Measure valley rafters from span, pitch, and overhangs. See run, rise, factors, and adjusted lengths. Build cleaner roofs using clearer numbers and visual checks.
Use one consistent unit for all linear values.
Effective Common Run = (Span ÷ 2 or Direct Common Run) + Overhang
Pitch Ratio = Pitch Rise ÷ Pitch Base
Plan Factor = 1 ÷ sin(Included Angle ÷ 2)
Valley Factor = √(Plan Factor² + Pitch Ratio²)
Rise = Effective Common Run × Pitch Ratio
Plan Run = Effective Common Run × Plan Factor
Theoretical Valley Length = Effective Common Run × Valley Factor
Ridge Deduction = (Ridge Thickness ÷ 2) × Valley Factor
Cut Length = (Theoretical Length - Ridge Deduction) + Tail Allowance
Final Order Length = Cut Length × (1 + Waste% ÷ 100)
These formulas work best for equal-pitch roof valleys and a centered ridge layout.
| Example | Unit | Span | Overhang | Pitch | Included Angle | Theoretical Length | Final Order Length |
|---|---|---|---|---|---|---|---|
| Garage roof | ft | 24.000 | 1.500 | 6/12 | 90.000° | 20.250 ft | 21.427 ft |
| Workshop roof | ft | 30.000 | 2.000 | 8/12 | 90.000° | 26.579 ft | 28.656 ft |
| Shed roof | m | 8.000 | 0.400 | 5/12 | 100.000° | 6.029 m | 6.264 m |
A valley rafter runs along the inside roof corner where two sloping planes meet. It supports jack rafters from both sides and usually ends up longer than a common rafter because its path is diagonal in plan.
The included angle changes the plan path of the valley. A tighter or wider corner changes the diagonal distance before pitch is even considered, so valley length changes even when span and pitch stay the same.
No. This version assumes equal-pitch roof planes meeting in a standard valley. Unequal pitches create a different spatial geometry and require a more detailed framing layout or a dedicated unequal-pitch valley calculation method.
Ridge deduction accounts for material that should not be included in the cut member because the ridge board or ridge beam occupies part of the theoretical centerline length. It helps move from geometry toward a practical framing cut.
Include overhang when you want the valley to extend to the full tail location. Leave it out if you only need the body length from the wall line to the ridge area. Use one consistent approach across your project.
Final order length is the cut length after a waste percentage is added. It is useful for estimating what to buy or stage on site, especially when trimming, checking crowns, or allowing for end waste.
Use feet, inches, meters, or millimeters. The calculator does not mix units for you, so every linear dimension must stay in the same selected unit from input through result.
The graph helps you see how pitch affects theoretical valley length while the run and corner angle remain fixed. That makes comparison faster when checking alternate roof slopes during planning or estimating.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.