Rafters Vs Joists

Rafters Vs Joists – Everything You Need to Know

When it comes to constructing a building or any other structure, one of the crucial components that must be given careful consideration is the support system. A building’s support system plays a crucial role in ensuring its durability, stability, and safety. Two common types of support systems found in most buildings are rafters and joists.

In this article, we will delve into the differences between rafters and joists, how they function in their respective roles, and which one is best suited for your specific project.

What are Rafters?

Rafters are structural members used in pitched roofs for supporting the roof’s weight and transferring it to the walls of the building. Rafters run from the ridge or the peak of the roof to the eaves, which are the edges of the roof. The sloped design of rafters allows for proper water drainage, making them ideal for roofing.

Rafters are usually made from wood or metal, and they must be strong enough to withstand the weight of the roof, any potential snow loads, and the wind. They come in different sizes and lengths depending on the roof’s span and pitch. Some common types of rafters include common rafters, hip rafters, valley rafters, and jack rafters.

What are Joists?

Joists are horizontal support members that run parallel to each other and are used to support the floor or ceiling of a building. They are typically made from wood, steel, or concrete, and their main function is to distribute the weight of the floor or ceiling evenly across the building’s walls or beams.

Joists come in various sizes and lengths depending on the load they are designed to bear. The size and spacing of the joists depend on the floor’s span, the type of building materials used, and the required deflection criteria. Some common types of joists include open-web steel joists, engineered wood joists, and timber joists.

Rafters Vs. Joists – Major Differences

1. Usage

Rafters are used in pitched roofs because of their sloping design. They’re designed to transfer the weight of the roof to the building’s walls, which in turn transmit the load to the substructure. Joists, on the other hand, are used to support the floors and ceilings of a building.

2. Orientation

Rafters are oriented diagonally, running from the ridge of the roof to the eaves. They are sloping and typically consist of a triangular cross-section. Joists, on the other hand, are oriented horizontally, running parallel to one another to create a level surface.

3. Shape and Size

Rafters usually have a triangular shape that’s wider at the base and narrow at the top. Its size varies depending on the span and pitch of the roof. On the other hand, joists have a rectangular cross-section and can be smaller in size than rafters, depending on the load they need to support.

4. Spacing

Rafters are spaced slightly wider than joists and are commonly spaced 16 to 24 inches apart. In contrast, joists are typically spaced closer together to support the floor or ceiling above.

5. Installation

Rafters are installed from the top of the roof down to the eaves of the building. They are placed on the building’s walls and then cut at an angle to create a slope. Joists, on the other hand, are installed from one end of the building to the other.

Which One is Better for Specific Projects?

The choice between rafters and joists depends on the specific project’s requirements. If you’re constructing a pitched roof, you will need rafters to support the roof’s weight and transfer it to the building’s walls. On the contrary, if you’re working on a construction project that involves floors and ceilings, you will need joists to provide support for the building’s structure.


Rafters and joists are both vital components of a building’s support system, and understanding their differences is crucial when deciding which one to use for a specific project. While rafters are used for pitched roofs, joists are used for supporting floors and ceilings. It’s important to match the support component with the project’s requirements to ensure that it remains strong, stable, and secure.

In summary, rafters and joists play a critical role in the strength and stability of a building, and choosing the right one is important for successful construction projects. Consult with a professional to determine which one best suits your project’s specifications and needs.