the eaves. Insulation stuffed into the eaves is the usual culprit.
- What ignoring it costs
- An attic that cannot exhaust runs hot in summer and holds moisture in winter. Hot decking ages shingles from underneath, and winter condensation on cold sheathing is frequently mistaken for a roof leak.
08Step flashing
- What it is
- Individual L-shaped pieces of metal, one per shingle course, woven up the joint where a roof plane meets a wall or a chimney.
- How it fails
- A continuous strip of metal caulked along the joint is the shortcut. It looks the same from the ground and fails within a few seasons as the sealant shrinks.
- What ignoring it costs
- The wall-to-roof joint is the most common leak point on any roof. Water that gets past it runs inside the wall cavity, so the stain appears well below and away from the actual defect.
09Counterflashing
- What it is
- Metal let into a cut groove in the masonry that laps down over the top of the step flashing, so water running down the brick is thrown outward rather than behind it.
- How it fails
- Surface-mounted counterflashing sealed to the brick face instead of cut into a reglet. The sealant is doing the waterproofing, and sealant is a maintenance item, not a flashing.
- What ignoring it costs
- A chimney with failed counterflashing wets the masonry from inside. Freeze-thaw then spalls the brick face, and at that point the repair is a mason’s job, not a roofer’s.
10Chimney cricket
- What it is
- A small peaked structure built on the up-slope side of a chimney to split the water and send it around both sides.
- How it fails
- It is missing. On a wide chimney the up-slope side becomes a shelf that collects water, leaves and ice.
- What ignoring it costs
- Standing water and debris behind a chimney is a permanent wet spot on the roof. It is also where ice builds first, and ice is heavy.
11Pipe boot
- What it is
- A metal flange with a rubber collar that seals around a plumbing vent stack.
- How it fails
- The rubber collar is the shortest-lived part of a roof. Sunlight hardens it, it cracks around the pipe, and it usually fails long before the shingles do.
- What ignoring it costs
- A cracked boot drops water straight down the vent stack into the wall or ceiling below. It is one of the cheapest repairs on a roof and one of the most common causes of interior damage, because nobody looks at it until there is a stain.
12Soffit and fascia
- What it is
- The fascia is the board the gutter hangs on. The soffit is the underside of the overhang, and vented soffit is the intake half of the attic ventilation system.
- How it fails
- Fascia rots from water coming off the roof edge behind the gutter. Soffit panels get pushed out by squirrels and birds, or painted over until the vents no longer breathe.
- What ignoring it costs
- A rotten fascia will not hold a gutter spike, so the gutter sags, so more water runs behind it. Blocked soffit intake disables the ridge vent at the same time.
13Valley
- What it is
- The line where two roof planes meet. It carries more water than any other part of the roof, because it drains both planes.
- How it fails
- Valleys fail from nails driven too close to the center line, from debris that dams up in the trough, and from a closed-cut valley where the cut is too tight to the center.
- What ignoring it costs
- A valley leak wets a large area of decking quickly, because the volume of water moving through it is high. Valleys and eaves are the two places ice and water shield is never optional.
14Box gutter
- What it is
- A gutter built into the cornice itself, lined rather than hung. Common on the older housing stock in neighborhoods like Hyde Park, Mt. Adams, Over-the-Rhine, Clifton, Norwood and Columbia-Tusculum.
- How it fails
- The lining fails at the seams and at the outlet. Because the trough is part of the structure, a failed lining drains into the cornice and the wall rather than over the edge.
- What ignoring it costs
- A leaking box gutter rots the framing it is built into. It is not a gutter replacement, it is a carpentry and lining job, and most volume roofers will decline it or quote it as a hung gutter, which is the wrong repair.
How to use this in a real conversation
- When somebody says the roof needs replacing, ask which of these components has failed and ask to see a photograph of it.
- When you are comparing written scopes, check whether each one names the ice and water coverage, the underlayment, the flashing plan and the nail count. A scope that only names the shingle is not comparable to one that names the assembly.
- When an adjuster writes a scope, check whether drip edge, starter, ice and water shield and ridge vent are on it. They are the line items most often missing from a first estimate.
- When a leak appears, start from where the water shows up and work back to the component: leaks are almost always a detail, not the field of the roof.
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SheetTool
Service areaGreater Cincinnati, Ohio and Northern Kentucky
Last reviewed11 September 2026