Roof Valley Length Calculator

Calculate the true length of a roof valley from roof width, pitch of each intersecting plane, and valley geometry to determine ice shield and W-metal flashing requirements

Enter roof width and pitch of each intersecting plane to calculate valley length

Quick presets

ft

Total Linear Feet

183 LF

Ridge: 40 LF • gable roof

PRO

Professional Calculator

Extended parameters for precise calculations

sq ft

Estimated Materials

60 bundles

Roof Area

1,792 sq ft

Squares

17.9

Detailed Breakdown

Roof Area1,792 sq ft
With Waste1,971 sq ft
Roofing Squares17.9
Bundles60
How to Use This Calculator
The Roof Valley Length Calculator determines the true geometric length of a roof valley from your roof width and the pitch of each intersecting roof plane. Unlike the general Valley Calculator that focuses on flashing methods and materials, this tool starts with the fundamental question: how long is the valley? Getting the accurate valley length is essential for ordering the right amount of ice and water shield, W-metal flashing, and valley shingles.

Dimensions tab: Enter the roof width at the valley — this is the width of the intersecting roof section measured perpendicular to the ridge. Select the pitch of each intersecting plane. For the most common case (equal pitches on both planes), the valley length formula is straightforward: Valley Length = sqrt(2 × run² + rise²). For unequal pitches, the calculator uses the more complex trigonometric solution. An overlap allowance of 2 feet (1 foot at each end) is standard to ensure flashing extends past the mathematical valley endpoints.

Multiple Valleys tab: Enter the total number of valleys on your roof. A simple cross-gable has 2 valleys, while a complex roof with dormers and wings can have 6-12 or more. If all valleys are the same length (equal width and pitch), select "all equal" and the calculator multiplies by your count. For mixed-length valleys, calculate the main valleys using the Dimensions tab and enter additional footage from smaller valleys in the supplemental field.

Materials Needed tab: The calculator converts total valley length into ice and water shield rolls and W-metal flashing footage. For ice shield, select the coverage width (36 inches centered is standard) and roll size (75-foot rolls are most common). For W-metal, each 10-foot section overlaps the next by 6 inches, so net coverage is 9.5 feet per section. The calculator rounds up to whole rolls and sections so you can order accurately.

The Formula
The roof valley length calculator uses these formulas:

Equal-Pitch Valley Length Run = Roof Width / 2 Rise = Run × (Pitch rise / 12) Valley Length = √(Run² + Run² + Rise²) = √(2 × Run² + Rise²) Example (30 ft wide, 6/12 pitch): Run = 15 ft, Rise = 15 × 6/12 = 7.5 ft Valley = √(225 + 225 + 56.25) = √506.25 = 22.5 ft

Simplified Equal-Pitch Formula Valley Length = Run × √(2 + (pitch/12)²) Example: 15 × √(2 + 0.25) = 15 × √2.25 = 15 × 1.5 = 22.5 ft

Unequal-Pitch Valley Length (Approximate) For pitches P1 and P2 on a width W: Run = W / 2 Rise1 = Run × P1/12 Rise2 = Run × P2/12 Avg Rise = (Rise1 + Rise2) / 2 Valley ≈ √(2 × Run² + Avg Rise²) Note: This is a simplified approximation. Exact unequal-pitch valley geometry requires solving for the intersection of two tilted planes.

Total Valley Length with Overlap Total per valley = Valley Length + Overlap Allowance (ft) Example: 22.5 + 2 = 24.5 ft per valley

Multiple Valleys Total valley footage = (Valley length × Valley count) + Additional valley feet Example: (24.5 × 2) + 0 = 49 ft total

Ice & Water Shield Rolls Rolls needed = ceiling(Total valley footage / Roll length) Example: ceiling(49 / 75) = 1 roll of 75 ft

W-Metal Sections Net coverage per 10 ft section = 10 − 0.5 ft overlap = 9.5 ft Sections = ceiling(Total valley footage / 9.5) Example: ceiling(49 / 9.5) = 6 sections (60 LF purchased) Cost = Sections × 10 ft × $/LF

W-Metal Cost by Material Aluminum: 6 sections × 10 ft × $5/LF = $300 Galvanized: 6 sections × 10 ft × $4/LF = $240 Copper: 6 sections × 10 ft × $20/LF = $1,200
Example Calculation
Example: Cross-Gable Roof — 30 ft Wide, Equal 6/12 Pitch, 2 Valleys

Lisa has a cross-gable roof where a 30-foot-wide perpendicular wing intersects the main roof. Both roof planes are 6/12 pitch. She needs to know the valley length to order ice and water shield and aluminum W-metal flashing.

Step 1: Valley Geometry
• Roof width at valley: 30 ft
• Run (half width): 15 ft
• Pitch: 6/12 on both planes (equal pitch)
• Rise: 15 × 6/12 = 7.5 ft

Step 2: Valley Length Calculation
• Valley = √(Run² + Run² + Rise²)
• Valley = √(15² + 15² + 7.5²)
• Valley = √(225 + 225 + 56.25)
• Valley = √506.25
• Valley = 22.5 ft per valley

Step 3: Total Valley Length
• 2 valleys × 22.5 ft = 45 ft
• Overlap allowance: 2 ft per valley × 2 = 4 ft
• Total: 45 + 4 = 49 ft of valley to cover

Step 4: Ice & Water Shield
• Coverage: 36" wide centered in each valley (standard)
• Roll size: 75 ft rolls
• Rolls needed: 49 ft / 75 ft = 0.65 → 1 roll
• Remaining 26 ft can be used for eave ice shield
• Cost: 1 roll × $85 = $85

Step 5: W-Metal Flashing
• Aluminum W-metal, 10 ft sections, 6" overlap
• Net per section: 9.5 ft
• Sections: 49 / 9.5 = 5.2 → 6 sections (60 LF purchased)
• Cost: 60 LF × $5/LF = $300

Step 6: Total Valley Material Cost
• Ice & water shield: $85
• W-metal: $300
• Sealant and nails: $25
• Total: $410 for both valleys

Lisa orders 1 roll of ice and water shield (75 ft) and 6 ten-foot sections of aluminum W-metal. The leftover 26 feet of ice shield will cover approximately 8 linear feet of eave protection on the north-facing side of her roof.

Frequently Asked Questions

How do you calculate the length of a roof valley?
For an equal-pitch valley, the valley length is the hypotenuse of a triangle formed by the horizontal run, the horizontal run again (because the valley runs diagonally in plan view at 45 degrees), and the rise. The formula is: Valley Length = sqrt(run² + run² + rise²). For a 30-foot wide roof at 6/12 pitch, the run is 15 feet and the rise is 7.5 feet. Valley length = sqrt(225 + 225 + 56.25) = sqrt(506.25) = 22.5 feet. Add 1 foot at the top and 1 foot at the bottom for overlap, giving a total of 24.5 feet. For unequal-pitch intersections, the geometry is more complex because the valley no longer runs at 45 degrees in plan view, requiring separate rise calculations for each plane.
How much ice and water shield do I need for a roof valley?
Ice and water shield for valleys is calculated by total valley length times the width of coverage. A single 36-inch-wide roll centered in the valley covers 18 inches on each side of the centerline, which is the minimum code requirement. Most professionals and shingle manufacturers recommend this as a minimum. Each 75-foot roll covers 75 linear feet of valley. If your total valley length is 50 feet, one roll is sufficient. For a complex roof with 100 feet of total valley, you need two rolls. Remember that ice and water shield is also required along eaves in cold climates (typically the first 3 feet from the eave edge), so plan your total roll count to cover both valleys and eaves.
What is the difference between a valley calculator and a valley length calculator?
The valley length calculator focuses on determining the true geometric length of a roof valley from roof dimensions and pitch — it answers the question "how long is my valley?" The general valley calculator focuses on the flashing materials and methods needed to waterproof a valley — it answers "what materials do I need for my valley?" The valley length calculator is the starting point because you need to know the valley length before you can calculate how much ice shield, W-metal, or valley shingles are required. This tool combines both: first calculating the precise valley length from your roof geometry, then converting that length into specific material quantities.
What happens when two roof planes have different pitches at a valley?
When two roof planes with different pitches meet at a valley, the valley line no longer runs at a 45-degree angle in the horizontal plan view. The steeper plane has a higher rise than the shallower plane, which changes the valley geometry. The valley runs from the point where both ridges intersect down to where both eaves meet, and its length is longer than it would be if both pitches were equal to the steeper pitch. In practice, unequal-pitch valleys are less common in residential construction and typically occur where a main roof meets a porch or addition with a different pitch. The framing is more complex and the shingles must be carefully cut to maintain a straight valley line despite the asymmetry.
Should I use W-metal valley flashing or a woven/closed-cut valley?
W-metal valley flashing is the most durable and maintainable valley method. The pre-formed metal channel (typically aluminum or galvanized steel with a center crimp) provides a clear water channel that can be inspected and repaired easily. W-metal valleys cost $4-$8 per linear foot more than woven or closed-cut methods but can last the life of the roof. Woven valleys interlace shingles across the valley centerline and work well but make it difficult to replace individual shingles later. Closed-cut valleys overlay one plane of shingles over the other and trim along a line — they look clean but the cut edge can allow water intrusion if not sealed. For longevity and ease of maintenance, W-metal is the professional recommendation, especially on lower-pitch roofs where water moves slowly through the valley.

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Related Guides & Resources

How to Measure a Roof Understanding Roof Pitch Roof Pitch Chart