Roof Valley Flashing Details: Open, Woven, and Closed-Cut Methods Compared

By The Roofing Brief Team. Last reviewed: October 2026.

Roof valley flashing details come down to three competing flashing methods over a code valley lining: open-metal valleys (a metal lining at least 24 inches wide under IRC R905.2.8.2, exposed in the center with shingles trimmed back on each side), woven valleys (field shingles interlaced from both planes across the valley), and closed-cut valleys (shingles from one plane run across, shingles from the other plane cut along a chalk line 2 inches off center). All three are permitted under IRC R905.2.8.2 when the shingle maker allows them. They differ in what the maker’s warranty accepts, how much water the channel carries, and where they fail. Open-metal handles the most water and is what IKO recommends for Cambridge shingles, woven is fastest but GAF does not allow it with laminated shingles, and closed-cut is what CertainTeed prefers for Landmark.

The short answer: pick the method, then build the stack

Valleys are where two roof planes meet at an internal angle, as on an L-shaped gable or a hip roof with intersecting wings, and concentrate water flow. A valley collects runoff from both planes, so a valley draining 200 sq ft of roof on each side in a 2 inch per hour rain carries about 500 gallons per hour at the eave (worked below). That concentrated flow drives every failure mode in valley design: granule erosion, ice damming, wind-driven water under shingles, and shingle mat fatigue at the fold line. The three roofing valley techniques (open-metal, woven, closed-cut) each address that concentrated flow differently. The underlayment stack below the visible surface is what actually keeps water out of the deck, and the 2021 IRC sets its minimums in R905.2.8.2.

The three valley methods compared

Method	What it looks like	Lining (maker spec)	Cut and cement (maker spec)	Maker acceptance	Best for
Open-metal	Exposed metal channel, 6 in wide at the top on IKO’s spec	36 in ice and water protector, then 24 in min metal liner (IKO Cambridge; IRC R905.2.8.2)	Trim to chalk line, 2 in corner triangle, 3 in band of plastic cement (IKO)	IKO recommends it for Cambridge	Heavy snow, long valleys, slate or designer shingle
Woven	Shingles interlaced, no visible metal	36 in liner per maker	No cutting	Not with GAF laminated shingles; accepted by OC Duration and CertainTeed Landmark	3-tab installs and shingles whose maker allows it
Closed-cut	Single straight cut line, no visible metal	36 in self-adhered liner or 36 in min 50 lb smooth roll roofing (OC Duration)	Cut 2 in off center; 3 to 4 in cement bead (OC) or 2 in strip (CertainTeed)	Preferred by CertainTeed for Landmark	Architectural shingles where open metal is not wanted

Maker specs are from the Owens Corning Duration instructions, CertainTeed’s Landmark instructions and IKO’s Cambridge instructions (01/21). The shingle on your roof controls; the warranty can depend on it.

Open-metal valleys

Open-metal is the original residential valley method and still the standard on premium and commercial installs. A pre-formed valley lining runs the length of the valley and channels water to the eave; field shingles are trimmed back to a chalk line so a strip of metal stays exposed in the center. The 2021 IRC requires an open metal valley lining to be at least 24 inches wide and made of one of the corrosion-resistant metals in Table R905.2.8.2, and IKO’s Cambridge instructions call for a minimum 24 inch wide, minimum 28 gauge pre-finished or galvanized liner (the IRC table’s galvanized minimum is 26 gauge, so the code is the stricter rule for galvanized steel).

The pre-formed valley metal options

Valley lining metal	IRC minimum thickness or weight (Table R905.2.8.2)
Galvanized steel	0.0179 in, 26 gauge, zinc coated G90
Aluminum	0.024 in
Cold-rolled copper	0.0216 in nominal, ASTM B370, 16 oz per sq ft
High-yield copper	0.0162 in nominal, ASTM B370, 12 oz per sq ft
Lead-coated copper	0.0216 in nominal, ASTM B101, 16 oz per sq ft
Stainless steel	28 gauge
Zinc alloy	0.027 in
Lead	2.5 lb
Painted terne	20 lb

• Painted or galvanized steel. The mainstream residential option; the IRC floor is 0.0179 in (26 gauge G90).
• Galvalume steel. 55% aluminum-zinc coated steel; for metal roof panels the IRC’s corrosion table (R905.10.3(2)) lists ASTM A792 AZ50 as the minimum.
• 16 oz copper. The historical standard, at the IRC’s 16 oz per sq ft cold-rolled minimum. Develops a patina that homeowners either love or hate.
• Aluminum. Lightweight; the IRC floor is 0.024 in. Less rigid than steel at the same thickness and can ripple between fasteners on long valleys.

Open-metal install sequence

1. Strip the valley clear and center a 36 inch wide strip of ice and water shield in the valley, full length, fastening at the edges with only enough nails to hold it (IKO Cambridge step 7).
2. Run a second course of underlayment (synthetic or 30 lb felt) over the ice and water shield where the shingle maker calls for it.
3. Center the pre-formed valley metal, at least 24 inches wide per IRC R905.2.8.2, on the valley line. Overlap splices with sealant under the lap, upper piece over lower.
4. Fasten the metal at the edges only, with only enough nails to hold it in place per IKO, or with cleats along the edges. Do not nail through the metal field where water runs.
5. Snap two chalk lines the full length of the valley, 6 inches apart at the top and widening 1/8 inch per foot toward the bottom (IKO). This is where the shingles will be trimmed.
6. Run field shingles across the valley toward the chalk line. Trim each shingle along the chalk line with a utility knife at the time of install, and keep nails at least 2 inches back from the chalk line (IKO).
7. Bed the cut shingle end in asphalt plastic roof cement (ASTM D4586). IKO’s Cambridge instructions and the Owens Corning Roofer’s Guide specify a 3 inch wide band for open valleys. Without it, wind-driven water can track under the cut edge and reach the underlayment.
8. Clip the upper corner of each cut shingle at a 45-degree angle to deflect water back toward the center of the valley. The clip size varies by maker; see the next section.

IKO’s taper means the exposed channel grows with valley length: a 16 ft valley runs from 6 inches at the top to 8 inches at the eave (6 + 16 x 1/8), and a 24 ft valley to 9 inches. The widening gives the bottom of the valley, which carries water from the whole run, the widest channel. On low-slope roofs, note that IRC R905.2.2 allows asphalt shingles only at 2:12 or steeper and requires double underlayment from 2:12 up to 4:12.

The clipped-corner detail

Shingle makers call for clipping the upper corner of each shingle along the valley cut at about a 45-degree angle, but they do not agree on the size. CertainTeed’s Landmark instructions say to cut 2 inches diagonally off the upper corner of each trimmed shingle. IKO’s Cambridge instructions call for a 2 inch triangle off the corner on open metal valleys. The Owens Corning Roofer’s Guide to Roof Installation says to clip 1 inch from the upper portion of the shingle on a 45 degree angle, and GAF’s Pro Field Guide (RESGN103) says to clip shingle corners without stating a size. Use the dimension in the instructions for the shingle you are installing. The clip diverts water that tracks down the cut shingle edge back toward the center of the valley instead of letting it run along the cut line and back under the shingles above.

Woven valleys

Woven valleys are the fastest method. Field shingles from both roof planes are interlaced across the valley: a shingle from the left plane, then a shingle from the right plane, then a shingle from the left plane, weaving up the valley. No metal is visible and no cutting is required; Owens Corning’s Duration instructions still call for the 36 inch valley liner underneath.

Whether woven valleys are allowed with architectural shingle roofs depends on the maker. GAF’s Pro Field Guide (RESGN103) says not to use woven valleys for Timberline or any laminated shingle, because laminated shingles are too thick to conform to the valley center and can “bridge” above it, where wind-driven rain can get underneath and the shingle can blow off. GAF limits woven valleys to Slateline and its 3-tab Royal Sovereign and Marquis WeatherMax shingles. Owens Corning’s Duration instructions (Pub. 10014283-L, July 2026) list woven and metal valleys as acceptable alternatives to closed-cut, and CertainTeed’s Landmark instructions prefer closed-cut but also accept open and woven valleys.

When woven still makes sense

• 3-tab field shingles such as GAF Royal Sovereign and Marquis WeatherMax, which GAF permits woven
• GAF Slateline, the one laminated-look GAF line its field guide allows woven
• Owens Corning Duration and CertainTeed Landmark, whose instructions accept woven valleys
• Budget reroofs where skipping the valley metal and the cut line saves labor, if the maker allows it

Why woven is falling out of favor

• GAF does not allow woven valleys with Timberline or other laminated shingles, and woven valleys do not qualify for its Golden Pledge and Silver Pledge warranties except with Slateline, Marquis WeatherMax or Royal Sovereign, per the RESGN103 Pro Field Guide
• Thick laminated shingles can bridge across the valley instead of sitting flush, which GAF links to wind-driven leaks and blow-offs
• Replacing a woven valley requires tearing into both roof planes, not just one

Closed-cut valleys

Closed-cut is the method CertainTeed prefers for Landmark and the first one Owens Corning describes for Duration. The sequence: shingles from one roof plane run uncut across the valley center and at least 12 inches onto the other plane; shingles from the other plane stop 2 inches short of the valley center along a chalk line. No metal is visible. No shingle is folded across the valley center.

Which plane gets the uncut shingles

The makers that address this put the smaller roof plane underneath. GAF’s Pro Field Guide (RESGN103) says to install the larger roof plane over the smaller one, so the larger water flow runs over the valley cut. CertainTeed’s Landmark instructions likewise lay shingles on the smaller roof area across the valley first, at least 12 inches onto the adjoining plane. Owens Corning’s Duration instructions say to lay all shingles on one side across the centerline by at least 12 inches without naming which side, and to weave the first course on both sides.

Closed-cut install sequence

1. Install a 36 inch wide liner centered on the valley line, full length: self-adhered underlayment, or 36 inch minimum 50 lb smooth roll roofing to ASTM D3909 or D6380 (OC Duration).
2. Install the field underlayment over the deck and lap it onto the liner as the shingle maker directs.
3. Run shingles on the under plane across the valley, extending at least 12 inches onto the opposite plane. Do not nail within 6 inches of the valley centerline (OC and CertainTeed).
4. Snap a chalk line 2 inches off the valley center on the cut plane side.
5. Run shingles on the cut plane up to the chalk line, trimming each as installed without cutting the shingles underneath.
6. Clip the upper outer corner of each cut shingle at a 45-degree angle to deflect water back to center (2 inches on CertainTeed Landmark).
7. Bed the cut edge in roof cement: a 3 to 4 inch continuous bead per Owens Corning, or a 2 inch wide strip per CertainTeed, and press the shingles firmly into it.

Cut-back, corner-clip and cement widths differ from maker to maker. For the maker-by-maker valley cut dimensions, see our shingle-cutting guide.

The nail-free zone

No nails within 6 inches of the valley centerline on closed-cut valleys, per both the Owens Corning Duration and CertainTeed Landmark instructions; on IKO’s open-metal detail, no nail closer than 2 inches from the chalk line. Nails in the valley flow path are direct leak paths because water collects around the nail head and shank. The shingle laps and the cement bead hold the shingles down within the no-nail zone.

Valley method decision table

If	Then	Source
Shingle is GAF Timberline or any laminated GAF line except Slateline	Closed-cut or open-metal; no woven	GAF RESGN103
Shingle is IKO Cambridge	Open-metal recommended; closed valleys acceptable but check the limited warranty	IKO Cambridge step 7
Shingle is CertainTeed Landmark	Closed-cut preferred; open and woven accepted	CertainTeed Landmark instructions
Valley is open metal	Lining at least 24 in wide, galvanized at least 0.0179 in (26 ga G90) or another Table R905.2.8.2 metal	IRC R905.2.8.2
Valley is closed with smooth roll roofing	One ply at least 36 in wide (ASTM D6380), or an ASTM D1970 self-adhering membrane	IRC R905.2.8.2
Roof slope is 2:12 to under 4:12	Double underlayment required for asphalt shingles	IRC R905.2.2
Valley ends at a sidewall	Install kickout flashing to divert water away from the wall	IRC R903.2.1
Copper flashings or runoff anywhere upslope	Keep galvanized and Galvalume metal out of its runoff	Metal Sales Classic Rib guide

Underlayment stack: where most valley leaks come from

The visible flashing method (open-metal, woven, closed-cut) is what inspectors and homeowners see. The underlayment stack is what keeps water out of the deck when the surface lets water past, which is why all three maker instructions above start the valley with a 36 inch liner before any shingle goes down.

A recommended valley underlayment stack

Layer	Spec	Width	Coverage
Deck	Solid sheathing (IRC R905.2.1 requires solidly sheathed decks for asphalt shingles)	n/a	n/a
Ice and water shield	ASTM D1970 self-adhering membrane	36 inches (OC, CertainTeed, IKO)	Full valley length, eave to ridge
Underlayment (felt or synthetic)	15 or 30 lb felt, or synthetic per ASTM D8257	Field rolls	Lapped onto the valley liner per the shingle maker
Valley metal (open-metal only)	Table R905.2.8.2 metal, e.g. 26 ga G90 galvanized or 16 oz copper	24 inches minimum (IRC R905.2.8.2)	Full valley length, splices lapped upper over lower
Field shingles	Per manufacturer install instructions	n/a	To chalk line

The IRC does not make ice and water shield mandatory in every valley. 2021 IRC R905.2.8.2 lists it as one permitted option, an ASTM D1970 self-adhering membrane in lieu of the lining, alongside metal and roll roofing linings, while the R905.1.2 ice barrier rule applies at eaves in areas with a history of ice damming. This guide recommends the membrane in every valley, in any climate, because it self-seals around fasteners and provides redundancy if the surface flashing fails. See our ice and water shield guide for product comparison.

Ice and water shield placement details

A commonly cited valley mistake is membrane that is too narrow. Our peel-and-stick underlayment guide compares the self-adhering membranes used in valleys. The 2021 IRC sets a 36 inch minimum width for a smooth roll roofing closed-valley lining and 24 inches for a metal lining (R905.2.8.2), and for open valleys lined with two plies of mineral-surfaced roll roofing, an 18 inch bottom ply under a 36 inch top ply. For a self-adhering membrane, 36 inches centered on the valley, which works out to 18 inches on each side, is the spec in the Owens Corning, CertainTeed and IKO instructions.

Two-layer membrane method

The IRC’s own two-layer rule is for eaves, not valleys: R905.1.2 allows an ice barrier of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet, extending from the eave to at least 24 inches inside the exterior wall line, and at least 36 inches along the slope on roofs of 8:12 or steeper. In a valley, the two-ply option the code names is mineral-surfaced roll roofing (18 inch bottom ply, 36 inch top ply) for open valleys.

Eave intersection detail

Where the valley meets the eave, lap the membranes shingle-fashion so water always runs from an upper layer onto a lower one: the eave ice barrier required by R905.1.2 (to at least 24 inches inside the wall line) goes down first, and the 36 inch valley liner laps over it. Reverse order and water tracks under the membrane at the intersection.

Water flow math: why valley width matters

The simplest valley water calculation: total flow at the eave end equals the contributing area on both planes times the rainfall rate. A valley with 200 square feet of contributing roof on each plane (400 sq ft total), in a 2-inch-per-hour rain, carries 400 x (2/12) = 66.7 cubic feet per hour, or about 499 gallons per hour (8.3 gallons per minute) at 7.48 gallons per cubic foot.

That flow concentrates into the narrow channel at the bottom of the valley, which is why IKO widens its open-metal channel 1/8 inch per foot toward the eave and why the IRC requires a 24 inch metal lining under an open valley. The wider the exposed flow channel and the smoother the surface (metal vs shingle), the more peak flow the system carries before water reaches the shingle edges. On long valleys with large contributing area, open metal is the method built for that flow, and it is the one where water runs on metal rather than on cut shingle edges.

Common valley failure modes

1. Underlayment too narrow

Standard residential reroofs sometimes use a 18-inch wide roll of ice and water shield instead of 36-inch. The membrane only covers the bottom 9 inches of the valley on each side, against the 18 inches per side that a 36 inch liner gives. Wind-driven water reaches the deck above the membrane and the deck rots over time. The repair is a full valley tear-out.

2. Nails in the flow path

Any nail driven within 6 inches of the valley centerline on a closed-cut valley breaks the Owens Corning and CertainTeed rule and can become a leak point over time. Rushed nailing by feel, without checking the centerline distance, is the usual cause.

3. Missing or thin cement bead on closed-cut

The cut shingle edge in a closed-cut valley must be bedded in roof cement: 3 to 4 inches wide per Owens Corning, 2 inches per CertainTeed. Without the cement, wind-driven water tracks up under the cut edge and reaches the underlayment.

4. Woven valley bridging

Laminated shingles woven across a valley can bridge, sitting above the valley surface instead of flush. GAF’s Pro Field Guide warns that wind-driven rain can get under a bridged shingle and that bridging lifts the shingle into the wind, where it can blow off. Check whether the shingle maker allows woven valleys before using one.

5. Open-metal pan nailed through the field

Pre-formed valley metal is fastened at the edges, with only enough nails to hold it per IKO’s Cambridge instructions, or with cleats, not nailed through the metal field. Field nails punch holes through the metal where the 36 inch liner is the only layer left, and pin the pan against thermal movement of about 1/8 inch (steel) to 1/4 inch (aluminum) over a 16 ft run.

6. Kickout flashing missing at the bottom of the valley

Where a valley meets a sidewall (a wall on one of the planes), kickout flashing diverts the concentrated valley flow away from the wall. IRC R903.2.1 requires a flashing that diverts water away from where the eave of a sloped roof meets a vertical sidewall, in metal no thinner than 0.019 inch (No. 26 galvanized). Without it, the valley dumps directly into the wall cavity, a frequent source of “mystery” interior wall stains traced to a roof leak.

7. Mixing aluminum and copper

Aluminum valley metal in contact with copper flashings elsewhere on the roof causes galvanic corrosion, and the aluminum corrodes first. Metal Sales’ Classic Rib guide likewise says galvanized and Galvalume panels should not touch, or take runoff from, copper, lead or uncoated steel. Pick one metal family and stick with it across all flashings on the same roof, chimney flashing included. Our guide to chimney flashing for a metal roof covers which flashing metals are compatible with each panel type. Where copper has already been damaged by contact with another metal, see our guide to copper roof repair.

Metal valley pan: nailed vs cleated

Pre-formed valley metal has two accepted fastening methods. IKO’s Cambridge instructions fasten the liner at its edges with only enough nails to hold it in place. Cleats (or clips) are small metal tabs nailed to the deck along the edge of the pan; the pan slides under the tabs and is held without being penetrated. This allows thermal expansion: at roughly 6.5 millionths of an inch per inch per degree F for steel and 12.8 for aluminum, a 16-foot (192 inch) pan moves about 1/8 inch (steel) or 1/4 inch (aluminum) over a 100 degree F seasonal swing.

If you nail the pan edge, the pan must have slotted holes or the nails must sit at the outer edge so the pan can still move, and the nails stay out of the water path: IKO keeps shingle nails at least 2 inches back from the chalk line. On longer valleys, where the expansion math grows with length (a 24 ft aluminum pan moves about 3/8 inch over the same swing), cleats are the safer choice.

Valley flashing on metal roofs

A metal roof valley flashing detail uses the same pre-formed pan concept as open-metal asphalt valleys, with two differences: the pan is typically integral to the panel system (matching the panel profile), and the field panels are not trimmed at a chalk line but are field-cut to match the valley angle. Fabral’s 5 Rib guide field-cuts the panels 4 to 6 inches back from the valley and fastens them through the valley flashing on both sides of each main rib. For metal roof shingles, IRC R905.4.6 requires valley flashing that extends at least 8 inches each way from the centerline, a splash diverter rib at least 3/4 inch high, end laps of at least 4 inches, and a 36 inch underlayment under it.

The critical detail on metal roof valleys is the panel-to-valley seal. Fabral’s 5 Rib installation guide calls for sealant and a 1 x 1 in foam sealer strip where the panel laps the valley, and a 6 inch lap with sealant under it where valley flashing pieces overlap. Without the seal, capillary action draws water up between the panel and pan. See our corrugated metal roofing guide for the panel-side detail. Standing seam roofs handle the valley differently, hooking the panel hems onto offset cleats so the panels can float. For that method, see our standing seam valley details.

Repair vs replace a leaking valley

If a valley is leaking and the rest of the roof is sound, HomeGuide’s roof leak repair guide (February 9, 2026) puts valley leak repairs at $300 to $1,500 for materials and labor, usually removing and reinstalling the valley flashing, and its flashing repair guide (January 13, 2026) gives $200 to $850 for a roof valley.

• Localized repair. Cement bead refresh, replacement of one or two shingles, or a valley metal patch when the liner below is intact. Expect the low end of HomeGuide’s $200 to $850 flashing range, and note its $250 to $300 typical minimum job fee.
• Full valley rebuild. Strip both planes back from the valley center, install a new 36 inch liner and flashing, re-shingle. This is the removal-and-reinstall work HomeGuide’s $300 to $1,500 valley leak figure describes.
• Full reroof recommended (valley failure plus worn field shingles). When the field shingles are near the end of their warranty term, a valley failure is usually a symptom of underlayment age affecting the whole roof, and IRC R908.3.1.1 bars a recover over a roof that is already two layers deep.

For homeowner diagnosis of valley leaks vs other roof leaks, see our how to fix a roof leak guide. Also see the broader roof flashing guide for the flashing system context.

Frequently asked questions

What’s the best valley flashing method?

Open-metal for heavy snow climates, long valleys, and any roof with designer or slate-look shingles; IKO recommends metal valleys for Cambridge. Closed-cut for architectural shingles where open metal is not wanted; CertainTeed prefers it for Landmark. Woven only where the shingle maker allows it: GAF does not allow woven valleys with laminated shingles, while Owens Corning (Duration) and CertainTeed (Landmark) accept them. Follow the instructions for the shingle on your roof.

Do I need ice and water shield in a valley?

Not always by code. The 2021 IRC (R905.2.8.2) permits several valley linings, installed per the shingle manufacturer’s instructions: metal at least 24 inches wide, two plies of mineral-surfaced roll roofing, one ply of smooth roll roofing at least 36 inches wide for closed valleys, or ASTM D1970 self-adhering membrane in lieu of the lining. The ice barrier rule in R905.1.2 covers eaves in areas with a history of ice damming, not valleys. Check the shingle instructions and any local amendment. Owens Corning, CertainTeed and IKO all specify a 36 inch wide liner centered on the valley line, full length from eave to ridge.

How wide should the exposed metal strip be on an open-metal valley?

IKO’s Cambridge instructions snap two chalk lines 6 inches apart at the top of the valley, widening 1/8 inch per foot toward the bottom, so a 16 foot valley is 8 inches wide at the eave and a 24 foot valley is 9 inches. The metal lining under it must be at least 24 inches wide under IRC R905.2.8.2. Use your shingle maker’s dimensions where they differ.

Can I add valley metal under existing shingles without a full reroof?

Not without opening the valley. Open-metal valley install requires stripping both planes back from the valley center, installing the 36 inch liner and at least 24 inch metal, then reinstalling shingles, which is the removal-and-reinstall valley repair HomeGuide prices at $300 to $1,500. Localized cement-bead repairs and shingle replacement can extend a closed-cut valley without a full rebuild when the liner below is sound.

Are nails allowed in the valley?

Not within 6 inches of the valley centerline on either plane of a closed-cut valley, per Owens Corning’s Duration and CertainTeed’s Landmark instructions; IKO keeps nails at least 2 inches back from the chalk line on open-metal valleys. Any nail in the concentrated flow path can become a leak source over time. The shingles in the no-nail zone are held by laps and roof cement.

Bottom line

Valley flashing is the part of the roof system where small details compound into big problems. A 36-inch liner instead of 18-inch, a clipped corner on each cut shingle, metal fastened only at the edges, and a properly sequenced eave intersection each cost minutes during install, against a valley leak repair HomeGuide prices at $300 to $1,500.

If you’re hiring out, write “minimum 36-inch valley liner, closed-cut or open-metal method per manufacturer specification, open-metal lining at least 24 inches wide per IRC R905.2.8.2” into the contract and check the eave intersection on completion day. If you’re DIY, the underlayment stack is what matters most. Pick open-metal or closed-cut (skip woven on laminated shingles unless the maker’s instructions allow it), get the 36 inch membrane right, keep nails 6 inches off center, and bed every cut edge in roof cement.