About this calculator
This calculator determines pitch angle and height differences for rooftop equipment installations on pitched roofs. It returns the total height difference across the curb and the high- and low-side offsets from the curb centerline needed to create a level mounting surface. Use it when preparing equipment submittals or installation drawings for rooftop units on non-flat roof sections.
Use when You need to determine equipment mounting requirements on a pitched roof.
Formula Angle = arctan(Rise ÷ Run); Height Difference = Pitch × Curb Width
Variables
Rise Roof rise value (e.g. 4 in a 4:12 pitch)Run Roof run value (default 12)Curb Width Equipment curb width in inches
Rise
Roof rise per the run below
Run (default 12)
Horizontal run in inches (typically 12)
Curb Width (in)
Equipment curb width perpendicular to ridge
Pitch —
Angle (°) —
High Side (in) —
Low Side (in) —
Total Diff (in) —

How is pitched roof equipment mounting calculated?

When HVAC rooftop units are installed on pitched (sloped) roofs, the equipment curb must be built with different heights at each corner to create a level mounting surface. The height difference across the curb depends on the roof pitch and the curb width in the direction of the slope. Roof pitch is expressed as rise over run (e.g., 4:12 means 4 inches of rise per 12 inches of horizontal run). The pitch angle in degrees is: Angle = arctan(Rise ÷ Run). The height difference across the curb is: ΔH = Pitch × Curb Width, where Pitch is the slope expressed as a decimal (rise/run). For example, a 4:12 pitch across a 48-inch curb requires a height difference of 16 inches between the high and low sides. This information is critical for equipment submittals and installation drawings to ensure proper condensate drainage and structural support on non-flat roof sections.

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