Valley Rafter Calculator

About the Valley Rafter Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the common rafter run of the intersecting roof plane.
2. Enter the roof pitch (rise per 12″ of run).
3. Enter any overhang at the valley eave.
4. Read the valley rafter line length, total length, and cut angles.
5. For unequal pitches, enter the secondary pitch for adjusted calculations.

Example Calculation

Tips & Best Practices

• Valley rafters carry the load from valley jack rafters on both sides—size them larger than common rafters.
• Install valley rafters before jack rafters so clients bearing against the valley are properly supported.
• Use a strip of metal flashing (valley flashing) over the valley before sheathing for extra leak protection.
• The valley rafter backing bevel is the same as the hip backing bevel for equal pitches.
• For unequal-pitch valleys, the geometry changes—the valley no longer sits at 45° in plan view.
• Check that valley jacks are cut with the correct compound angle before nailing to avoid gaps in sheathing.

Valley Framing Methods

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.

Drainage at Valleys

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.

Structural Considerations

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.

Frequently Asked Questions

• Is a valley rafter the same calculation as a hip rafter?

  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.

• How do I frame an unequal-pitch valley?

  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.

• What size lumber for a valley rafter?

  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.

• How do I prevent valley leaks?

  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.

• What are valley jack rafters?

  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.

• Can I use a valley board instead of a valley rafter?

  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.