Roof joist, rafter, and truss are common terms in the construction industry. But what do they mean? And how do they differ from one another? Let’s look at this handy guide to help you get your bearings straight.
Roof joists are the horizontal support beams that run parallel to the roof’s top edge. The rafters are the boards that connect the ends of two adjacent joists. They form the ridges of a roof when you look up from below. Trusses are structural members that form triangular units used to support the roof.
Read on to learn more about roof joists, rafters, and trusses, including their main differences.
What Is a Roof Joist?
Horizontal Beam That Supports the House’s Roof Load
A roof joist is a horizontal beam that supports the weight of your house’s roof load. It runs perpendicular (90°) to the walls’ framing members and is fastened to them with nails or screws.
Can Be Made of Wood or Metal
Roof joists can be made of wood or metal. They are attached to rafters, which are, in turn, attached to the wall or ceiling of your home.
Roof Joist Spacing
The spacing of the joists usually depends on the type of roof you have. In all cases, however, it is important to have proper spacing so that your roof is properly supported and you don’t have any problems with leakage or other structural issues.
So to help distribute the load of the structure, a typical roof joist is spaced 16 inches apart on-center. But it can vary depending on the size and span of the roof.
The most common spacing for roof joists is 16 inches on-center, which provides adequate support for most roofs. But there are some situations where you might need to add more support by using a 24-inch-on-center spacing or even more if your building has a hefty load.
Roof Joist Span
The roof joist span is the distance between a beam’s centerline and its support. This is usually measured in feet, inches, and fractions of an inch. The span of a roof joist depends on the size, thickness, and spacing of the members used to construct the roof trusses.
A building code specifies how long a roof joist can be before you need to install additional support members called struts or rafters.
These supports are installed at regular intervals along all roof joists so that they can bear more weight without buckling under pressure from wind loads or snow loads on top of them.
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What Is a Roof Rafter?
Structural Beams That Create the Slope of the Roof on a Building
Roof rafters are structural beams that create the slope of the roof on a building. The rafters run parallel to each other and support the roof’s weight. They are usually made from two pieces of wood combined with a series of screws or nails.
Runs from the Roof’s Peak to the Wall Plate of the Building
Each rafter runs from the roof’s peak to the wall plate or barge board at the edge of the building. Some roofs have a ridge beam. A ridge beam is a horizontal beam that rests on top of the rafters and increases their strength.
Traditional Method of Roof Framing
The traditional method of framing a roof is with rafters. This is also referred to as stick framing, and a professional carpenter will cut and construct it on the job site. A rafter’s major components are as follows:
- Common rafter
- Ridge board
- Plumb cut
- Birdsmouth cut
- Collar tie
- Double top plates
- Ceiling joist
- Wall stud
- Tail cut
Typically, the rafters that create the roof’s slope are wider than those used to form the trusses. Although 2x4s are most frequently used in trusses, 2x12s, 2x10s, and 2x8s are commonly used in rafters. Insulation is positioned between the rafter boards and the drywall in a finished space.
Insulation is typically installed between joists in an unfinished space, such as an attic.
Pros and Cons of Using Roof Rafter
Many homeowners still use rafters to build their roofs, even though the percentage of homeowners using trusses has increased over the past 50 years. Rafters have many advantages being the traditional method of providing structure. These advantages include:
1. Create Extra Space in Your Home
You’ll be happy to learn that using rafters will increase your house’s space. You might even convert your attic into a second bedroom or office space. And if you don’t need more room, you can open the lower floor to create a vaulted ceiling.
2. Ideal for Last-Minute Projects
They’re good for “spur of the moment” projects because rafters don’t need to be bought or erected in advance.
So rafters may be the best option if you’re looking for flexibility in terms of timeframe, available space in your home, or transporting construction materials. Of course, it’s essential to seek advice from a roofing specialist before making a final choice.
3. Useful Anywhere
Rafters usually are built on-site, making them ideal for construction in difficult-to-reach areas. The material transport method is also pretty flexible and can be done by truck, boat, or helicopter.
Although using rafters to frame your roof is a common and high-quality option, you should be aware of a few drawbacks. Using rafters has some downsides, such as:
1. Cost More Than Trusses
Rafters are often more expensive than trusses when labor and material costs are considered. Trusses might be a better option if you’re on a tight budget.
2. Could Take More Time to Construct
Construction will take longer if you have to deal with bad weather. The entire building must be covered with tarps until the rain stops.
What Is a Roof Truss?
Trusses are structural members that form triangular units for supporting loads. You can construct them from wood, steel (usually galvanized), or aluminum. There are a variety of different types of trusses, which you can use for various purposes and settings:
1. King Post Truss
One common type of truss is the king post truss, which has one vertical post or beam that connects the top and bottom chords of the truss. It looks like a king sitting on his throne, with two smaller beams on either side.
This truss often supports roofs in small buildings, such as sheds or gazebos.
2. Queen Post Truss
Another common type of truss is the queen post truss, with two diagonal members connecting the top corners of the vertical posts. This truss supports larger roofs, such as those on barns or warehouses.
3. Pratt Truss
Pratt trusses support the roofs of long buildings, such as factories or train stations. They have diagonal members that all slope down from the left to the right side of the truss.
Thomas and Caleb Pratt, who were brothers, invented prat trusses. They patented it in 1844, although they used it before then. You can use these trusses for bridges and buildings. But, they are expensive and difficult to install.
4. Warren Truss
The Warren truss and its variations, patented in 1846 by British engineers James Warren and Willoughby Monzoni, are a metal truss bridge frequently constructed in the 19th century and early 20th century.
Builders from the middle of the 19th through the 20th century used the Warren truss in construction across the United States.
While it is commonly known as Warren by historians and early textbooks, you can also refer to it as an equilateral structure. Its panel length and the diagonals are identical, making an equilateral triangle.
5. Howe Truss
William Howe created the Howe truss in 1840 and saw widespread use as a bridge in the mid-to-late 1800s.
In geometry, Howe trusses are essentially the reverse of Pratt trusses. You can visualize a Howe truss-like structure by turning a Pratt truss upside down.
Despite the diagonal braces taking up the opposing or vacant joints, the overall structure is still relatively the same. The diagonal members’ change in position has a significant structural impact.
Again, what’s the difference between a roof rafter vs. a truss? The most significant difference between roof rafters vs. trusses is that trusses are prefabricated while rafters are being built on-site. The triangular webbing of trusses enables it to provide roof support and tie the home’s outside walls together.
Roof Truss Pros and Cons
Builders utilize trusses in around 80% of new residential roof installations for a reason. They provide many benefits and will give your house the proper structural stability. A truss system roof has the following benefits:
1. Trusses Possess More Strength and Span
Truss roofs are superior to rafters in terms of strength and span. While truss spans can be up to 60 ft. long, rafter spans are only approximately 30 ft. Additionally, truss roofs’ webbing offers exceptional structural strength.
2. Trusses Are Easy to Put Together
Compared to rafters, trusses are simpler to build. You can even do a DIY home project with trusses. Luckily, the instructions with truss sets help you in spacing and securing. They usually number or mark various shapes and sizes of trusses for easy identification.
3. Trusses Cost Less Than Rafters
The price of a prefabricated truss package is typically between 30 and 50% less expensive than the materials and labor expenses associated with constructing rafters on-site.
4. Truss Fabrication Enables Superior Accuracy
You can construct a truss roof with fewer mistakes because you will build it in a controlled environment. Plus, you can digitally measure the parts and cut them once you enter the specifications into the software.
Trusses aren’t flawless, as nice as they sounded above. Otherwise, there’d be no need for the debate between roof trusses and rafters. Trusses also have drawbacks, including:
1. Assembled Trusses Are Massive
Assembled trusses must be delivered on a semi due to their huge size, which increases the shipping cost. And to get to the roof, you might even need to rent a crane or boom.
2. Less Adaptable
Trusses have structural webbing, limiting your options for what you can do with the area above and below them. Most likely, you won’t be able to turn your attic into an office or an extra bedroom.
Roof Rafter Vs. Truss – Which Costs Less?
Generally, trusses are less expensive than rafters. In reality, prefabricated truss packages are between 30 and 50% cheaper than the supplies and labor required to construct rafters on-site.
Because they are more affordable than rafters, they have grown in popularity over the past 50 years. Most experts will advise choosing trusses if cost is a major consideration when deciding between a roof truss and rafter.
Roof Joists vs. Rafters
Roof joists and rafters are both structural elements that support the roof, but they’re different.
They are the primary supporting members of a roof. You attach the roof joists to the walls. This way, the joists can support the rafters, which transfer the roof’s weight down to the walls. You can use other framing members, but these are the most common.
Rafters are boards that run parallel to each other, perpendicular to the wall plates. They support roofing material, including asphalt shingles and wood shakes.
Roof Joists vs. Trusses
Joists and trusses are two different types of framing.
Roof joists are the horizontal beams that support the weight of a roof. You can make them from lumber, metal, or engineered wood. Trusses are structural triangles that consist of steel and/or wood. You can use them as an alternative to rafters or structural triangles.
You can use trusses in new construction where you can quickly assemble on-site and do not require large amounts of lumber. They also offer greater strength than traditional framed roofs because they have fewer joints and connections between pieces.
Frequently Asked Questions – Difference Between Roof Joist Vs. Rafter Vs. Truss
What Is the Difference Between Truss and Rafters?
While you usually construct rafters on-site, trusses are prefabricated wooden structures. The triangular web of structural pieces known as trusses connects the house’s outside walls while supporting the roof.
What’s the Difference Between Ceiling Joists and Rafters?
Buildings are constructed using joists and rafters, although the two have a few differences. Knowing the distinction between the two is crucial for those in the construction industry. And it can be helpful for homeowners to be aware of it in case repairs or upkeep are required.
The core difference between joists and rafters is that rafters are used for steeply sloping roofs, whereas joists are often more horizontal to the ground.
What Is the Difference Between Joist and Truss?
A joist is a horizontal beam supporting the floor; it bears a load that the floor is built to bear. A truss supports the roof.
What Are the Beams in a Roof Called?
The exposed beams you can see in a building’s ceiling and at its apex are called rafters.
Can You Replace Trusses with Rafters?
Although replacing trusses with rafters is technically feasible, it is essential to consult a structural expert before making any decisions.
When remodeling a roof area, you run the risk of compromising the structure, so you should not only seek advice from a pro but also carefully plan your project before getting started. Failure to do so could result in high costs and potentially dangerous issues with your roof.
Can You Screw Into Roof Joists?
Although it is risky, it is possible to screw into your truss. Even though you wouldn’t be drilling or cutting, you would still change how the truss is stressed.
However, only fasten screws into your truss if they are essential, and before you do so, speak with the engineer who designed your attic.
Does My Ceiling Have Joists?
Hold a flashlight directly above the ceiling, slightly inclined to the surface. This will enable the light to spread across the ceiling almost horizontally. Check the paint for irregularities. Where there’s a nail, there’s a joist. This typically indicates the presence of a nail behind the plaster.
Conclusion – Roof Joist Vs. Rafter Vs. Truss
So, what is the difference between roof joist, rafter, and truss? These structures all serve the same purpose: to support the weight of your roof. But they’re different in how they do it:
- A roof joist is a platform made from wood or metal to which you attach your roofing material.
- A rafter is a horizontal beam that supports the structure of a roof.
- A truss comprises many small beams shaped like triangles and used to support heavy loads.
We hope you find this article helpful. Next time you’re working on your roof, check out our guide to ensure you get the right support for your building.