valley rafter

A valley rafter is a structural member that spans diagonally from the wall plate or supporting beam to the ridge, positioned where two roof slopes meet at an inside angle. Unlike common rafters that run perpendicular to the ridge, the valley rafter sits in the valley itself, bearing the concentrated load where water naturally drains and where two roof planes intersect. It carries shear and bending stress more severe than ordinary rafters because it must support not just its own roof section but also help distribute the loads from both adjacent slopes meeting at that line.

Valley rafters are typically larger in dimension than common rafters in the same roof because of these combined loads. A roof valley on a simple saltbox or intersecting gable might use a single 2x10 or 2x12 valley rafter, depending on span and snow load. The rafter sits at an angle that bisects the valley angle, typically 45 degrees on standard rectangular roof intersections, though this varies with roof pitch and geometry. The valley rafter must be precisely cut and positioned because it dictates the angles for jack rafters and secondary members that frame against it on both sides.

Two main variants exist: the open valley rafter leaves the valley unsealed for visual effect and drainage clarity, while the closed valley (or woven valley) rafter is covered by shingles from both roof planes overlapping in a specific pattern. In closed valleys, the rafter itself remains hidden, but its correct sizing and installation directly affects how well water sheds. In open valleys, the rafter is often a finished material like copper or galvanized sheet metal shaped as a gutter, not a structural lumber member, though the structural valley rafter still sits underneath supporting the roof framing.

Valley rafter failures typically stem from undersizing relative to load, inadequate bearing support at the ridge or wall plate, or poor connection at intersections where jack rafters join. Settlement or deflection in the valley rafter causes cracking in drywall below and visible sagging at the roof line. Water penetration at valley leaks is common because the valley collects and concentrates runoff; improper flashing or shingle installation aggravates this. In older framing, valley rafters installed with insufficient depth or lateral bracing can twist or bow under load, especially in high-snow regions.

The term "valley rafter" comes directly from its location in the valley of the roof; it is also called a "valley beam" when it carries multiple jack rafters, or sometimes confused with a "hip rafter," which is its counterpart on outside angles. The distinction matters for load calculation and joist layout. Carpenters must understand that the valley rafter is a primary structural element, not a secondary trim member, and its installation precedes and determines the framing of dozens of smaller members around it.