Joist Vs Rafter

As a homeowner or builder, choosing the right framing system for your structure can make or break your project. Two common types of framing systems used in construction are the joist and the rafter. Though they are both used to support roofing and flooring systems, there are key differences between the two. In this article, we will explore the differences between joists and rafters, their strengths and limitations, and frequently asked questions.

What are Joists?

Joists are horizontal, load-bearing structures that are typically used to support decking, flooring, and ceilings. They are usually made from wood or steel and run perpendicular to the walls, distributing the weight of the floor or roof to the load-bearing walls or supports. Joists come in various shapes and sizes, but the most common types are standard, engineered and SIP (structural insulated panels) joists.

Standard Joists

Standard joists are made from dimensional lumber such as 2×4 or 2×6, which are the most popular dimensions used to build floors and ceilings. They are inexpensive and widely available, making them the go-to option for many homeowners and builders.

Engineered Joists

Engineered joists are a more recent invention and are designed to provide the same strength as traditional lumber joists but at a lower cost. They are made from composite materials such as laminated veneer lumber (LVL) and steel, which are often pre-constructed and designed to span longer distances. Engineered joists are also more uniform in size, making them easier to work with than standard joists.

SIP Joists

SIP (structural insulated panels) joists are typically used for supporting floors and roofs. They are made up of two outer skins of oriented strand board (OSB) and a core of rigid insulation such as polyisocyanurate or polystyrene. SIP’s are more energy-efficient than traditional joists because of their airtight design, making them an excellent choice for passive homes.

What are Rafters?

Rafters, on the other hand, are inclined, load-bearing structures that run from the ridge beam to the wall plate. They support the weight of the roof and transfer it to the load-bearing exterior walls. Rafters are typically made from wood but can be made from steel or concrete. They come in different shapes and sizes, but the most common type is the common rafter.

Common Rafters

Common rafters are typically made from dimensional lumber and are designed to support the weight of the roof. They extend at an angle from the ridge beam to the outside wall, transferring the weight of the roof to the exterior walls.

Strengths of Joists

Joists are commonly used in the construction of decks, floors and ceilings because of their ability to span lengths of up to 20 feet without sagging. They are also more cost-effective than rafters due to their simple design and materials.

Strengths of Rafters

Rafters are typically stronger and able to hold more weight than joists. They can be used to create different roof shapes, such as the gable or hip, and are ideal for constructing vaulted ceilings.

Limitations of Joists

Joists are generally limited to the spans they can cover without additional support. In addition, traditional lumber joists may sag over time, making them less desirable in certain applications.

Limitations of Rafters

While rafters can span longer distances, they require additional support in the form of ridge beams and load-bearing walls. This can add to the overall cost of construction and limit design flexibility.


Q: How do I determine the size of joists or rafters for my project?

A: The size of joists or rafters required is dependent on several factors, including the load the structure will carry and the spacing between supports. It’s best to consult with a structural engineer or architect to determine what size and type of framing system are best for your project.

Q: Can I use joists or rafters interchangeably?

A: No, joists and rafters are separate framing systems with different functions. Joists are used to support flooring and ceilings while rafters support the roof. While they may have some similarities, using one system in place of the other can result in structural failure.

Q: What is the maximum span of a joist or rafter?

A: The maximum span of a joist or rafter typically depends on its size and spacing. For example, a 2×8 joist spaced 16 inches apart could span up to 12 feet while a 2×4 rafter spaced 24 inches apart could span up to 8 feet. Always consult with a structural engineer or architect to determine the maximum span for your project.


In conclusion, joists and rafters are both integral framing systems used in the construction of floors, ceilings, and roofs. While they have some similarities, they each have unique strengths and limitations that should be considered before choosing which system to use. Always consult with a professional when undertaking any structural project to ensure that the chosen system can support the intended load and span.