Pergola Roof Slope Calculator

To calculate the slope of a pergola roof, you need to know the horizontal run (span) and your desired pitch or angle. Using standard trigonometry, the formula for rise is: Rise = Run × tan(Angle). Using our pergola roof slope calculator automates this math, allowing you to instantly find the required height difference between the front and back posts.

The "best" pitch entirely depends on the roofing material you select. For solid covers like corrugated polycarbonate panels, a minimum 5% slope (roughly a 3-degree angle or 0.6/12 pitch) is required for proper drainage. For traditional asphalt shingles, a 2/12 pitch or steeper is mandatory. For open-slat pergolas, the roof can be completely flat.

If you are designing a nearly flat roof and want to avoid water pooling, a common rule of thumb is to provide a minimum fall of 1/4 inch per foot of horizontal run. However, 1/2 inch per foot is generally much safer to ensure swift water shedding and to prevent debris buildup.

If you know your pitch ratio (for example, 2/12) and your total horizontal run, you simply multiply the run by the pitch fraction. For a 12-foot run at a 2/12 pitch, the math is: 12 feet × (2/12) = 2 feet. Therefore, the total rise or height difference required is 24 inches.

While often used interchangeably, they represent the steepness in different formats. Roof pitch is specifically expressed as a fraction indicating vertical rise over a 12-inch horizontal run (e.g., 4/12). Roof slope is usually expressed as a geometrical angle in degrees or as a percentage.

To find the exact cut length of the rafter, you must know the total horizontal footprint (including overhang) and the roof angle. Using trigonometry: Rafter Length = Run ÷ cos(Angle). Alternatively, you can use the Pythagorean theorem (a² + b² = c²) if you already know the run and the total rise.

Yes, this tool doubles perfectly as a lean to roof slope calculator. Enter the horizontal distance from your house wall (ledger board) out to the front posts as the run, add your roof overhang, and calculate the required rise. This rise tells you exactly how high up the wall the ledger board must be attached relative to the front posts.

Most leading manufacturers of polycarbonate panels stipulate a minimum slope of 5% (which translates to approximately 3 degrees or a 0.6/12 pitch) to ensure effective water run-off and to prevent capillary action from pulling water up through the overlapping seams.

The overhang adds to the total horizontal footprint of the roof. If you calculate the rafter length using only the distance between the posts, your rafters will end perfectly flush with the posts. By adding the overhang to the horizontal run, the calculator ensures the generated rafter length includes that extra cantilevered span.

To calculate the angle mathematically, use the inverse tangent function based on your rise and run: Angle = arctan(Rise ÷ Run). For example, if you have a 12-inch rise over a 120-inch run, arctan(12/120) yields approximately 5.7 degrees. Or, simply use the "Total Rise" input in our roof angle calculator to get the result instantly.