So exactly what is the difference between a rafter and a joist? Many of us have likely heard these terms without really understanding what they are and what they do. Like many other components, rafters and joists are weight-bearing, which means they hold something up like a roof, ceiling, or floor.
Rafters and joists are not only some of the most common weight-bearing components but also vitally important. Rafters and joists are used in a style of carpentry the professionals call “stick building”.
Today we will discuss what rafters and joists are, what they do, and where they might be used in a structure.
What Is a Rafter?
A rafter is a component of a roof that supports the roof decking and roof covering, like shingles or tiles. Rafters will typically have some amount of pitch, meaning they slope towards the ground.
Essentially any structure with a pitched roof will incorporate rafters in some form. Normally, a rafter will extend from the very top of a sloped roof to the lowest edge of a roof, usually where the gutters are located.
Rafters provide the structural support of the roof while shedding water away from the structure. The use of rafters dates back hundreds of years and can be found in ancient buildings still intact.
In conjunction with other roof components such as collar ties, strongbacks, ridge boards, and joists, rafters create the shape and structural integrity of a roof.
What is a Joist?
Joists are weight-bearing components of construction framing that provide structural support for ceilings and floors. Joists are usually installed horizontally and spaced evenly between two weight-bearing vertical walls.
In cooperation with rafters, joists typically support not only the weight of the structure but also hold the structure together. As part of a roof, joists provide the horizontal support for ceiling drywall and tie rafters together.
In a floor, joists provide the horizontal support for the subfloor, floor coverings, and serve as the connection point between a structure’s masonry foundation and the subfloor.
Are Rafters and Joists the Same Thing?
No, but they do work together to provide support for a roof. A rafter provides the slope, or pitch of a roof and serves as a connection point for roof decking and shingles.
In roof framing, joists connect rafters together at ceiling level to prevent them from separating under the weight of the roof. In older construction, rafters and joists were an integral part of essentially any wooden structure.
Before trusses were commonly used in residential construction, rafters and joists, along with other framing components were standard. This is most evident in a ranch-style home, which is why the design became so popular.
The use of rafters and joists usually requires more bearing points for support, which is why most ranch-style homes have a weight-bearing wall near the center of the structure. In modern construction, trusses often replace the majority of rafters and joists, but both rafters and joists are still commonly used either alone or in conjunction with trusses.
Where Would I Use a Rafter?
In a basic four-sided structure, rafters would typically extend from the lowest part of the roof to the highest. This provides structural support for the entire roof from the very top (ridge) to the very bottom (fascia).
Rafters are perfect for any roof with a pitch including commercial buildings, residential homes, barns, and storage buildings. Rafters are usually made on-site and installed in rows with even spacing. Rafters can be made to accommodate roofs with very steep pitches as well as buildings with nearly flat roofs.
How Do I Make a Rafter?
Rafters start out as a simple pieces of lumber. In most situations, this is a small board like a 2” x 4”, but some homes will need a rafter built from much larger lumber like a 2” x 12”. As mentioned earlier, these boards dictate the pitch or slope of the roof.
This pitch is determined by the angle cut into both ends of the rafter. Pitch is just a ratio of height to length, so to make a rafter the carpenter determines the required roof pitch based on the plans and using a saw, cuts a ridge pocket on the highest end and a plate notch (also called a “bird’s mouth”) on the lowest end.
The rafter is then nailed into place using at least three (3) #12 nails or larger on both ends. The process is then repeated, with additional rafters added (usually every 24”) from one end of the structure to the other.
These rafters are then tied together using other components like a ridge board at the very top and sub-fascia at the bottom. This method keeps the rafters evenly spaced, straight, and interconnected.
Where Would I Use a Joist?
Joists can be used as part of a roof or floor, depending on the application. Joists are usually installed parallel to the ground and evenly spaced like rafters. This provides consistent support for floors and ceilings while creating a bearing point for weight bearing walls.
The size of the joists will be determined by the distance between bearing points. For example, the width of the joist will be determined by the distance between two bearing supports (like piers or foundation walls) and the species of lumber being used. Span tables (a chart showing the strength of certain wood species) are used to determine this distance, based on the amount of weight these boards can support.
How Do I Make a Joist?
Joists are quite simple to construct, as they are typically just cut to length. To illustrate, let’s say the building is 24’ wide with bearing support (either a foundation wall or pier) all along the perimeter and in the center.
This means a joist must span 12’ outward from the center in both directions. Professionals will consult the span tables and may determine that one 2” x 12” spruce joist will span this distance if placed 16” apart. The carpenter will then simply cut the joist to length and nail it on edge to the band joist and the sill plate of the masonry foundation.
Can I Use Rafters and Joists Together?
Yes. In fact, in most cases, they will be used together to form a very strong structure. Other components like collar ties augment this design to provide additional strength and support. In a typical roof system, rafters provide vertical support for the roof and ceiling joists provide horizontal support.
This prevents the roof from squatting under the weight of the roof system and shingles or tiles. In recent decades the use of trusses has become more and more common, replacing the rafter/joist configuration. This is often due to the speed, customization, and ease of installing trusses.
How Are Rafters and Joists Different From Trusses?
Although both techniques provide the same basic function, trusses are much stronger than rafters and joists. A roof truss can be described as a rafter, cripple, joist, and collar tie all in a single, preformed unit. Where rafters and joists are made on-site, trusses are built in a factory setting.
Design engineers use computer software to design simple to elaborate configurations of trusses, based on the architectural plan. Trusses can be designed to span distances that are not possible using rafters and joists.
These are designed to replace rafters and joists with components such as chords, struts, and gussets. Trusses are designed similarly to an expansion bridge, where the structure uses the interior design of the truss for support. This can greatly reduce the number and size of bearing supports required, similar to an expansion bridge crossing a river.
Trusses can also be built to support a floor. They offer a wide range of benefits, including greater design flexibility and strength. For example, in a structure using floor joists, the maximum span will be determined by the width of the joist.
Since joists are milled from a single tree, their size is limited. With trusses, however, the individual components can be made from small lumber, allowing for much more design freedom. Trusses can be built to essentially any size required for the application and can even incorporate special design features. For example, joists should remain intact to preserve their structural integrity, because if a notch or hole is needed it will weaken the joist.
Trusses do not have this limitation, so they can be designed to include a chase for things like HVAC ductwork and cables. Trusses can also be any size, which makes them ideal for custom designs that may need special consideration.
Are Rafters and Joists Better Than Trusses?
Generally speaking, trusses offer more design options than a stick-built structure using rafters and joists. Trusses can be designed for nearly any application and can incorporate many custom features.
However, trusses must not be altered in any way after they are built, so when special considerations are required, often rafters and joists are a better option. Rafters and joists are still commonly used in modern construction, especially when the design is unique, or has design elements that must be customized.
In practice, most new construction will combine the convenience and speed of trusses where possible, and then employ rafters and joists as needed for a strong, durable building.