Roof Valley Length Calculator
Estimate roof-valley length from horizontal run and pitch, then calculate total stock pieces, overlap allowance and material cost.
Default project preview
5 pieces
Length per valley
20.12 ft
Base valley length
40.2 ft
Order length
45.1 ft
Purchased length
50 ft
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Horizontal valley run
Enter this project assumption in ft. The current imperial default is 18 ft.
Roof rise per 12 run
Enter this project assumption in in/12. The current imperial default is 6 in/12.
Number of valleys
Enter this project assumption in valleys. The current imperial default is 2 valleys. Minimum: 1.
Lap and cutting allowance
Enter this project assumption in %. The current imperial default is 12 %. Minimum: 0. Maximum: 50.
Stock flashing length
Enter this project assumption in ft. The current imperial default is 10 ft.
Price per stock piece
Enter this project assumption in per piece. The current imperial default is 24 per piece. Minimum: 0.
How to read the outputs
Length per valley
slope-adjusted length. Default preview: 20.12 ft.
Base valley length
2 valleys. Default preview: 40.2 ft.
Order length
12% extra. Default preview: 45.1 ft.
Purchased length
rounded stock pieces. Default preview: 50 ft.
Offcut allowance
after stock rounding. Default preview: 4.9 ft.
Material allowance
pieces × entered price. Default preview: $120.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Valley length = horizontal run × √(1 + slope ratio²). Total length = one valley × count. Lap and cutting allowance are added, then stock pieces are rounded up.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Measure total roof-valley run from plan geometry and slope so valley membrane, metal or shingles can be ordered by actual sloped length.
Calculation steps
- 1Measure each valley plan run.
- 2Apply the roof-slope multiplier.
- 3Add required end laps and transitions.
- 4Round membrane or metal to sold lengths or rolls.
Worked numerical example
A valley with a 14 ft horizontal plan run on a 6:12 roof uses a 1.118 slope multiplier, giving 15.65 ft. Two matching valleys total 31.3 ft before laps and waste.
Purchasing and rounding
Separate open-valley metal, membrane and shingle quantities because they have different widths and lap rules.
Interpret the result
Length is only one input; valley detail, width and underlayment sequence follow the selected roofing system.
Page-specific assumptions
- Straight valleys.
- Uniform slope along each run.
- Plan run is measured correctly.
Common mistakes
- Using horizontal length as sloped length.
- Adding one waste factor to products with different lap requirements.
- Ignoring valley intersections.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Roof Valley Length Calculator questions
Is a roof valley always longer than its horizontal run?+
Yes, when it rises with the roof slope. The steeper the roof, the longer the sloped valley.
Does this handle unequal roof pitches?+
No. Unequal intersecting slopes can change the valley geometry and should be measured from plans or in the field.
Why include extra length?+
Valley materials may need end laps, trimming, turn-ups and waste at ridge or eave transitions.