Calculator Inputs
Example Data Table
| Roof 1 Pitch | Roof 2 Pitch | Plan Angle | Open Flashing Angle | Valley Pitch | Use Case |
|---|---|---|---|---|---|
| 4 / 12 | 4 / 12 | 90° | 154.16° | 2.83 / 12 | Balanced porch valley |
| 6 / 12 | 8 / 12 | 90° | 138.09° | 4.80 / 12 | Unequal main roof valley |
| 5 / 12 | 7 / 12 | 110° | 136.97° | 3.32 / 12 | Wide plan junction |
| 3 / 12 | 9 / 12 | 75° | 144.47° | 2.99 / 12 | Low roof to steep wing |
Formula Used
Pitch is first converted into slope ratio. For a 6 over 12 roof, the slope ratio is 6 ÷ 12.
Roof plane normal vectors are built as follows:
n1 = (s1, 0, 1)
n2 = (s2 × cos θ, s2 × sin θ, 1)
Here, s1 and s2 are roof slope ratios. The symbol θ is the plan angle between roof run directions.
The angle between roof plane normals is:
A = acos((n1 · n2) ÷ (|n1| × |n2|))
The open valley flashing angle is:
Open angle = 180° − A
The valley line direction comes from the cross product:
v = n1 × n2
The valley pitch is calculated from the vertical and horizontal parts of that vector:
Valley pitch = 12 × |vz| ÷ √(vx² + vy²)
Bend allowance is estimated as:
Bend allowance = bend radians × (inside radius + K-factor × thickness)
How to Use This Calculator
- Measure roof pitch one as rise over twelve inches of run.
- Measure roof pitch two using the same method.
- Enter the plan angle between the two roof run directions.
- Enter the sloped valley length from ridge point to lower outlet.
- Add flashing side width, hem width, stock length, lap, and waste.
- Enter material thickness, bend radius, and K-factor when bend allowance matters.
- Press the calculate button.
- Review the open flashing angle, valley pitch, miter references, and material estimate.
- Use the CSV or PDF button to download the report.
Valley Flashing Angle Guide
Roof Geometry Basics
Accurate valley flashing begins with clear roof geometry. Two roof planes meet along one water line. That line is rarely simple. Pitch, plan angle, and roof symmetry all change the bend. This calculator converts those inputs into practical layout values. It supports equal and unequal roofs. It also shows the valley slope, the open flashing angle, and the projected bearing.
Why the Angle Matters
A valley carries concentrated runoff. Small fitting errors can lift edges. They can also leave gaps under shingles, tiles, or metal panels. A correct open angle helps the sheet sit flat against both planes. A correct valley slope helps estimate water speed and overlap needs. The result is better drainage and fewer site adjustments.
Plane Method
The tool treats each roof face as a plane. Each pitch is changed into a slope angle. The plan angle defines how the two fall directions meet in plan view. Plane normals are compared to find the fold between the surfaces. Their cross direction gives the actual valley line. From that line, the calculator reports valley rise per twelve, valley angle, and bearing from roof one.
Field Measurement
Use measured values when possible. Do not rely only on drawings. Framing can move during construction. Roof sheathing can also vary slightly near hips and valleys. Measure pitch with a level, digital gauge, or known rise and run. Measure the plan angle from the roof layout. For standard perpendicular roof wings, use ninety degrees.
Material Planning
The material section helps with ordering. Enter valley length, strip width, lap, stock length, and waste. The calculator estimates total cut length, piece count, and sheet area. These values are planning aids. They do not replace local code, manufacturer instructions, or drainage design. Wider valleys may be needed for heavy rain, low slopes, debris, or tile roofs.
Cutting Checks
Review the results before cutting. The open angle is the included angle for the flashing. The center bend reference is half that value. Some brakes use different angle scales. Always confirm the brake reading with a small test piece. Mark the center line cleanly. Keep laps pointed downhill. Seal only where the roofing system allows. Good layout saves time and protects the roof envelope. Record each project report for later inspection, quoting, and client review.
FAQs
What is a valley flashing angle?
It is the included bend angle needed for flashing where two roof planes meet. The angle depends on both roof pitches and the plan angle between the roof runs.
Is the open angle the same as the brake setting?
Not always. The open angle describes the final included shape. Some brakes use the amount bent from flat. Check your brake scale and test a small piece first.
Can I use this for unequal roof pitches?
Yes. The calculator accepts different pitches for each roof plane. It also shows separate plan miter references for each side of the valley.
What does plan angle mean?
Plan angle is the horizontal angle between the two roof run directions. A common perpendicular roof intersection often uses ninety degrees.
Why is valley pitch lower than roof pitch?
The valley travels diagonally across the roof planes. Because of that diagonal path, its rise per run is often lower than the adjoining roof pitches.
Does this replace roof code requirements?
No. It is a layout and estimating aid. Always follow local rules, manufacturer details, underlayment needs, and drainage requirements for the roof system.
What is K-factor?
K-factor estimates where the neutral bend line sits inside metal thickness. It helps calculate bend allowance for a more accurate blank width.
Why add waste allowance?
Waste covers trimming, mistakes, laps, damage, and layout changes. A small allowance helps avoid short orders during field installation.