Roofing / Panel dimensions
Roof Panel Cut-Length Calculator
Find roof sheet cut length from horizontal run and rise or pitch. Include eave extension, ridge setback, and end laps for panels joined along the slope.
Find your panel cut length
Measure one roof slope, then account for eave extension, ridge setback, and any joints along the slope.
Slope geometry
Horizontal distance from the eave reference to the ridge reference on ONE slope; exclude the extension below.
Each method retains its last entered value. Check the selected rise or pitch after switching.
Vertical difference between those same eave and ridge references.
Allowances and panel joints
Added beyond the eave reference. Enter 0 if already included in your run.
Deducted below the ridge reference. Use the project or manufacturer detail.
One continuous panel, or equal-length panels joined end to end in a single strip. This is not the count across the roof.
Panel cut length
- Cut length per panel · equal lengths
- 4.522136 m
- Total cut length · one strip
- 4.522136 m
- Panels along slope
- 1
Geometry and overlap
- Base slope length
- 4.472136 m
- Covered length after allowances
- 4.522136 m
- Total extra length in end laps
- 0 m
- Roof angle
- 26.5651 °
Method
How to calculate roof panel length
Measure the horizontal run R and vertical rise H between the eave and ridge references for one slope. Do not enter the full building span for a single side of a gable roof. If using pitch p:12, the rise is R × p ÷ 12.
- Base slope length S = √(R² + H²)
- Covered length C = S + eave extension − ridge setback
- Total end-lap length = (number of panels − 1) × lap per joint
- Total cut length = C + total end-lap length
- Equal cut length per panel = total cut length ÷ number of panels
Extension, setback, and laps are lengths measured along the roof slope, not horizontal projections. Ridge setback must be smaller than the base slope length. For multiple panels, each lap must be shorter than the covered length so every equal panel extends beyond its overlap.
Worked example
A 4 m run with 6:12 pitch
A 4 m horizontal run at 6:12 pitch has a 2 m rise and a base slope length of √20 = approximately 4.472136 m. Add a 0.1 m eave extension and subtract a 0.05 m ridge setback to obtain 4.522136 m of covered length.
One continuous panel needs that same geometric cut length. For two equal panels with a stated 0.2 m end lap, the total cut length is 4.722136 m and each panel is approximately 2.361068 m. Two such panels minus one overlap cover 4.522136 m.
The example allowances and overlap are illustrative, not product recommendations. Do not round a fabrication order down from the displayed estimate; confirm supplier increments and final site dimensions.
End laps and side laps serve different dimensions
End laps occur where panels join from eave toward ridge. Side laps occur between neighboring strips across the roof. Use the Roof Sheet Effective Width Calculator for usable width and the number of strips across a roof.
This tool reports one strip only. It does not multiply by roof width, combine both sides of a gable, position joints on supports, or select stock lengths. Equal-length panels may not suit your support layout.
Assumptions and limitations
- One planar, constant slope with rectangular panels running up the slope.
- No hip or valley cuts, curved roofs, penetrations, ridge caps, or flashings.
- End-lap length is supplied by the user; no minimum slope or lap requirement is inferred.
- Cutting kerf, wastage, thermal movement, manufacturing tolerances, and order rounding are excluded.
- Switching units preserves entered physical lengths. Switching rise/pitch methods recalls that method’s last input, which should be checked.
Roof panel cut-length FAQ
Is horizontal run the full width of the building?
No. It is the horizontal distance for the single roof slope being calculated. For a symmetrical gable it is often half the reference span, but use the actual eave and ridge positions in your drawings.
What does a 6:12 pitch mean?
The roof rises 6 units for every 12 horizontal units. The ratio is unitless: it works with metres, feet, or any consistent length unit.
Should I enter a horizontal overhang as the eave extension?
No. The extension field expects its length along the slope. Alternatively, include the horizontal overhang in the run and the corresponding rise, then enter zero extension to avoid counting it twice.
Why divide by the number of panels?
This version assumes equal cut lengths in one strip. Total covered length plus one overlap per joint is divided evenly. A real cut schedule may need unequal panels to align joints with supports.
Does one panel need an end-lap allowance?
No. A continuous panel has no end joint. The calculator hides the lap input and applies zero total end lap when panel count is one.