How to Order Custom Flashing for a Non-Standard Roof Pitch

Stock flashing is designed around a handful of common roof pitches — typically 4/12, 6/12, and 8/12. When you are working with a 3/12 shallow-slope, a 10/12 steep commercial pitch, or an unusual pitch dictated by structural drainage requirements, standard profiles won't fit correctly. The leg angles will be wrong, the overlap dimensions won't seal, and the piece won't sit flush against the roof surface. This guide explains how to measure, specify, and order flashing for any pitch.

Why Pitch Changes the Flashing Geometry

Flashing pieces that sit at the intersection of a roof surface and a wall (sidewall flashing, counter flashing, diverter flashing) have two legs: one that lies on the roof surface and one that runs up the wall. The angle between those legs must match the actual angle between the roof and the wall — which changes with roof pitch.

For a vertical wall and a sloped roof, the angle between them is: 90° + arctan(rise/run). For a 4/12 pitch that's approximately 108.4°. For a 12/12 pitch it's 135°. A piece bent for 4/12 will gap at the wall on a 12/12 roof, creating a water infiltration path at the most critical joint in the system.

Converting Pitch to Degrees

You can calculate the slope angle for any pitch with: arctan(rise ÷ run) × (180 ÷ π). On a phone calculator: enter rise ÷ run, press the inverse tangent (tan⁻¹) button.

Measuring Pitch in the Field

If you don't have the original architectural drawings, measure pitch directly:

• Pitch gauge / angle finder: Most reliable. Hold flush to the roof surface and read the angle directly.
• Level and tape method: Hold a level horizontal at any point on the roof surface. Measure 12" horizontally from where the level touches the roof, then measure vertically up to the roof surface at that point. That vertical measurement is the rise. Rise/12 = pitch in standard notation.
• Phone inclinometer apps: Adequate for verification but less precise than a dedicated pitch gauge on textured surfaces.

Sidewall Flashing for Non-Standard Pitches

Sidewall flashing is an L-shaped sheet metal assembly fabricated in overlapping sections — the horizontal leg integrates with the roofing field and the vertical leg mounts behind the wall cladding or counterflashing.

For non-standard pitches, the critical dimensions are:

• Horizontal leg length: 4" minimum onto the roof field; typically 6"–10" to accommodate membrane weld width or felt integration plus the required lap.
• Vertical leg height: Minimum 4" above the roof surface — measured perpendicular to the roof, not vertically. On steep pitches this means the actual metal leg needs to be longer than 4" since it runs at an angle.
• Section width: Sized to span the wall run with adequate lap. Standard sections are 8"–12" wide; longer sections require additional fastening to prevent thermal buckling.

For standard pitches, a 90° bend is close enough. For pitches below 4/12 or above 8/12, specify the exact bend angle so the piece lies flat on the roof and flush against the wall without forcing.

Counter Flashing for Non-Standard Pitches

Counter flashing covers the top of sidewall flashing and is embedded in the wall (reglet) or surface-applied with sealant. For non-standard pitches, the counter flashing must slope correctly to drain water away from the wall and maintain a minimum lap over the sidewall flashing below.

The bottom edge of the counter flashing should maintain a 2" minimum lap over the top of the sidewall flashing at all points. On steep pitches this means the counter flashing bottom edge must be cut at the pitch angle, not horizontally — otherwise the overlap is inconsistent up the run.

Diverter Flashing

Diverter flashing is installed at the bottom of a sidewall where the roof edge meets a wall. It directs water from the roof slope into the gutter rather than letting it run against the wall cladding or behind the fascia.

For non-standard pitches the deflection angle — how aggressively the piece deflects water — must account for the actual water flow direction. On shallow pitches (3/12, 4/12), water moves mostly horizontally and requires a smaller deflection angle. On steep pitches (10/12, 12/12), water is moving fast and nearly vertically, and requires a deeper diverter flashing to redirect it into the gutter rather than over it.

Valley Flashing for Non-Standard Pitches

Valley flashing spans the internal angle where two roof planes meet. The valley angle depends on the pitches of both planes. For equal pitches on both sides, valley angle = 180° − 2 × slope angle. For unequal pitches, the valley angle is asymmetric and must be specified precisely.

Common mistake: ordering valley flashing fabricated for a standard 6/12 equal pitch on a building with mixed pitches (one side 4/12, one side 8/12). The centerline of the valley migrates, the leg widths on each side are wrong, and water channels to the short-leg side.

What to Include in a Custom Flashing Order

A complete specification for non-standard flashing should include:

1. Flashing type (sidewall flashing, counter flashing, diverter flashing, valley, head)
2. Exact bend angle(s) in degrees
3. Leg dimensions for each leg in inches
4. Hem type on exposed edges
5. Material (gauge, alloy, coating)
6. Quantity (pieces or linear footage)
7. Piece length for sidewall flashing individual pieces

Trimgy's drawing canvas lets you draw the exact cross-section with the correct angles — for any pitch — get the developed width automatically, and place a confirmed order with freight included. No back-and-forth on dimensions. Draw your flashing profile now →