About

Determining the correct roof slope is a critical step in architectural design and waterproofing. Professional roofers and architects use three distinct dialects to describe steepness: ratio, degrees, and percentage. Misinterpreting these values results in structural failure, particularly under heavy snow loads, or the installation of incompatible roofing materials. For instance, standard asphalt shingles often fail on slopes below 2:12, leading to water ingress.

This utility translates between these standards instantly. It assesses the geometric relationship between the rise (vertical height) and the run (horizontal distance). Furthermore, it evaluates the Snow Load Risk. Low-slope roofs retain snow significantly longer, increasing the dead load on trusses and necessitating specific membrane solutions like EPDM or TPO rather than traditional tiling.

Formulas

The relationship between the roof elements relies on right-angle trigonometry. We define the pitch angle θ based on the vertical rise and horizontal run.

1. Calculate Angle (Degrees):

θ = arctan(riserun) × 180π

2. Calculate Percentage:

grade = (riserun) × 100

3. Convert Degrees to Pitch (x:12):

x = tan(θ) × 12

When θ < 10°, water drainage velocity decreases efficiently, requiring distinct waterproofing protocols.

Reference Data

Pitch (Ratio)	Angle (°)	Grade (%)	Common Application	Snow Risk
0:12	0.0°	0%	Flat / Commercial	CRITICAL
1:12	4.8°	8.3%	Industrial / Sheds	HIGH
2:12	9.5°	16.7%	Low-Slope Residential	HIGH
4:12	18.4°	33.3%	Standard Ranch	MODERATE
6:12	26.6°	50.0%	Conventional Gable	LOW
9:12	36.9°	75.0%	Steep Residential	LOW
12:12	45.0°	100%	Tudor / Gothic	MINIMAL
18:12	56.3°	150%	Mansard / A-Frame	MINIMAL

Frequently Asked Questions

The 2:12 pitch (approx 9.5 degrees) is widely considered the minimum slope for standard asphalt shingles. Below this threshold, wind-driven rain or ice dams can push water underneath the shingles. Codes typically require double-layer underlayment for pitches between 2:12 and 4:12, and specialized membranes (like rubber or bitumen) for anything flatter.

Steeper roofs increase surface area relative to the building footprint, requiring more materials. Additionally, slopes above 7:12 often require "steep charge" labor costs because workers need safety harnesses and scaffolding, which slows down installation.

While often used interchangeably, "pitch" technically refers to the ratio of total rise to total span (width of the building), whereas "slope" refers to the ratio of rise to the run (half the span). In modern US carpentry, however, "pitch" is almost universally used to describe the rise per 12 inches of run (e.g., 4:12).

No. A 100% slope means the rise equals the run (e.g., 12 inches up for 12 inches over). This corresponds to a 45-degree angle. A vertical wall has an undefined or infinite slope.