Roof Slope Calculator

About Roof Slope Calculation

The Roof Slope Calculator determines roof pitch in degrees, percent, and ratio. Enter any two known values - Base B, Height H, Slope length L, or pitch angle - to calculate the remaining dimensions and slope values automatically. The Metric version uses millimetres and expresses the pitch ratio as 1:x. The Imperial version uses feet and inches and expresses roof pitch as x:12, the standard format commonly used in the United States. Both versions provide a dimensioned roof slope diagram that updates automatically.

Tips and Tricks

• Input rule: enter any two of the four parameters B, H, L, and roof pitch. The remaining values are calculated automatically. In Metric, dimensions are entered in millimetres. In Imperial, feet and inches are entered in separate fields. Decimal inches are accepted, while calculated lengths are displayed to the nearest 1/8 inch.
• Pitch formats: Metric supports degrees, percent, and the 1:x ratio. Imperial supports roof pitch x:12, degrees, and percent. For example, a 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. This corresponds to approximately 26.57° or 50% slope.
• Calculation formulas: the roof geometry forms a right triangle. The slope angle is α = arctan(H / B), and the slope length is L = √(B² + H²). Percent slope is calculated as 100 × H / B. For the Metric ratio 1:x, the value of x = B / H. For Imperial roof pitch x:12, the value of x = 12 × H / B.
• Typical roof pitches: a 2:12 pitch corresponds to approximately 9.46°, a 4:12 pitch to 18.43°, a 6:12 pitch to 26.57°, and a 12:12 pitch to 45°. These are geometric conversions, not universal minimum slopes for roofing materials.
• Roofing materials: minimum roof pitch depends on the selected covering and installation system. In US practice, asphalt shingles generally require at least a 2:12 pitch. Slopes from 2:12 to below 4:12 require special underlayment and installation procedures. Metal roofing, clay and concrete tiles, slate, and membrane systems have their own requirements. Always check the manufacturer's installation instructions.
• Measuring roof slope: measure Base B horizontally and Height H vertically. Do not confuse the horizontal run with the sloped length. If the slope length L and pitch angle are known, enter those two values to calculate the horizontal and vertical dimensions.
• Design considerations: roof pitch affects drainage, roofing material selection, snow accumulation, and installation requirements. For European projects, consider EN 1991-1-3 for snow loads, EN 1991-1-4 for wind loads, and EN 12056-3 for roof drainage. For projects in the United States, follow applicable local building codes and roofing manufacturer specifications.

FAQs

What is roof slope?

Roof slope, also called roof pitch, describes the inclination of a roof surface relative to the horizontal. It can be expressed in degrees, percent, or as a ratio. The Metric calculator uses the 1:x ratio, while the Imperial calculator uses x:12, representing inches of rise per 12 inches of horizontal run.

How do I calculate roof slope in degrees?

Measure the horizontal Base B and vertical Height H in the same units. Divide H by B and calculate the arctangent: α = arctan(H / B). For example, a 6:12 roof pitch has a rise-to-run ratio of 6/12 = 0.5, corresponding to approximately 26.57°.

How do I measure roof slope safely?

Whenever possible, measure from the ground or an accessible interior location using a level and measuring tape. Determine the horizontal distance B and vertical rise H without walking on the roof. The calculator uses these dimensions to determine the slope angle and length.

What qualifies as a low-slope roof?

The definition depends on the roofing system. For asphalt shingles in US residential construction, a pitch from 2:12 to below 4:12 is considered a low-slope application and requires special installation procedures. Standard asphalt shingles should not be installed below 2:12. Other roofing systems, including membranes and metal panels, have different slope classifications and requirements.

What is the minimum slope for a metal roof?

The minimum slope depends on the metal roofing system, panel profile, seam type, and installation method. Some standing-seam systems are suitable for relatively low slopes, while other metal panels require steeper pitches. Check the manufacturer's specified minimum slope and the applicable building regulations before selecting a roofing system.