What Is a Rafter?

A rafter is a structural component used in the construction of a roof. It is a sloping beam that supports the weight of the roof and transfers it to the walls or other supporting structures of a building. Rafters are typically installed in a series running from the ridge or peak of the roof down to the eaves.

Rafters are typically made of wood or steel and are spaced at regular intervals across the width of the roof. They are designed to carry the load of the roof covering, such as shingles or tiles, as well as any additional weight from snow, wind, or other environmental factors.

The shape and size of rafters can vary depending on the design and requirements of the roof. Common types of rafters include common rafters, hip rafters, valley rafters, and jack rafters, each serving a specific purpose in roof construction.

Overall, rafters play a crucial role in providing structural support and stability to a roof, ensuring it can withstand the forces it may encounter and maintain the integrity of the building.

Basic Principles of Rafters Design

When designing rafters, there are several basic principles that need to be considered to ensure the structural integrity and stability of a roof. Here are the key principles:

• Load Calculation: Determine the design loads that the rafters need to support. These loads include the weight of the roof covering materials, snow loads, wind loads, and any additional loads such as solar panels or HVAC equipment. The load calculations are typically based on local building codes and standards.
• Span and Pitch: Determine the rafter span, which is the horizontal distance between the supporting walls or beams. The span and the pitch (slope) of the roof will influence the size and spacing of the rafters. A steeper pitch may require shorter spans and larger rafters to handle the increased loads.
• Rafter Size: Based on the calculated loads and span, select an appropriate rafter size. Rafters are typically made of dimensional lumber such as 2x4, 2x6, or larger members, depending on the load requirements. The size of the rafter is determined by its depth (height) and width.
• Spacing: Determine the spacing between rafters. This is influenced by factors such as the rafter size, the roof covering material, and local building codes. The spacing should be such that it adequately supports the loads without excessive deflection or sagging.
• Bracing and Support: Rafters need to be adequately braced and supported to prevent them from buckling or twisting. This includes installing collar ties or rafter ties near the top of the rafters to resist spreading and sagging. Additional support may be required at mid-span with the use of ridge beams or purlins.
• Connections: Ensure proper connections between the rafters and the supporting walls or beams. This typically involves using appropriate hardware such as joist hangers, rafter ties, and brackets. The connections should be strong and capable of transferring the loads between the rafters and the supporting structure.
• Considerations for Special Situations: Some roof designs may require additional considerations. For example, hip roofs, where all sides slope down to the walls, may require additional bracing and structural elements to handle the complex load paths.

Types of Rafters

Rafters are an essential component of a roof structure, providing support and stability. They are inclined beams that run from the ridge of the roof down to the eaves. The specific type of rafter used in a roof depends on various factors such as the design, load requirements, and architectural style.

Here are some common types of rafters:

• Common Rafters: Also known as principal rafters or main rafters, common rafters are the primary rafters that run from the ridge board down to the wall plates. They form the main framework of the roof structure.
• Hip Rafters: Hip rafters are diagonal rafters that connect the corners of the building to the ridge. They are typically used in hipped roofs, which have sloping ends and a pyramid-like shape.
• Valley Rafters: Valley rafters are diagonal rafters that run along the internal intersection of two roof planes forming a valley. They connect the ridge to the eaves along the valley line.
• Jack Rafters: Jack rafters are shorter rafters that run from the ridge or hip rafters to the wall plates but do not extend all the way to the eaves. They are positioned between the common rafters and provide support for the roof covering.
• Cripple Rafters: Cripple rafters, also known as cripples or jack legs, are shorter rafters that are installed above or below an opening in the roof, such as a dormer window or chimney. They help transfer the load around the opening.
• Purlins: Purlins are horizontal beams that are laid across the rafters to provide additional support and prevent them from sagging. They are commonly used in roofs with longer spans or in areas with heavy snow loads.
• Collar Ties: Collar ties are horizontal members that connect two opposing rafters near the ridge. They help to resist the outward thrust and provide additional stability to the roof structure.
• Rafter Ties: Rafter ties, also known as ceiling joists, are horizontal members that connect the lower ends of opposing rafters at the bottom of the roof. They prevent the roof from spreading apart and help distribute the load to the walls.