Shed Roof Pitch Calculator: Solve the Run, Rise, and Rafter-Length Inputs Before Cuts
For a shed roof, record the ledger height, plate height, and total horizontal run first. Subtract the heights to get the rise, then divide rise by horizontal run to describe the roof plane. That geometry can calculate pitch and diagonal length, but it does not by itself lay out a birdsmouth, a plumb cut, or a roof covering.
Inputs a shed roof calculator needs
Use heights measured from the same reference level and the horizontal distance between their relevant bearing locations. Keep every measurement in one unit. The height difference is the vertical rise; the horizontal distance is the run. These three records describe one right triangle.
| Record | Use in the geometry | Keep separate from it |
|---|---|---|
| Ledger height | One endpoint for finding the vertical difference. | Its attachment and flashing details. |
| Plate height | The other endpoint for finding the vertical difference. | Its framing and bearing details. |
| Total horizontal run | The horizontal side of the same roof-plane triangle. | Any later layout decision that changes where a cut or overhang begins. |
Calculate the geometry in a visible order
- Calculate
rise = ledger height - plate height, using the magnitude of the difference. - Calculate
slope = rise / horizontal run. - Express pitch as
(rise / horizontal run) x 12if an X:12 format is useful. - Calculate the triangle's diagonal as
sqrt(rise^2 + horizontal run^2).
The roof pitch calculator will convert matching rise and horizontal-run inputs into pitch, angle, percent slope, and rafter factor. For a separate run-to-diagonal check, use the roof pitch multiplier chart.
Rafter length is a geometry result, not a cut list
The diagonal above is the length of the recorded right triangle. Before treating it as a rafter dimension, identify whether the recorded run already accounts for the project’s actual bearing points, ridge or ledger condition, and any intended overhang. A different defined run produces a different diagonal even when the roof plane’s pitch is unchanged.
A reported shed-roof question combined ledger and plate heights with a total run, then also asked for birdsmouth and plumb-cut dimensions. That is a useful distinction: the first three inputs can establish a triangle, while cut layout needs the additional framing decisions that define the bearing and cuts. See the source discussion for that reported scenario.
Do not let a pitch result decide the roof covering
A calculator describes the roof plane; it cannot approve a roofing assembly. In a separate reported low-slope roof discussion, participants raised concern about heavy rain backing into seams on one stated panel roof. That is context for keeping drainage path, product system, and current manufacturer instructions outside a geometry result, not a universal roof rule. Read the source discussion, then consult the named product documentation and applicable project requirements.
Shed roof pitch calculator FAQ
Can ledger and plate heights find shed roof pitch?
Yes, when they are measured from the same datum and describe the same roof plane. Their difference is the rise; pair it with the matching horizontal run to calculate slope.
Does a shed roof pitch calculation lay out a birdsmouth?
No. It establishes the roof triangle. Birdsmouth layout requires framing choices about the bearing and cut, beyond the triangle's rise and run.
Does the calculated pitch select a roof covering?
No. Pitch is a geometric input. A covering decision also depends on the named product system, its current instructions, drainage conditions, and applicable project requirements.
Method and source notes
- Geometry method: use one roof plane, matching measurement units,
rise = |ledger height - plate height|, anddiagonal = sqrt(rise^2 + run^2). - The two linked community discussions provide reported situations only. They do not replace project drawings, manufacturer instructions, or qualified design and framing advice.