What Are Rafters?
Dec 22, 2021
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.
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.