What Is Valley Flashing on a Roof?

What a roof valley is, and what the flashing does

Any roof that is more than a simple box has at least one valley.

When an L-shaped house, a dormer, a garage or a porch roof joins the main roof, the two slopes meet in a V-shaped inside corner. That corner is the valley.

Its opposite is a hip, which is an outside corner that sheds water away to each side.

Because both slopes drain into it, a valley carries far more water than the same length of open roof. It is also a little flatter than the slopes around it, so water moves more slowly there.

A valley lining is the extra layer that makes this channel watertight. The code calls it a valley lining; roofers call it valley flashing, valley metal or the valley liner. All three names mean the layer laid in the valley before the shingles ¹.

This page goes deep on valleys only. For flashing at walls, chimneys, pipes and skylights, see our guide to roof flashing.

How a valley is built, layer by layer

A valley is built from the deck up, and every layer laps the one below it in the direction water flows. The roofer first sweeps the valley clean and sets any nail heads that stick up, so nothing punctures the layers above ⁴.

Next comes underlayment centered in the valley. The roofing contractors' association, NRCA, describes a 36-inch-wide sheet of asphalt felt or polymer-modified bitumen centered in the valley before the valley is built ².

Many crews now use a full-width self-adhered membrane here, which seals around nails. The code allows that membrane, if it meets ASTM D1970, in place of the lining material on a shingle roof ¹.

Then the valley lining goes in: metal for an open valley, or roll roofing or membrane for a closed one. Only after that do the shingles go on. The code is plain about the order: "Valley linings shall be installed in accordance with the manufacturer's instructions before applying shingles" ¹.

The last rule is about nails. Water runs down the center line, so that is where a nail hole would leak. NRCA advises keeping fasteners at least 8 inches from a valley's center ². Shingle makers set their own limits for their product, and those govern the warranty.

Every layer laps the one below in the direction water runs, so the order in the diagram is the order the crew builds it.

The three ways to shingle a valley

There are three basic valley methods for asphalt shingle roofs: open, closed-cut and woven ². A fourth, sometimes called a Long Island valley, uses a row of shingles laid on edge and only suits some laminated shingles, with maker approval varying ⁴.

Open, closed-cut and woven valleys in more detail

Open valley. The metal stays in view, and the shingles stop short of the center on both sides, leaving a clear channel for water, ice and debris.

NRCA suggests valley metal with a center rib at least 1 inch high, in pieces no longer than 10 feet ³.

The rib stops water from one slope rushing across and up under the shingles on the other slope.

Fine Homebuilding notes open valleys are the most decorative and pair well with heavy laminated shingles, but take prefabricated metal and careful sealing at the shingle edges ⁴.

Closed-cut valley. The roofer shingles one slope first, running each shingle at least 12 inches onto the other slope.

Then the second slope is shingled over it and each shingle is cut along a line about 2 inches from the center ².

NRCA's 2013 article adds that the ends are clipped at the top corner and set in roof cement ³.

The clip steers water back into the valley instead of letting it run sideways under the cut edge. NRCA says to start on the slope with the shortest run to the ridge ².

Woven valley. Shingles from both slopes overlap each course, like a basket weave. Fine Homebuilding rates it the most weather-resistant pattern, but slow, because both slopes must go on at once ⁴.

NRCA's 2013 article says woven valleys collect more debris, snow and ice and that NRCA does not recommend them ³. They also do not suit thick laminated shingles, which will not bend tight into the valley ².

Example: one shingle maker's open valley

Here is how IKO's printed instructions for its Cambridge laminated shingle describe an open metal valley. They are one maker's rules for one product, shown to make the steps concrete ⁵:

A valley 20 feet long would be 6 inches wide at the top and 6 plus 20 × 1/8 = 8.5 inches wide at the bottom.

The widening is there because the valley carries more water the lower it goes. Other shingle makers set different numbers, so the bundle wrapper for your shingle is the rule that counts.

Where you find valleys on a house

You will find a valley anywhere two roof planes meet in an inside corner: where the wings of an L- or T-shaped house join, on both sides of a dormer, where a porch or garage roof ties into the main roof, and where a cross gable sits on a long roof.

A roof with many dormers and wings can have several hundred feet of valley.

Valleys also form in small places. A cricket, the little peaked roof built behind a chimney, creates two short valleys.

NRCA notes that short, shallow valleys at crickets and small dormers are among the few places some roofers still weave shingles ³.

Wherever the valley is, its bottom end dumps a lot of water onto the eave, which is why the gutter below a valley often overflows first.

What a well-built valley does for you

• Keeps the busiest drain on the roof dry. The lining is a second barrier under the spot that sees the most water.
• Handles ice and snow. In cold climates, snow slides into valleys and melts slowly. A membrane under the lining seals around nails if water backs up ².
• Lets debris wash out. An open valley gives leaves and grit a clear path off the roof ³.
• Protects the deck. A valley leak runs down the inside corner and can rot the sheathing and the valley rafter before it shows on a ceiling.
• Can look sharp. Copper or painted open valleys are a style choice as well as a drain ⁴.

Trade-offs of each valley method

What drives the cost of valley work

• Total valley length. Roofers measure valleys in linear feet. A roof with several dormers has much more than a simple L.
• Method. An open valley needs bent metal and more trim work. A closed-cut valley uses shingles and a liner. A woven valley takes the most labor time ².
• Metal type. The code allows aluminum, copper, galvanized steel, lead, lead-coated copper, painted terne, stainless steel and zinc alloy ¹. Copper and stainless cost more than painted steel.
• Membrane. A full-width self-adhered membrane in the valley costs more than felt but adds a seal around nails.
• Slope and height. Steep or tall roofs need more safety setup and slow the work.
• Deck repair. Old valley leaks often leave soft sheathing that must be replaced once the roof is open.
• For a whole-roof figure, use our roof cost calculator or read the roofing cost guide. We do not publish valley prices without sourced, dated quotes.

How a roofer redoes a valley during a reroof

1. Tear off the old shingles and the old valley lining down to the deck.
2. Check the deck along the valley for soft or stained wood and replace damaged sheathing.
3. Sweep the valley and set any raised nail heads ⁴.
4. Center underlayment or self-adhered membrane in the valley, at least 36 inches wide ².
5. Install the valley lining the shingle maker calls for: metal for an open valley, roll roofing or membrane for a closed one ¹.
6. Shingle the valley by the chosen method, keeping nails away from the center line ².
7. Clip the top corners and set the cut ends in roof cement where the instructions require it ^{3 5}.
8. Check the bottom of the valley where it meets the eave and gutter, and clear all scraps out of the channel.

This is work for a licensed roofing contractor. Valleys sit on sloped inside corners where footing is poor, and OSHA requires fall protection for roofing work at heights.

Looking after your valleys

• Keep valleys clear of leaves, pine needles and branches. Debris holds water against the shingles and dams the flow ³.
• Look from the ground, or with binoculars, after storms for lifted shingles, cracked cut edges or bent metal.
• In winter, watch for ice building up at the bottom of valleys over the eave.
• Keep the gutter under each valley clean, because it takes the heaviest flow.
• Do not walk in valleys or scrape them clean yourself; let a roofer do it from proper footing.

Why valleys leak

NRCA lists the forces that work on a valley: debris from trees and wind, snow and ice, heavy rain, swings in temperature, wind and strong sun ³. On top of those, most valley leaks trace back to a few installation mistakes.

Nails too close to the center. A nail driven near the center line sits right where water runs. NRCA's guide is at least 8 inches from the center ².

Cut ends not clipped or sealed. On a closed-cut or open valley, an unclipped top corner lets water run sideways along the shingle's top edge and under it ^{3 5}.

Wrong method for the shingle. Weaving thick laminated shingles leaves bridges and humps that trap water ².

Missing or narrow lining. A valley built with no liner, or one narrower than the code allows, has nothing behind the shingles when water gets past them ¹.

Rusted or worn metal. Thin or uncoated metal can corrode, which is why the code sets minimum thickness and coating for each metal ¹.

Ice and backed-up water. In cold areas, ice at the bottom of a valley can push water uphill under the shingles ². Our guide to ice and water shield explains the membrane that guards against it.

What the 2021 IRC says about valleys (R905.2.8.2)

Section R905.2.8.2 of the 2021 International Residential Code, as adopted in Maryland, reads: "Valley linings shall be installed in accordance with the manufacturer's instructions before applying shingles. Valley linings of the following types shall be permitted" ¹. The three permitted types, quoted:

Sources: [1]

Table R905.2.8.2: valley lining metals

The minimum thickness, gage or weight for each metal allowed in an open metal valley, from the 2021 IRC ¹:

Valley rules for other roof coverings

Other roof types have their own valley rules in the same chapter of the 2021 IRC ¹:

States and cities adopt the IRC on their own schedule and can amend it, so check which edition your building department enforces. Panel metal roofs follow the panel maker's valley details; see metal roofing.

Valley flashing compared with related roof parts

Common misconceptions

Myth A closed valley has no flashing.

Reality It does. The lining is just covered by shingles. The code requires a lining for closed valleys too ¹.

Myth Woven valleys work with any shingle.

Reality NRCA limits weaving to strip shingles on slopes of at least 4 in 12 and advises against it with laminated shingles ².

Myth More roof cement fixes a leaking valley.

Reality Cement is a small part of the detail. A leaking valley usually needs the lining, nailing or cut edges redone.

Myth Any metal will do in an open valley.

Reality The code lists the allowed metals with a minimum thickness for each ¹, and the shingle maker may set its own minimum ⁵.

Myth Valley rules are the same for every roof.

Reality Tile, slate, wood and metal shingles each have their own valley widths in the code ¹.

How location changes valley choices

The valley rules quoted here come from the 2021 IRC as adopted in Maryland. Your state or city may use a different edition or local amendments, so your building department and permit set the rule where you live ¹.

Climate matters too. In snowy places, valleys fill with snow that melts slowly and can freeze at the eave. NRCA advises self-adhering membrane or tapered metal in valleys on lower slopes where snow and ice build up ².

In wooded areas, open valleys clear leaves better ³. On low-slope roofs, NRCA points to open valleys ³. Use our roof pitch calculator to find your slope, or read what roof pitch means.

What to check on a quote

• Which valley method will you use, open, closed-cut or woven, and does the shingle maker approve it for my shingle?
• What lining goes in the valley, and how wide is it? Does it meet R905.2.8.2 ¹?
• Is self-adhered membrane going full width in every valley, under the lining?
• If open: what metal, what thickness, and does it have a center rib ³?
• Will the old valley metal be torn off and replaced, or reused?
• How do you handle soft deck boards you find along the valleys?
• Is the work covered by the shingle maker's warranty with this valley method ⁵?

When valley flashing matters most

Valleys matter on every reroof, but they deserve extra attention if you have a stain on a ceiling or wall under an inside corner of the roof, a roof with several dormers or wings, heavy trees nearby, or a cold climate with snow.

They also matter when you switch to heavy architectural shingles, since the valley method may need to change. See architectural shingles.

If you suspect a valley leak, start with a roof inspection. A single valley can often be rebuilt as a repair, but if the shingles are near the end of their life, redoing valleys during a full reroofing job is usually the cleaner fix.

Keep going

• Roof repairFixing a leaking valley
• Roof replacementNew valleys as part of a full reroof
• Roof underlaymentThe layer under the valley lining
• Asphalt shinglesThe covering most valleys sit under

Related terms

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.

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.

Roof Underlayment

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

Step Flashing

Step flashing is a set of small, L-shaped pieces of metal, one for each row of shingles, woven into the roof where a sloped roof runs up beside a wall, a dormer or a chimney side, so water on the roof is sent back onto the shingles and away from the wall.

Architectural Shingles

Architectural shingles are asphalt roof shingles made from two or more layers bonded together, so each shingle is thicker than a flat strip shingle and gives the roof a textured, three-dimensional look.

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.

Roof Pitch

Roof pitch is the steepness of a roof, written as the inches it rises for every 12 inches it runs horizontally, so a 6/12 roof climbs 6 inches per foot.

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: Slate Roofing, Clay Tile Roofing, Wood Shakes, Roof Underlayment, Ice and Water Shield, Roof Flashing, Chimney Flashing