What Are Roof Rafters? How Rafters Work in Roof Framing (2024)

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Dec 22, 2021

What Are Roof Rafters? How Rafters Work in Roof Framing (1)

When working in roof construction, it’s important to understand the structure of the building and how each component provides support. Rafters are a key part of roof framing and require precise measurements and cuts in order to properly distribute weight and keep the building safe.

In addition to rafters, some roof designs opt for a truss system, which uses compression to bear more weight. However, the interior ceiling design becomes more limited. Find out the key differences between rafters and trusses, plus the pros and cons of each.

How House Framing Works $75.00 USD | 1H 12M Preview Course

What is a rafter?

A rafter is a structural component of a roof on a building. Traditional rafters frame out the roof and connect to the exterior walls. This system is also called stick framing. Typically built on site, rafters are cut to 2 x 10 pieces and land on a ridge board, which runs across the length of the building. Ceiling joists are then used to connect the rafters and exterior walls. The result is a vaulted ceiling that can be filled with insulation and drywall to finish the space. Alternatively, it can be left as open space in an attic.

Types of rafters

When building a rafter roof, you’ll typically find two common types used: principal and common rafters. The principal rafters are the largest pieces used on each side of the roof structure. Common rafters are smaller and placed in between the principal rafters. Together, they provide enough structural support to serve as the frame of the roof.

There are also some lesser-used rafters including:

  • Auxiliary rafters. These are sometimes used to support a principal rafter.
  • Hip rafters. These span from the building’s outside corners to the ridge board at a 45-degree angle.
  • Valley rafters. These rafters are located at the building’s inside corners at a 45-degree angle.
  • Compass rafters. These are curved at the top (and sometimes the bottom) and are usually used for ornamentation.

Even if you don’t regularly use each of these types of rafters in your projects, it’s helpful to be familiar with them on the rare occasion they’re needed.

Rafters vs. trusses: What’s the difference?

An alternative method for building a roof is using a truss system. Rather than laying rafter boards in the shape of an A-frame, a truss roof design looks like a web of support beams that criss-cross to better distribute the weight of the wood. The bottom of the rafters are connected by beams spanning the length of the room.

Trusses use compression and tension to provide strength and stability. Additionally, they’re built off-site in a factory and then delivered to the job site. The delivery adds to the overall project cost, but trusses are usually less expensive than rafters since they’re made in bulk. One downside is that any errors at the factory can slow the project since you’re not in control of making cuts on-site. Trusses also take multiple people to install, often requiring a forklift to do the heavy lifting.

When working in roof construction, it’s important to understand the structure of the building and how each component provides support. Rafters are a key part of roof framing and require precise measurements and cuts in order to properly distribute weight and keep the building safe.

In addition to rafters, some roof designs opt for a truss system, which uses compression to bear more weight. However, the interior ceiling design becomes more limited. Find out the key differences between rafters and trusses, plus the pros and cons of each.

How House Framing Works$75.00 USD | 1H 12MPreview Course

What is a rafter?

A rafter is a structural component of a roof on a building. Traditional rafters frame out the roof and connect to the exterior walls. This system is also called stick framing. Typically built on site, rafters are cut to 2 x 10 pieces and land on a ridge board, which runs across the length of the building. Ceiling joists are then used to connect the rafters and exterior walls. The result is a vaulted ceiling that can be filled with insulation and drywall to finish the space. Alternatively, it can be left as open space in an attic.

Types of rafters

When building a rafter roof, you’ll typically find two common types used: principal and common rafters. The principal rafters are the largest pieces used on each side of the roof structure. Common rafters are smaller and placed in between the principal rafters. Together, they provide enough structural support to serve as the frame of the roof.

There are also some lesser-used rafters including:

  • Auxiliary rafters. These are sometimes used to support a principal rafter.
  • Hip rafters. These span from the building’s outside corners to the ridge board at a 45-degree angle.
  • Valley rafters. These rafters are located at the building’s inside corners at a 45-degree angle.
  • Compass rafters. These are curved at the top (and sometimes the bottom) and are usually used for ornamentation.

Even if you don’t regularly use each of these types of rafters in your projects, it’s helpful to be familiar with them on the rare occasion they’re needed.

Rafters vs. trusses: What’s the difference?

An alternative method for building a roof is using a truss system. Rather than laying rafter boards in the shape of an A-frame, a truss roof design looks like a web of support beams that criss-cross to better distribute the weight of the wood. The bottom of the rafters are connected by beams spanning the length of the room.

Trusses use compression and tension to provide strength and stability. Additionally, they’re built off-site in a factory and then delivered to the job site. The delivery adds to the overall project cost, but trusses are usually less expensive than rafters since they’re made in bulk. One downside is that any errors at the factory can slow the project since you’re not in control of making cuts on-site. Trusses also take multiple people to install, often requiring a forklift to do the heavy lifting.

Another possible downside of trusses is that they create no room for vaulted ceilings since a bottom piece must provide structural support, meaning the home needs flat ceilings. It’s also impossible to include a spacious attic, which can limit storage potential for the homeowner.

Advantages of rafters

There are several pros to using rafters instead of a truss roof.

  • More space. Using rafters allows for attic space or cathedral ceilings because there’s open space to develop as the homeowner sees fit. The upper area could even be converted into additional living space.
  • Allows for insulation. Insulation, which can be left as-is or covered with drywall, can be rolled in between the rafter beams for better energy efficiency.
  • Built on-site. Rafters are cut on-site, which means they can be cut to size and adjusted as needed.
  • Less lead time. Because rafters are cut to size at the job site, you don’t have to rely on third-party factories to cut and deliver the materials. This can also save costs on heavy load deliveries.

Overall, rafters are a time-tested solution that provide flexibility in the design process.

Disadvantages of rafters

There are a few drawbacks to using rafters:

  • More expensive. Ultimately, rafters are more expensive than trusses because they’re crafted on-site by tradespeople rather than off-site in a factory.
  • Lengthens construction time. Contributing to the cost factor is the amount of time it takes for rafters to be cut to the right size on-site, compared to trusses, which are manufactured in bulk.
  • Requires a professional. It takes skill to do the carpentry work involved with cutting rafters. With trusses, you’ll simply have your pre-cut materials delivered to the job site.
  • Weaker than trusses. While rafters are structurally sound, trusses are stronger. Not only can trusses bear more weight, they can handle a greater distance between each one.

All of these factors should be considered when choosing between rafters or trusses for a building project. Trusses typically win when price is a deciding factor. But there is a lot more design flexibility with rafters since there’s room for attic storage or cathedral ceilings.

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications, like how house framing works. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.

What Are Roof Rafters? How Rafters Work in Roof Framing (4)

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Jordan Smith is the CEO and Founder of Smith House Company, a design+build firm based in Austin, Texas. He learned to build and weld while working on the farm with his grandad, and after earning a degree in Welding and Materials Engineering, he spent the next 10 years working in the heavy construction building everything from robots to ships to offshore oil rigs before transitioning to residential construction. After spending a couple of years working with industry leading builders in Austin, Jordan and his wife Veronica struck out on their own to form Smith House Co.Smith House Co. strives to build more beautiful, functional and resilient spaces which are self sustaining and harmonize with their natural surroundings.

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What Are Roof Rafters? How Rafters Work in Roof Framing (2024)

FAQs

What Are Roof Rafters? How Rafters Work in Roof Framing? ›

A rafter is defined as one of a series of sloped structural pieces (typically wooden beams) that extend from the hip or ridge to eave, wall plate, or downslope perimeter. They're designed to support the roof deck, shingles, and everything else that goes with the roof. Rafters are the traditional way to frame a roof.

How do roof rafters work? ›

Rafters are structural elements that form the framework of a roof. They are inclined beams that extend from the ridge of the roof to the eaves and support the roof covering material, such as shingles or tiles.

How do you work out roof rafters? ›

Just multiply the rise or run by the appropriate rafter length factor corresponding to your roof pitch. For example, if your roof pitch is 30ﹾ and your run is 2.4m then reading from the table the rafter length factor is 1.15. Multiply this factor by your run which gives your rafter length.

What does a roof rafter look like? ›

Rafters are long wooden boards that form a stick framing pattern. These are typically cut on the job site into 2x10 or 2x12 beams. Rafters slope down from the peak of the roof and ceiling joists secure them to the exterior walls.

How are rafters framed to each other? ›

Rafters shall be framed not more than 11/2-inches (38 mm) offset from each other to ridge board or directly opposite from each other with a gusset plate as a tie. Ridge board shall be not less than 1-inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter.

What holds rafters in place? ›

Rafter ties are always required unless the roof has a structural (self-supporting) ridge, or is built using engineered trusses. A lack of rafter ties is a serious structural issue in a conventionally framed roof. In most homes, the ceiling joists also serve as the rafter ties.

Do I need rafter ties on every rafter? ›

A ceiling joist or rafter tie is required at every rafter.

What do roof rafters sit on? ›

The rafters sit on a wall plate which is an efficient means of spreading the load exerted by the roof structure down through the walls without creating pressure points where each rafter meets the wall.

Do rafters hold up roof? ›

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.

Are roof rafters and roof joists the same thing? ›

A rafter is a structural member that supports the roof deck. They are almost always not horizontal. A joist is a structural member that supports a structural portion of a building; usually a floor or a ceiling. They are almost always horizontal.

Do roof rafters need bracing? ›

If the bracing provided is not used entirely, positioned incorrectly or not securely fixed, it can result in a distortion or failure of individual trusses or in a worst case scenario – the whole roof so it's safe to say they're pretty important!

What is the space between rafters on a roof? ›

Roof rafters shall be spaced either 16 inches or 24 inches apart (on-center (o.c.). The roof design is based on a simple gable end roof with the ridge line at the center of the building width, or a sloped shed type.

What connects two rafters? ›

Collar beam

These are horizontal beams connecting two rafters that intersect at the ridge of the roof. Not to be confused with the purlins.

What do roof rafters rest on? ›

A ridge board is a non-structural member that serves as a prop for opposing rafters to rest against and connect to. Ridge boards can only be used in roofs with slopes from 3:12 up to 12:12.

Do rafters have to line up with each other? ›

Rafters on opposite sides of the ridge should be installed directly opposite each other in pairs -- although, if you see a few that don't align, it's really not a defect. Rafters sometimes have to be moved a little to accommodate components of other home systems.

Do roof rafters need blocking? ›

Rafters and ceiling joists having a depth-to-thickness ratio exceeding 6 to 1 based on nominal dimensions shall be supported laterally by solid blocking, diagonal bridging (wood or metal) or a continuous 1-inch by 3-inch (25 mm by 76 mm) wood strip nailed across the rafters or ceiling joists at intervals not exceeding ...

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