It is common to read that “rafters are the way they used to frame roofs. Now everyone uses trusses.” First, that is plain wrong, and an oversimplification at best.
Secondly, rafters can have important advantages over trusses in certain building projects; If you’re planning an out-of-the-ordinary, custom roof design or want to make use of the attic space, then rafters might be more appropriate.
In this guide, we will define roof trusses and rafters and explain the pros and cons of each for comparison. We will also outline the best uses of each, so you can decide whether rafters or trusses are the right choice for your building project.
Quick Overview: Rafters and Trusses:
It is certainly true that trusses are more commonly used than rafters. They’re more economical to build and offer the same or greater roof strength. There’s a lot to like.
However trusses don’t give you the opportunity for creativity in home design that rafters allow. There’s more on the pros and cons below, but first let’s define the terms:
What Is a Roof Rafter?
Rafters are the traditional means of framing a roof.
Building with them is known as stick framing. This means that each rafter is built on the job site using dimensional lumber. Every piece is measured, cut and fastened together to form the rafter. Building is labor-intensive.
This diagram shows the major components of a traditional rafter, though slightly different styles can be built.
- Rafter Boards: The boards creating the slope of the roof are wider than lumber used to build trusses – 2x8s, 2x10s and 2x12s are most common in rafters vs. 2x4s in trusses. Rafter boards are fastened to the ridge board at the peak and the top plate where they join the wall.
In finished space, insulation is placed between the rafter boards and drywall or other material is fastened to the face of the boards.
- Ridge Board: The ridge board must be the same width as the rafter boards. In heavy-duty construction, a ridge beam might be used instead.
- Collar Ties: These horizontally installed boards give the trusses strength and stability.
- Ceiling Joist: This board forms the bottom member of each rafter and, of course, the ceiling of the space below.
In unfinished space (an attic), insulation is typically laid between the joists. The joists might then be covered with OSB or plywood to form an attic floor for storage. The are similar to trusses in this regard.
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What Is a Roof Truss?
One good roof truss definition is, “a prefabricated wooden structure that integrates a triangular webbing of structural members to provide support for the roof above while tying the outside walls of the house together.”
The diagram illustrates the “webbing” design of wood truss components.
Trusses and rafters have many common parts including the sloping rafter boards and a bottom joists that form the ceiling of the space below.
The major functional difference between the two is that trusses are built mostly with 2x4s in place of the wider dimensional boards. To compensate for using materials that aren’t as strong, more material is used.
The key practical difference is that trusses are built in a factory (pre-fab) under ideal conditions. Cuts are automated for tremendous precision.
Comparison of Pros and Cons
While trusses and rafters can handle similar loads, there are clear distinctions in essential areas that will help you choose which is right for your building project.
Price: When the cost of a prefabricated truss package is compared to the material and labor costs to build rafters on site, the cost of trusses is 30% to 50% less.
Accuracy: There are fewer mistakes made in the fabrication of trusses. They are built in a controlled environment. Specifications are loaded into software, and truss components are digitally measured and cut. As noted, the precision is impressive. Every one of them exactly meets the specs.
Time and Weather Risks: Truss installation takes about a day on most 2,500 square foot homes. The trusses get installed, and the roof sheathing goes on, protecting the rest of the structure from weather. And the roof is ready for the installation of shingles.
On the same home, rafters might take a week, depending on the size of the crew. This slows the project, and the framing is susceptible to weather for much longer.
Pro tip: You can only take advantage of the short installation time for trusses if you get them ordered well ahead of when they’re needed. Lead time is usually 3-4 weeks, but could be longer during busy construction months.
Excellent Strength and Span: The webbing effect of the truss members creates outstanding structural strength, even though smaller lumber is used.
Truss spans can reach 60 feet.
According to the American Wood Council, the maximum span for rafters is about 30 feet, and that’s with a relatively low load of 20lbs per square foot. As the load goes up for snow in northern climates or for heavy roofing material like tile, the span length decreases.
DIY Friendly: Truss installation is much easier than building rafters one at a time. Your truss package will come with detailed installation instructions for spacing and fastening. If there are trusses of differing sizes or type, such as gable trusses, they will be numbered or otherwise marked for easy identification.
If trusses were perfect, this discussion would be useless. Here are a few cons for trusses.
Weight and Size: Assembled trusses are big and heavy. This means a couple things. You’ll need them delivered on a semi, and that raises shipping costs (though trusses are still the economical choice).
You might need to rent a boom or crane to get them to the roof. If you’re building in the spring, the boom or crane company might say, “We can’t get our equipment onto the worksite until the muddy ground dries up.” That could cause unexpected delays not common to building with rafters.
Their sheer weight makes them more difficult to work with, especially when they span 40 feet and beyond.
Access: Building a mountain getaway or a home on an island? Good luck getting the trusses to your property! Transporting them will either be impossible or the cost will be exorbitant.
Ceiling Design Limitations: The structural webbing of trusses limits what you can do with the space beneath and between them. Forget about a finished attic.
And most homes with trusses have flat ceilings. Scissor trusses are available for cathedral ceilings, but the angle of the ceiling can’t be as steep as when rafters are used.
Many of the pros reflect truss cons, so we’ll make them brief.
More Space or More Drama: Have you ever stood in the attic of a home with a roof pitch of 6/12 or steeper? There is an enormous amount of unused space in there! Using rafters takes advantage of the area beneath the roof and above the ceiling of the lower floor. Use your attic as a bedroom or office, for example.
Don’t need it for living space? Consider opening it to the lower floor to create a truly dramatic vaulted ceiling.
Use them Anywhere: Since rafters are stick built on the job site, they’re ideal for hard-to-reach building locations. The material can be brought in on a pickup truck or boat, if necessary. We’ve seen rafter material transported by helicopter. Pricey, yes, but at least possible compared with getting trusses to that sort of remote building site.
Zero Lead Time: Going to work on a project “spur of the moment”? Then rafters are a good choice, since they don’t have to be ordered and built ahead of time.
Small Projects: Many builders choose rafters for sheds, small additions or cabins to save the hassle of ordering and waiting on trusses.
Again, these have been alluded to.
Cost: When materials and the much-higher labor rate is factored in, rafters are the clear loser in the cost battle.
Pace: The pace of building slows noticeably once the walls are up and work shifts to hand-building each and every rafter. If inclement weather rolls in, expect delays as the entire structure gets tarped to wait out the rain. At worst, heavy rains during construction can lead to damage to the home’s structure that requires an insurance claim. We’ve seen that too.
Skill Required: Factoring angles on stick-built rafters is complex. Frankly, many licensed builders today would struggle to get measurements and angles correct to produce 20 or more rafters exactly alike – and exactly like the specifications.
Best Uses for Trusses Vs. Rafters
In a nutshell, trusses are right for most projects. Rafters are a better choice for specialty projects.
Use trusses when:
- Your home-site is easily accessed.
- You don’t prefer finished attic space.
- Scissor trusses will provide enough steepness for any vaulted ceilings in the plan (consult your architect to determine that).
- Cost is a significant priority in design.
Use rafters when:
- You want to maximize living space within the home’s structure.
- A steeply vaulted ceiling is preferred.
- The building site cannot be reached by an affordable method of transportation for trusses.
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