Roof Slope Calculator

Pitch units
Roof dimensions

Calculations

INPUT DATA

RESULTS

mm
mm
mm
°
%
1:x
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About Roof Slope Calculation

The results are approximate. Before use, verify the calculations against the applicable standards and consult a specialist. The developer is not responsible for the consequences of use without project verification.

The Roof Slope Calculator finds the pitch of any sloped roof and converts it between degrees, percent, and a 1:x ratio. Enter any two known values - Base B (mm), Height H (mm), Slope length L (mm), or pitch angle - and the calculator derives the rest automatically. Use it to size a single-slope (shed) roof triangle, check a low-slope roof, or prepare a quick roof slope chart for a project. Press Draw to get a visual diagram with dimensions and the angle.

Tips and Tricks

  • Input rule: type values in centimetres and fill in exactly two of B, H, L, or the pitch. The remaining fields and the angle conversions will populate automatically.
  • Conversions you may need: percent slope = 100 × H / B; degrees = arctan(H / B); ratio 1:x where x = B / H (for H < B). The roof slope multiplier (roof slope factor) that converts plan lengths to sloped lengths is L / B.
  • What people often use: for sketches and take-offs, many users work in percent; for structural notes, degrees are common; the 1:x ratio is handy on site when marking lines.
  • Typical ranges (approximate): very low-slope ≤ 5° (~0-8.8% ≈ 1:∞ to 1:11.4); low-slope 5-12° (~8.8-21% ≈ 1:11.4 to 1:4.7); medium 12-30° (~21-57.7% ≈ 1:4.7 to 1:1.73); steep 30-45° (~57.7-100% ≈ 1:1.73 to 1:1); very steep > 45° (>100%).
  • Materials and minimum roof slope: the required pitch depends on the product. Metal roofs, membranes and shingles have different limits; always check the manufacturer data and local code before choosing a minimum roof slope.
  • Good practice for measuring: measure Base and Height horizontally and vertically (not along the slope). If you only know L and the angle, enter those two and convert.
  • Normative context (EU): when selecting slope, consider loads and detailing per EN 1991-1-3 (snow), EN 1991-1-4 (wind), and drainage guidance such as EN 12056-3. These affect safe roof pitch choices, especially for low-slope roofing.

FAQs

What is roof slope?

Roof slope (roof pitch) describes how steep a roof is. It can be expressed as degrees, percent rise over run, or a 1:x ratio - all of which this calculator shows simultaneously.

How do I calculate roof slope in degrees?

Measure Base B and Height H in centimetres, then compute α = arctan(H / B). Enter B and H in the calculator and it will return the angle in degrees, percent, and 1:x automatically.

How do I measure roof slope safely?

Whenever possible, measure from the ground or loft: take a horizontal Base (B) and vertical Height (H) using a level and a tape. Avoid measuring along the sloped surface; the tool converts your horizontal and vertical dimensions into an accurate pitch.

What qualifies as a low-slope roof?

In practical terms, roofs up to about 5° are very low-slope, and 5-12° are commonly called low-slope. That corresponds roughly to ~9-21% or about 1:11 to 1:4.7; detailing, drainage and materials for these slopes require special attention.

What is the minimum slope for a metal roof?

It varies by profile and fastening. Many standing-seam and profiled metal systems allow relatively low pitches, while others need a steeper angle for drainage. Use the calculator to test scenarios, then follow the product documentation and local building rules.