Parts of a Roof, Explained: Flashing, Valleys, Underlayment & More

Chimney flashing is the set of metal pieces that seals the gap where a masonry chimney passes through the roof. It is a system, not one part: an apron across the front, step flashing up both sides, a back pan or cricket on the uphill side, and counter flashing set into the brick over all of it.

PurposeKeeps rain out of the gap where the chimney passes through the roof, one of the most leak-prone openings on any roof.

Where it goes

Around all four sides of the chimney, where the brick or stucco meets the roof deck and shingles.

Materials

Copper, lead-coated copper, stainless steel and painted or galvanized steel. Aluminum is common elsewhere on a roof, but it is a poor choice where metal is set into mortar, because wet mortar is alkaline and corrodes it.

Why it matters

Brick, mortar and the roof move at different rates. Two-piece flashing lets them move without opening a gap, which is why caulk alone always fails at a chimney.

Code

The IRC requires flashing at every roof-and-wall intersection and roof opening (R903.2.1), and a cricket behind wide chimneys (R903.2.2).

How it’s installed

1. Shingles are run up to the front of the chimney and the apron (front base flashing) is set over them and bent up the face of the brick.
2. Step flashing is woven up each side, one piece per shingle course (see step flashing).
3. A back pan or a framed cricket is installed on the uphill side, tucked under the shingles above it so water runs over it, never behind it.
4. Counter flashing is let into the mortar joints and folded down over the top edges of all the base pieces (see counter flashing).
5. Corners are folded or soldered so there are no open seams at the four corners of the chimney.

A chimney cricket (also called a saddle) is a small peaked ridge built on the uphill side of a chimney. It splits the water, snow and leaves coming down the roof and sends them around both sides of the chimney instead of letting them pile up against the brick.

PurposeKeeps water, snow and leaves from piling up behind a wide chimney by sending them around both sides of it.

Where it goes

Behind the chimney, on the side that faces the ridge. Chimneys that sit right on the ridge do not need one.

When it’s required

IRC R903.2.2 requires a cricket or saddle on the ridge side of any chimney or penetration more than 30 inches wide, measured across the slope.

How it’s built

Usually framed in wood on the roof deck and sheathed, then covered with a waterproof membrane and either sheet metal or shingles. Small crickets can be formed entirely from metal.

Materials

Copper, painted or galvanized steel, or shingles over ice and water shield to match the roof.

How tall should a cricket be?

For masonry chimneys, IRC Table R1003.20 sets the cricket’s height as a share of the chimney’s width, based on how steep the roof is. The steeper the roof, the taller the cricket.

Source: IRC Table R1003.20, chimney crickets. Your town’s adopted code edition governs.

Counter flashing is the upper layer of metal that covers the top edge of base and step flashing. It is attached to the wall or chimney, and it laps down over the flashing below so water running down the masonry is shed onto the roof instead of getting behind it.

PurposeCovers the top edge of the base and step flashing so water running down a chimney or wall can’t get behind it, while letting the roof and the masonry move separately.

How it works

Base flashing is fastened to the roof; counter flashing is fastened to the wall. They overlap but are not fastened to each other, so the roof and the masonry can move separately without tearing the seal.

On brick or stone

The best practice is a reglet: a groove cut into a mortar joint. The top edge of the metal is bent into the groove, wedged and sealed. On the sloped sides it is stepped to follow the mortar courses.

Surface-mounted

Some counter flashing is fastened to the face of the masonry with a termination bar and sealant. It works, but it depends on sealant and needs more upkeep than a reglet.

On siding walls

On a framed wall, the house wrap and siding do the job of counter flashing: they lap over the vertical leg of the step flashing (IRC R905.2.8.3).

Step flashing is a series of small L-shaped metal pieces installed one per shingle course where a sloped roof runs along a vertical wall or the side of a chimney. Each piece laps over the one below it, so water is always shed onto the next shingle down.

PurposeSeals the joint where a roof runs along a wall, one shingle course at a time, so water is always shed back onto the roof.

Where it goes

Along every place where a roof slope runs beside a wall: dormer sides, a lower roof against a two-story wall, and both sides of a chimney.

Code minimum

IRC R905.2.8.3: base flashing against a vertical sidewall must be continuous or step flashing, at least 4 inches high and 4 inches wide, and must direct water onto the roof or into the gutter.

Materials

Prefinished or mill aluminum, galvanized or painted steel, and copper. Copper is used against masonry and on copper-detailed roofs.

Partner part

Step flashing always needs a cover: counter flashing on masonry, or house wrap and siding on a framed wall.

How it’s installed

1. Each piece is set just above the shingle it sits on, with one leg on the roof and one leg up the wall.
2. The next shingle covers the roof leg, and the next piece of step flashing laps over the one below.
3. Pieces are nailed to the roof deck only, high on the roof leg, never to the wall, so the roof and wall can move independently.
4. The house wrap and siding (or the counter flashing on masonry) come down over the wall leg.
5. The first piece at the bottom of the wall is a kick-out, so the water collected by the step flashing is turned into the gutter.

Headwall flashing (also called apron flashing) is a continuous strip of metal that seals the joint where the top edge of a roof meets a wall, such as where a porch roof, bay window roof or lower roof runs into the house wall above it.

PurposeSeals the joint where the top of a roof meets a wall, so water running down the wall lands on top of the roof instead of behind it.

Where it goes

Along the horizontal line where the top of a roof slope meets a wall: porch roofs, bay windows, shed dormers and the front of a chimney.

How it’s shaped

One leg lies over the top course of shingles; the other leg runs up the wall, behind the house wrap and siding or under counter flashing. The bottom edge is often hemmed for stiffness.

Materials

Aluminum, painted or galvanized steel, or copper.

Ends

Where the headwall meets a sidewall, the flashing turns the corner and ties into the step flashing and kick-out.

Kick-out flashing (also called a diverter) is a small angled piece of metal at the bottom of a roof-to-wall joint. It turns the water running down the step flashing out and away from the wall, into the gutter, instead of letting it pour behind the siding.

PurposeTurns the water collected along a roof-to-wall joint into the gutter, so it can’t pour behind the siding or stucco.

Where it goes

At every spot where a roof edge stops against a wall that keeps going down past it, such as the bottom of a dormer side or where a garage roof meets a two-story wall.

Code

IRC R903.2.1 requires flashing that diverts water away where the eave of a sloped roof meets a vertical sidewall. Metal flashing must be corrosion resistant and at least 0.019 inch thick (No. 26 galvanized sheet).

Materials

Pre-formed aluminum or plastic kick-outs, or soldered copper on copper-flashed roofs.

Why it matters

It is a small part, but missing kick-outs are one of the details most often linked to hidden rot behind siding and stucco right below a roof edge.

How it’s installed

1. The kick-out is installed first, at the eave, before the first piece of step flashing.
2. Its wall leg goes behind the house wrap; the step flashing laps over it.
3. The angled “kick” sits over the gutter so the water lands in the gutter, not on the wall.

A pipe boot is the flashing that seals around a plumbing vent pipe where it comes up through the roof. Its flat base is woven into the shingles and its collar fits snugly around the pipe.

PurposeKeeps rain from running down the outside of a plumbing vent pipe into the attic, while the pipe vents sewer gases out of the house and lets the drains flow.

Rubber (neoprene/EPDM)

The most common type: a rubber collar on a metal or plastic base. The rubber is exposed to sun and weather and often cracks or splits years before the shingles wear out.

Lead

A soft lead sleeve that is folded down into the top of the pipe. Long-lasting, but it can be chewed by squirrels.

Copper

A formed and soldered copper sleeve and base. Very long-lasting and a natural match for copper flashing details.

Where it goes

Every plumbing vent stack on the roof. Exhaust vents and other penetrations get their own matched flashing.

How it’s installed

1. The lower half of the base sits on top of the shingles below the pipe.
2. The upper half of the base and the sides go under the shingles above, so water runs over the flashing.
3. Nails are placed where the next course of shingles covers them, and the collar is seated tight around the pipe.

Skylight flashing is the matched kit of sill, side (step) and head flashing pieces, made by the skylight manufacturer, that seals a skylight frame into the roof.

PurposeSeals the skylight frame into the roof so rain runs around the skylight instead of into the opening.

Why a kit

Each skylight model is designed with its own flashing kit, and the right kit depends on the roofing material: low-profile shingles, thicker tile or shake, or metal.

Under the kit

A waterproof membrane collar around the opening is the first line of defense if water ever gets past the metal.

Cricket rule

Unit skylights installed and flashed to the manufacturer’s instructions are the one exception to the IRC’s 30-inch cricket rule (R903.2.2).

Lifespan tip

When a roof is replaced, the skylight’s flashing should be replaced with it. Old flashing rarely fits a new roof correctly.

How it’s installed

1. A waterproof membrane is installed around the opening before the skylight is set.
2. The sill (bottom) flashing goes on first, over the shingles below.
3. Step flashing pieces run up each side with the shingle courses.
4. The head flashing goes on last and is tucked under the shingles above.

A roof valley is the inward-angled seam where two roof slopes meet. Because both slopes drain into it, a valley carries more water than any other part of the roof, which is why it gets extra protection underneath and, on many roofs, a metal lining on top.

PurposeCarries the water from two roof slopes down to the gutter without letting it get under the roofing.

Under every valley

A self-adhering waterproof membrane runs down the full length of the valley before anything else goes on.

Open valley (code)

IRC R905.2.8.2: an open valley lined with metal must be at least 24 inches wide and made of a corrosion-resistant metal.

Closed valley (code)

A closed valley can be lined with one ply of smooth roll roofing at least 36 inches wide, or with a self-adhering polymer-modified bitumen membrane.

Materials

Copper, aluminum, galvanized steel or prefinished steel for open valleys. Slate and cedar roofs are traditionally detailed with copper.

The four valley types

Why pitch and length decide the valley

There is no single right valley for every roof. Two things matter most:

• Pitch. A valley always runs flatter than the roof around it. Where two equal slopes meet at a right angle, the valley’s pitch is about 70% of the roof’s (a 6:12 roof has a valley of roughly 4¼:12), so water slows down there. Some valley methods also have slope limits; GAF, for example, accepts its closed-cut “California” valley only on slopes of 4:12 or more.
• Length. A long valley gathers water from more roof, so far more water reaches the bottom than runs past the top. That’s why open valleys are laid out wider at the eave than at the ridge: one common layout starts 6 inches wide at the top and widens at least 1/8 inch per foot of valley toward the eave.

Minimum metal thickness for open valleys

The IRC lists the corrosion-resistant metals allowed for open valley linings in Table R905.2.8.2. The three used most on homes:

• Copper: 16 ounces per square foot
• Aluminum: 0.024 inch
• Galvanized steel: 0.0179 inch (26 gauge)

Roof decking (also called sheathing) is the layer of plywood, OSB or wood boards fastened to the rafters. Every other layer of the roof is nailed into it, so a roof is only as solid as its deck.

PurposeGives every other layer of the roof a solid surface to nail into and carries the roof’s weight to the rafters.

Materials

Plywood or OSB panels on most homes. Older homes often have solid wood plank decking, and some cedar roofs are laid on spaced boards.

What we look for

Rot, soft spots, delamination, sagging between rafters and gaps between old boards that are too wide to nail shingles into.

Why it matters

Shingle and membrane warranties assume a sound, dry deck. Nails driven into soft wood don’t hold.

Drip edge is an L- or T-shaped metal strip installed along the eaves and rakes (the bottom and sloped side edges) of a roof. It guides water off the roof edge and into the gutter, so it can’t curl back under the shingles and rot the fascia and the edge of the deck.

PurposeGuides water off the roof edge into the gutter and protects the fascia and the edge of the deck from rot.

Where it goes

Along every eave and rake of the roof, at the very edge of the deck, lapping over the top of the fascia board.

Code

IRC R905.2.8.5 requires drip edge at the eaves and rakes of shingle roofs.

Profiles

Type C (L-style): a simple 90-degree angle. Type D (T-style, “D-metal”): a T-shaped profile that holds water farther from the fascia; the Asphalt Roofing Manufacturers Association prefers it to Type C. Type F (gutter apron): a longer leading edge, useful over existing shingles and on rakes.

Materials

Prefinished or painted aluminum, galvanized or painted steel, and copper. Vinyl and plastic versions are meant for non-roofing uses, such as over doors and windows.

Laps & reach

Pieces overlap at least 2 inches; the metal extends at least 1/4 inch below the roof sheathing and at least 2 inches back onto the deck, nailed no more than 12 inches apart.

Layering at the eaves

The drip edge goes on first, and the ice and water shield or underlayment laps over it, so water running down the roof stays on top of the metal.

Layering at the rakes

The underlayment goes on first, and the drip edge goes over it, so wind-driven rain can’t get under the underlayment at the side edges.

How the drip edge and gutter apron hand the water off to the gutter is covered in our gutter guide: gutter apron and drip edge.

A fascia board is the long, flat board that runs along the roof edge across the ends of the rafters. It closes off the roof edge, carries the gutters, and gives the drip edge something to lap over.

PurposeCloses off the ends of the rafters, holds the gutters and backs the drip edge along the roof edge.

Where it is

Along the eaves at the bottom edge of the roof, just below the drip edge and in front of the soffit. The sloped gable ends have a matching rake board.

Works with

The drip edge laps over the top of the fascia so water lands in the gutter, the gutter hangers fasten through it, and the soffit meets it underneath.

Materials

Wood (often pine, fir or cedar), cellular PVC or composite trim board, fiber cement trim, and aluminum fascia wrap (also called capping): aluminum coil stock bent on a brake and fitted over the wood.

Signs of trouble

Peeling paint, soft or stained wood behind the gutters, gutters pulling away from the house, and gaps where birds or squirrels get into the eaves.

The fascia carries the whole gutter system. See how gutters are hung on it in our gutter guide: gutter hangers.

How it’s installed

1. The rafter tails are checked so the fascia can run in a straight, even line.
2. The fascia board is fastened across the ends of the rafters.
3. If the fascia is wrapped, the aluminum is formed to fit and fastened over sound wood, with joints lapped so they shed water.
4. The drip edge goes on along the roof edge, lapping over the top of the fascia, and the gutters are hung on it.

Ice and water shield is a self-adhering, rubberized-asphalt membrane applied directly to the roof deck in the areas most likely to leak. It sticks to the wood, seals around every nail driven through it, and stops water that backs up under the shingles from ice dams or wind-driven rain.

PurposeSeals the deck where water is most likely to back up or collect, so ice dams and wind-driven rain can’t reach the wood.

Where it goes

Eaves, valleys, around chimneys, skylights and pipes, along roof-to-wall joints and on low-slope sections.

Code (eaves)

Where the local code calls for an ice barrier, IRC R905.1.2 requires it to run from the roof edge to at least 24 inches inside the exterior wall line of the house.

Roll size & laps

Rolls are typically 36 inches wide. Grace Ice & Water Shield, for example, calls for side laps of at least 3.5 inches and end laps of at least 6 inches, always shingled so water runs over each lap.

Temperature

Standard membranes are installed in fair weather (Grace specifies 40°F and above). Metal and copper roofs need a high-temperature version (see below).

Why 24 inches often means two rows

The 24 inches is measured level, inside the warm wall, not up the slope. Add the roof overhang and the pitch, and on many homes the membrane has to run well past one 36-inch-wide roll. That’s why you often see two rows of ice and water shield at the eaves.

“Ice and water shield” is a brand name that stuck. Manufacturers also call these products leak barriers or self-adhered underlayments; GAF’s versions, for example, are sold as WeatherWatch and StormGuard leak barriers.

Roof underlayment is the water-resistant sheet that covers the entire roof deck under the shingles. It is the backup layer: if wind-driven rain or melting snow gets past the shingles, the underlayment sheds it down and off the roof.

PurposeAdds a second water-shedding layer under the shingles across the whole roof deck.

Felt paper

Asphalt-saturated felt, sold as No. 15 (ASTM D226 Type I) and the heavier No. 30 (ASTM D226 Type II), in 36-inch-wide rolls. The traditional underlayment, and still standard under slate and cedar.

Synthetic

Woven or spun polypropylene or polyester sheets. Lighter and far more tear-resistant than felt, they lie flatter and are safer to walk on. Laps and fastening follow the manufacturer’s printed lines.

Code laps (felt)

On shingle roofs sloped 4:12 or steeper, one layer, laid shingle-fashion from the eave, lapped 2 inches horizontally and 4 inches at the ends, with end laps staggered 6 feet apart.

Low slopes

From 2:12 up to 4:12, the IRC calls for two layers: a 19-inch starter strip, then 36-inch sheets each lapped 19 inches over the one below.

When felt paper is installed

Underlayment goes on right after the drip edge at the eaves and the ice and water shield, starting at the bottom of the roof and working up, so every sheet laps over the one below it. It overlaps the top edge of the ice and water shield and runs up to the ridge, then the drip edge is installed over it along the rakes and the shingles go on.

A starter strip is the first course of adhesive-backed shingles installed along the eaves and rakes before the main shingles. It gives the first row something to seal to and covers the joints and cutouts in that row, so wind can’t lift the roof edge.

PurposeSeals the roof edges so wind can’t lift the first row of shingles, and covers the gaps in that row.

Where it goes

Along the bottom edge (eaves) and the sloped side edges (rakes) of every roof plane.

Why it matters

Most shingle blow-offs start at the edges. A sealed starter course is what keeps the perimeter locked down in high wind.

Material

Pre-made starter shingles from the shingle manufacturer, with a factory adhesive strip placed where the first course needs it.

Ridge cap shingles are the thicker, pre-bent shingles that cover the peak of the roof (the ridge) and the outside corners where two slopes meet (the hips). They seal the highest, most wind-exposed points of the roof and finish its lines.

PurposeSeals and finishes the peaks and hips, the most wind-exposed points of the roof.

Where it goes

Along every ridge and hip, and over the ridge vent where there is one.

Material

Matching cap shingles made by the shingle manufacturer, sized and creased for ridges. Slate, tile and metal roofs have their own ridge pieces.

Why not cut shingles?

Cut-up field shingles are thinner and can crack when bent over the ridge. Purpose-made caps are designed for that spot.

Roof ventilation is the balanced intake-and-exhaust system that lets outside air enter low at the soffits and leave high at the ridge. It keeps the attic close to the outdoor temperature and dry, which protects the shingles, the deck and the house below.

PurposeKeeps the attic dry and close to the outdoor temperature, which protects the shingles and deck and helps prevent ice dams.

Intake

Vented soffits or continuous strip vents under the roof overhang let fresh air in at the bottom. Baffles keep insulation from blocking them.

Exhaust

A ridge vent is a continuous vent along the peak, covered by ridge cap shingles over a slot cut in the deck. Box vents and gable vents are other exhaust types.

Code baseline

IRC R806.2 sets a minimum net free vent area of 1 square foot for every 150 square feet of attic, with reductions allowed when vents are balanced between high and low.

Golden rule

Use one type of exhaust per attic. Mixing a ridge vent with box vents or a power fan can short-circuit the airflow.