Calculate valley rafter length and angles where two roof planes intersect. Supports equal-pitch and unequal-pitch valley configurations.
Valley rafters form where two roof planes meet at an inside corner, such as where an addition intersects the main roof or where an L-shaped building creates an interior roof valley. The geometry of a valley rafter mirrors that of a hip rafter—for equal-pitch roofs, the calculations are essentially the same but oriented in the opposite direction.
This valley rafter calculator computes the line length, compound angles, and bevel cuts for valleys based on the common rafter run and roof pitch. For equal-pitch intersecting roofs, the valley rafter runs at 45° in plan view, just like a hip, and uses 17 inches as the run constant on a framing square.
Accurate valley rafter dimensions are essential for watertight roof framing. A poorly fitted valley creates gaps that compromise sheathing support and can lead to leaks at one of the most vulnerable points on a roof.
This data-driven approach helps contractors minimize rework, avoid delays caused by material shortages, and deliver projects on time and within the agreed budget.
Valley intersections are among the most complex areas to frame. This calculator takes the guesswork out of valley rafter length and cut angles, helping you achieve tight joints and proper bearing at the ridge and plate intersections. Accurate figures enable contractors to prepare competitive bids with confidence, reducing the risk of underestimating costs or overcommitting on project timelines and deliverables.
For equal pitches: Valley Run = Common Run × √2 Valley Line Length = √(Valley Run² + Rise²) Valley Pitch per foot = Rise per 17″ of run
Result: 16'-2″ valley rafter length
With a 10-ft run at 8:12 pitch, rise = 10×(8/12) = 6.67 ft. Valley run = 10×√2 = 14.14 ft. Valley line = √(14.14²+6.67²) = 15.63 ft. Adding overhang on slope brings total to about 16.2 ft.
There are two main approaches to valley framing. The traditional method uses a single valley rafter running from the plate intersection to the ridge, with valley jack rafters filling in both sides. The California method (or blind valley) lays the intersecting roof on top of the existing roof plane without cutting into it, using a support ledger.
Valleys concentrate water from two roof planes, making them high-flow areas. Code requires special attention to valley flashing and underlayment. In areas with ice dams, ice-and-water shield membrane should extend at least 24 inches on each side of the valley centerline.
Valley rafters carry significant loads because they support jack rafters from two roof planes. In snow regions, drifting can create point loads in valleys that exceed normal design loads. An engineer may specify a larger valley rafter or additional supports for critical valley spans.
For equal-pitch roofs, yes. The valley rafter length and angles are geometrically identical to a hip rafter. The difference is that a valley forms an inside corner instead of an outside corner.
When the two roof planes have different pitches, the valley no longer sits at 45° in plan. You must calculate the plan angle from the two pitches and adjust all cuts accordingly. This is complex and often left to experienced framers.
Valley rafters should be at least the same size as common rafters, and ideally one size deeper. They carry tributary loads from both roof planes and must resist concentrated loads at the valley line.
Use proper valley flashing (W-metal or woven/cut valley shingles), ensure sheathing is tight at the valley, and maintain adequate slope for water to drain. Ice and water shield membrane is recommended in cold climates.
Valley jacks are short rafters that run from the ridge board down to the valley rafter. They decrease in length at a constant increment as they approach the valley rafter intersection.
A valley board is a sheathing-support member laid in the valley between the intersecting rafters. It's used in some framing methods where the structural valley rafter is omitted and the sheathing bridges the valley directly.