Five Common Types of Rafters Used for Roof Framing

The roof framing system relies on precise rafter types to transfer loads from the roof deck to the walls. Understanding the five common rafter types helps builders choose the right configuration for structural integrity, attic space, and installation efficiency. This article outlines each rafter type, where it’s best used, typical dimensions, and practical considerations for installation and cut details.

Common Rafters

Common rafters, also called plain rafters, run from the ridge to the wall plate and form the main slope of the roof. They support the roof deck and are the most frequently used rafter type in simple gable and shed roofs.

• Structure: Long, straight members angled to meet the ridge and wall.
• Position: Primary load-bearing members on each side of the roof plane.
• Cut details: Typically require birdsmouth cuts at the wall plate and a precise rise-run calculation for each bay.
• Applications: Standard pitched roofs; versatile in many residential designs.

Hip Rafters

Hip rafters extend from the corners of a roof up to the hip ridge, forming the sloping edges of a hip roof. They handle distinct shear forces where two roof planes meet at an external corner.

• Structure: Sloped members that run diagonally from the intersection of two planes toward the hip ridge.
• Position: Located along the outside edges of a hip roof, supporting shorter internal rafters.
• Cut details: Require precise bevels to match the roof planes; often cut with mitered joints at the hip and jack connections.
• Applications: Essential for hip roofs to maintain the continuous slope around the building footprint.

Valley Rafters

Valley rafters run from the valley intersection of two roof planes down to the eave. They carry loads from two planes merging at a valley and distribute them to the supporting frame below.

• Structure: Typically steeper than common rafters to carry combined loads from intersecting slopes.
• Position: Placed where two roof sections meet, commonly forming a “V” shape.
• Cut details: Valley cuts must align with both adjoining rafters; careful layout prevents misalignment and water pooling.
• Applications: Critical in roofs with intersecting wings or dormers, ensuring structural continuity at the valley.

Jack Rafters

Jack rafters, also known as dropped or secondary rafters, span from the ridge to a point along a common rafter or hip/valley rafter rather than to the wall plate.

• Structure: Shorter than common rafters, connecting the ridge line to the other primary rafters.
• Position: Used to fill gaps where roof geometry changes or to create soffits and overhangs.
• Cut details: Require precise end cuts and sometimes notches to seat correctly against adjacent members.
• Applications: Common in complex roof geometries and where attic space is limited.

Purlins

Purlins are horizontal or nearly horizontal members laid across the rafters to provide additional roof deck support and spacing for large or complex roof systems.

• Structure: Can be wood, steel, or engineered wood; they sit perpendicular to the rafters and tie the rafter system together.
• Position: Placed across rafters, typically in longer spans or multi-bay roofs.
• Cut details: Notches and fastener patterns must align with both rafters and deck sheathing.
• Applications: In business or residential constructions with long, open spans or non-standard roof shapes.

Key Considerations For Choosing Rafter Types

Selecting the appropriate rafter type depends on roof geometry, span, loads, and attic space goals. Structural integrity hinges on proper sizing, connection details, and adherence to local building codes. Environmental factors such as wind and snow loads influence rafter choice and spacing. In complex roofs, a structural engineer may specify a combination of rafter types, including hip, valley, and jack rafters, to optimize performance.

Practical Installation Tips

• Accurate layout: Begin with precise measurements for ridge height, wall plate heights, and bay spacing to avoid fit issues later.
• Birdsmouth cuts: Ensure clean, square cuts at the wall plate for common rafters to promote proper seating and load transfer.
• Bevels and joints: Use correct bevel angles for hip and valley rafters to maintain correct roof plane intersections.
• Secure connections: Use approved fasteners and brackets, especially where rafters meet hips, valleys, or purlins, to enhance wind resistance.
• Code compliance: Verify local codes for allowable spans, rafter sizes, and spacing, and obtain necessary permits before construction.

Understanding these five common rafter types helps builders design safer, more efficient roof systems. By aligning each rafter with the roof’s geometry and local code requirements, the structure achieves reliable load transfer, durable performance, and better overall quality.