About

Converting a roof's rise and run ratio into a percentage slope is a fundamental step in construction planning, particularly for drainage systems and material selection. A slope expressed as a percentage indicates how many units of vertical rise exist for every 100 units of horizontal run. This metric is critical because specific roofing materials have hard cut-offs for warranty and safety.

For instance, standard asphalt shingles typically require a minimum pitch to prevent water wicking under the tabs. Conversely, steep slopes impose safety rig requirements for work crews. This tool instantly converts the standard construction ratio (e.g., 4:12) into a precise percentage, flagging potential drainage issues for flat roofs.

Formulas

The slope percentage is calculated using the ratio of vertical change to horizontal distance:

Slope_% = RiseRun × 100

If the result is < 2%, the roof is considered "dead flat" and requires specialized membrane systems to handle ponding water.

Reference Data

Rise / Run (Ratio)	Slope Percentage	Angle (Degrees)	Common Application
1 / 12	8.3%	4.8°	Industrial / Built-up Roofs
2 / 12	16.7%	9.5°	Minimum for Metal Seam
4 / 12	33.3%	18.4°	Standard Low Residential
6 / 12	50.0%	26.6°	Standard Gable
9 / 12	75.0%	36.9°	Steep Residential
12 / 12	100.0%	45.0°	Half-Pitch / Church
18 / 12	150.0%	56.3°	A-Frame / Mansard
24 / 12	200.0%	63.4°	Decorative Towers

Frequently Asked Questions

Most manufacturers require a minimum slope of 2:12 (approx 17%). However, slopes between 2:12 and 4:12 often require a double layer of underlayment to comply with code. Below 2:12, shingles are not viable.

Percentage is preferred in civil engineering and drainage planning because it directly correlates to grade. A 1% slope drops 1 unit for every 100 units of distance, simplifying calculations for pipe flow and water runoff.

No. As long as the Rise and Run use the same unit (e.g., both in inches or both in meters), the ratio and resulting percentage will be identical.