What Type of Roof Valley Is Best for Shingles?
For architectural (laminated) shingles, a closed-cut valley or an open metal valley is a safer pick than a woven valley. The National Roofing Contractors Association (NRCA) said in a 2003 Professional Roofing article that it "does not recommend the use of woven valleys with laminated shingles" because weaving them "can create a relatively thick buildup of material" and an irregularly sloped surface that could interfere with drainage. Between open and closed-cut, the same article ties the choice to roof slope, climate and debris.
Atlas Roofing's Pinnacle Pristine installation instructions describe two valley methods, open and closed cut, and the word "woven" does not appear in the document. Both methods start with a 36 inch ice and water product centered in the valley, and the open method adds copper flashing at least 16 inches wide (or equivalent), narrower than the 24 inch minimum the 2021 International Residential Code sets for metal open valley lining. This guide covers why valleys leak, how the three valley types differ, what Atlas and the code require, and which type suits a roof in Irwin, North Huntingdon or nearby towns.
What is a roof valley, and why does it leak?
A roof valley is the inside angle where two roof slopes meet, forming a channel that carries water from both slopes down toward the eave. NRCA's 2003 article explains that the volume of water and the lower slope along valley lines make valleys particularly vulnerable to leakage. For other leak sources on a roof, see this guide to common causes of roof leaks.
Under Section R905.2.8.2 of the 2021 International Residential Code, valley linings go in according to the manufacturer's instructions before the shingles are applied. NRCA adds that underlayment fasteners should generally stay at least 8 inches from a valley's center.
What are the three types of roof valleys?
NRCA's 2003 Professional Roofing article names three basic valley types for asphalt shingle roofs: open, closed-cut and woven.
Open valley: the lining stays exposed down the center and the shingles stop short of it. The article recommends valley metal at least 18 inches wide, in pieces no longer than 10 feet, with a center rib at least 1 inch high where the adjoining slopes differ.
Closed-cut valley: shingles from one slope run at least 12 inches onto the other side, and shingles from the second slope are cut about 2 inches short of the center line. The code calls any valley covered with shingles a closed valley.
Woven valley: shingles from both slopes are woven together across the center. The article says this type is generally limited to strip shingles on valley slopes of at least 4:12.
Architectural shingles are laminated rather than strip shingles, so that limit and NRCA's laminated shingle advice point the same way. On NRCA's guidance, that leaves open and closed-cut valleys as the two to compare.
What does Atlas Roofing require for valleys on Pinnacle Pristine shingles?
Atlas's Pinnacle Pristine installation instructions say valleys must be installed before the shingles are applied, and they give exact steps for two methods. The details below come from the Pinnacle Pristine document, so check the instructions for any other Atlas shingle.
For an open valley, Atlas calls for a 36 inch ice and water product centered in the valley, then copper flashing at least 16 inches wide (or equivalent), also centered (the 2021 IRC sets a 24 inch minimum for metal open valley lining). The copper is secured every 24 inches along both edges with copper cleats or large head copper nails, and overlaps must be at least 12 inches. Shingles are trimmed to chalk lines 3 inches from the center line, with 2 inches cut diagonally off the upper corner to direct water into the valley, and each valley edge is set in a 3 inch band of asphalt plastic cement and nailed no closer than 6 inches from the center line.
For a closed cut valley, the steps are:
Apply a 36 inch wide ice and water product centered in the valley, nailing 2 inches from the outer edges only.
Run all shingles on one side across the center of the valley by at least 12 inches.
Nail at least 6 inches from the center line on the unshingled side, then strike a chalk line 2 inches from the center on that side.
Apply shingles on the unshingled side up to the chalk line and trim them, without cutting the underlying shingle.
Cut the upper corners of the shingle, cement and nail.
Atlas also recommends its self-adhering WeatherMaster products for critical areas such as valleys and eaves. Its Standard Limited Shingle Warranty does not apply to products not installed according to the printed instructions on the bundle wrappers, so these steps carry weight beyond workmanship.
Can architectural shingles be woven into a valley?
The written sources disagree. NRCA's 2003 article recommends against woven valleys with laminated shingles, while an undated excerpt from the Asphalt Roofing Manufacturers Association's laminated shingle manual says laminated shingles can be installed at valleys with the woven, open or closed-cut method.
For a Pinnacle Pristine roof, the maker's own document is the one to follow, and it describes only open and closed cut valleys. It does not ban weaving in writing, but it does not describe it either. With the warranty tied to the printed instructions, sticking to the two described methods is the cautious choice. For more on why installation details matter, see this guide on how poor installation leads to roofing failures.
What does the 2021 building code require under a roof valley?
Section R905.2.8.2 of the 2021 International Residential Code, as adopted in Pennsylvania, permits three kinds of valley lining. An open valley lined with metal needs lining "not less than 24 inches (610 mm) wide" in one of the corrosion-resistant metals in Table R905.2.8.2. Open valleys can instead use two plies of mineral-surfaced roll roofing, an 18 inch bottom layer under a top layer at least 36 inches wide.
A closed valley can use one ply of smooth roll roofing at least 36 inches wide, either open valley lining, or self-adhering polymer-modified bitumen underlayment complying with ASTM D1970. Atlas states that its WeatherMaster products conform to ASTM D1970.
Table R905.2.8.2 sets a minimum for each metal, including aluminum at 0.024 inch, galvanized steel at 0.0179 inch (26 gage, zinc coated G90), cold-rolled copper at 16 ounces per square foot, stainless steel at 28 gage and zinc alloy at 0.027 inch.
Widths do not line up. The code's metal open valley minimum is 24 inches, NRCA's 2003 article recommends at least 18 inches, and Atlas calls for copper at least 16 inches wide. Section R905.2.8 says flashing for asphalt shingles must comply with both that section and the shingle maker's approved installation instructions, and Atlas's own document says installation must comply with local building codes or the manufacturer's requirements, "whichever is stricter."
Where the code applies, treat its 24 inches as the floor and meet whichever of the code or Atlas's steps is stricter. Local permit questions go to your township or borough code office.
Which valley type works best on a Western PA roof?
Open valleys work well on lower slopes and where debris collects, according to NRCA's 2003 article. In areas prone to snow and ice buildup or regular freeze-thaw cycling, it says open valley construction should be enhanced with self-adhering modified bitumen underlayment or with valley metal tapered wider at the low point than the high point. If your valleys in Irwin or North Huntingdon hold snow and ice through the winter, that advice fits your roof. Atlas's 36 inch ice and water product, with self-adhering WeatherMaster as the maker's recommended choice for valleys, lines up with the first option.
For closed-cut valleys, NRCA suggests cutting the side with the longest run to the ridge or the steepest slope. Put together for an architectural shingle roof: an open metal valley at least 24 inches wide fits low-sloped valleys or ones that catch debris, a closed-cut valley is the other method Atlas describes, and either one should follow Atlas's printed steps.
Valley metal is a form of flashing, and this guide covers roof flashing problems that cause leaks. For the bigger picture, here are the layers of a roofing system.
FAQ
Can you weave architectural shingles in a valley?
NRCA's 2003 article advises against it, an undated ARMA laminated shingle manual excerpt lists woven as an option, and Atlas's Pinnacle Pristine instructions describe only open and closed cut valleys. Following the shingle maker's printed instructions keeps the warranty question simple.
How wide should valley flashing be?
For an open valley lined with metal, the 2021 IRC sets a 24 inch minimum. Atlas's 16 inch copper minimum and NRCA's 2003 recommendation of 18 inches both fall under that, so the code width is the one to meet, with your local code office as the final word.
Does a roof valley need ice and water shield?
Atlas's Pinnacle Pristine instructions call for a 36 inch ice and water product centered in the valley for both open and closed cut methods. The code also accepts ASTM D1970 self-adhering underlayment as a closed valley lining.
Is an open valley or a closed-cut valley better?
It depends on the roof's slope and how much debris lands in the valley, the two factors NRCA's 2003 guidance stresses for open valleys. Atlas gives full steps for both methods on Pinnacle Pristine, so either can be built to the maker's instructions.
Why do roof valleys leak?
Valleys carry water from two slopes along a lower pitch than the roof planes beside them, which NRCA's 2003 article says makes them particularly vulnerable. The installation rules target that weak spot: Atlas keeps shingle nails at least 6 inches from the valley center line, and NRCA's guidance generally keeps underlayment fasteners at least 8 inches from it.
Does the shingle warranty cover valley metal?
Atlas's Standard Limited Shingle Warranty excludes flashing, metal work and related work from the replacement costs it covers. Ask your installer how the valley work itself is covered.
Atlas roof replacements from Gerken Roofing
Gerken Roofing, a certified Atlas Shingle Pro+ Diamond contractor at 414 Main Street in Irwin, installs Atlas shingles exclusively on roof replacements and new roofs. Summit 60 synthetic underlayment and Weathermaster ice and water shield come with its Better and Best packages, and its shingles page says its crews follow Atlas's installation requirements from starter strips to hip and ridge caps so the roof qualifies for the full manufacturer warranty. Homeowners comparing Pinnacle Pristine colors can call The Pickle for a free, no-obligation quote.
Visit Gerken Roofing or call 724-515-5551 today. Contact us for more information.
Comments