Roof slope is the vertical height change divided by the horizontal distance between the same two points. For a metal roof, record that ratio, its direction and the measurement locations before comparing it with the selected panel system’s requirements. An angle or a conversion table alone cannot establish whether a roof assembly is suitable.
Define the measurement baseline
Use h for the magnitude of vertical rise and r for horizontal run. Express both in the same units and record the uphill or downhill direction separately. A 4:12 slope means 4 units of rise for 12 units of horizontal run; it equals 33.33% and approximately 18.43°. The length measured along a sheet is the inclined length, a different side of the triangle.
“Pitch” is often used for slope, especially in product literature. Older drawings may define pitch as rise divided by the whole span. Read the document’s definition before converting. On a symmetrical gable, the run from eave to ridge is half the full span; an offset ridge requires the actual run on each side. Hip and valley lines can have different inclinations from the main roof plane.
Convert ratios, percentages and angles
Let s = h/r. Then slope (%) = 100s; rise per 12 units = 12s; angle θ = arctan(s). For an angle supplied in degrees, s = tan(θ), with the calculator set to degrees. A stated 1-in-N gradient has s = 1/N when the document defines one vertical unit for N horizontal units.
| Rise per 12 horizontal units | Slope (%) | Angle (°) |
|---|---|---|
| 0.25:12 | 2.08 | 1.19 |
| 0.5:12 | 4.17 | 2.39 |
| 1:12 | 8.33 | 4.76 |
| 2:12 | 16.67 | 9.46 |
| 3:12 | 25.00 | 14.04 |
| 4:12 | 33.33 | 18.43 |
| 6:12 | 50.00 | 26.57 |
| 12:12 | 100.00 | 45.00 |
Scroll within the table to see all columns →
Values below are geometry only, not permitted minimum pitches for panel products. Percentages and angles are rounded to two decimal places. For a dimensioned setting-out calculation, retain the unrounded ratio until the final reporting step. Thus 5% is approximately 2.86°, whereas 5° is approximately 8.75%.
Measure without substituting sheet length for run
Prefer design dimensions or a survey made from safe access by a competent person. Do not climb onto a roof to obtain a simple pitch reading. Establish the two endpoints, horizontal direction and elevation reference. On an accessible sample or safely reached surface, a level held horizontally provides the run; measure the vertical gap to a consistent reference on the sloping surface at its other end. A digital inclinometer needs a checked zero and a straight reference aligned with the roof plane’s steepest descent; a diagonal reading can give a lower angle.
Profile ribs, laps, local dents and a tilted measuring tool can distort a short reading. Repeat measurements at identified positions and record the method and instrument resolution. A short measurement describes its local segment; it does not certify the whole roof. Where only inclined length L and vertical rise h are known for a straight planar segment, calculate r = √(L² − h²), then h/r. This assumes a right triangle and L greater than h.
Work through the dimensions
For a straight roof segment rising 300 mm over a horizontal run of 6,000 mm, s = 0.05. The result is 5%, 0.6:12 and approximately 2.86°. Its inclined length is √(6,000² + 300²) = 6,007.50 mm. Keep that distinction when checking dimensions; panel ordering must also resolve overhangs, laps and end details separately.
For a 4:12 plane with a horizontal run of 3,600 mm, the rise is 1,200 mm and the inclined length is approximately 3,794.73 mm. Dividing 1,200 by 3,794.73 would give about 31.62%, below the correct 33.33%. Neither worked example is a roof-system approval or a cutting schedule.
Compare nominal slope with local fall
A ridge-to-eave average can conceal a flat area or reverse fall between supports. Check elevations and drainage direction where the surface changes, including laps, penetrations and outlets. At low slopes, small support-level variations become significant. LYSAGHT’s flashing guidance specifically warns that support geometry and later changes can create zero or negative fall; its product recommendations remain specific to the documented system.
A drawing with 200 mm of overall fall over 10 m gives 2%. If two surveyed points 1 m apart on that route are at the same elevation, that segment has zero fall despite the positive overall average. Record and refer the discrepancy for assessment. Do not invent a universal tolerance or use a rounded conversion to accept a borderline condition. Design allowance for deflection, construction tolerances and drainage belongs with the project designer and system supplier.
Put the result into an actionable record
Record roof area and direction, drawing revision, endpoints, horizontal run, vertical difference, units, calculated ratio/percent/angle, measurement method and any local reversal. State whether the result is a design dimension, survey result or approximate screening measurement. Obtain the current requirements for the exact profile, seams, laps and application, then resolve discrepancies before procurement or installation.
The roof-system comparison covers standing-seam and corrugated selection, including system-specific slope context. Use the installation guide for work sequence and the lap requirements guide for joint details. The source notes identify the measurement and low-slope references used here.