If you have ever wondered what the difference is between a rafter and a truss, you’ve come to the right place. Both rafters and trusses can form the framework of a roof system, but they can be used very differently.
Rafters are just one component in a roof system, along with collar ties, ceiling joists, rafter ties, strong backs, and weight-bearing walls. Trusses, however, incorporate nearly all of these components into a ready-made unit.
When professionals discuss “stick building”, they are referring to the traditional method of building a roof system on-site using rafters, joists, and other components. Trusses are generally custom built in a factory based on the job they must perform, and delivered to the job site ready to install.
Do Rafters and Trusses Do the Same Thing?
In a sense, yes, rafters and trusses have the same purpose. However, while trusses can essentially stand alone and perform their function, rafters only make up one component of a roof system. Rafters establish the angle of the roof, which is expressed as a ratio of height to length.
For example, if a roof’s vertical measurement rises 4’ in height for every 12’ in length, the roof would be said to have a 4/12 pitch. This is important because the pitch will establish the look and functionality of the roof, not to mention the amount of roofing materials needed.
Trusses also establish this pitch, but the board in a truss that performs the function of a rafter is called a chord. This chord will usually extend from an exterior wall to the gable, just like a rafter. While rafters rely on joists and other components for support, trusses are mostly self-supporting. This makes trusses not only fast to install, but in most designs, easier.
Trusses use components similar to an expansion bridge to span much larger distances than are generally possible with rafters. This opens up many more design options, as often weight-bearing walls can be reduced or eliminated.
Most new homes built today incorporate trusses instead of rafters and other components, because they are much faster and simpler to install. However, in the majority of new construction, trusses will require the use of a crane. Smaller trusses used on smaller homes can sometimes be installed with simple labor, but in most situations, a crane will actually be more cost-effective.
What Are the Advantages Of Using Rafters?
Rafters have the advantage of being customizable on site. During normal construction, professional builders are primarily interested in speed while maintaining quality construction. As such, most will employ trusses wherever possible. It should be noted however, that trusses are not adjustable.
Some situations require small tweaks to the framing to close small gaps or square up a section. In these instances, building rafters and other roofing components on site saves time, effort, and most importantly, improves quality.
What Are the Disadvantages Of Using Rafters?
Building rafters and other roof components by hand is the traditional method of constructing a roof system. However, this process can be slow and require much more skill and experience than installing trusses.
Journeyman carpenters (the highest skill level) are often required to navigate the complex structural requirements of a stick built home. Some designs are so complex that the structure cannot be built without the use of trusses.
For example, trusses can be designed so that structural support can be hidden within a second floor system, eliminating the need for arches, beams, and posts. This is not generally possible using rafters, which is why older architecture popularized visible beams, weight bearing walls, and columns.
When and Where Would I Use Rafters?
In the vast majority of applications, rafters are used to either solve a complex support concern or when an existing structure is being modified. For example, most new construction projects will employ trusses because the structure only needs to support its own weight. This is especially evident when the design is relatively simple, like a gable roof.
However, in some remodeling projects, using trusses is very difficult or even impossible, because the original roof must be matched precisely. This can be an insurmountable problem if the original home has settled or was not correctly built. Because rafters can be modified on-site (assuming they still meet building code requirements), they are usually the best solution. This allows the builder to add roof components as needed to not only build the best-looking project but ensure the structure is structurally stable as well.
What Are the Advantages Of Using Trusses?
Trusses are fast to install, do not require advanced skills, and can be designed for most applications. This is why the vast majority of new homes are built with trusses. In fact, trusses can also be (and often are) used to build the floor system as well. Roof trusses depend entirely on their design for strength.
Trusses are designed by a computer, along with the help of a design engineer. This allows the designer to accommodate nearly every design request, from removing weight-bearing walls to hiding beams within a truss.
Trusses are constructed so that much of the required support comes from within the truss itself, as opposed to the direct support of other components. Trusses include both horizontal and vertical support, and depending on the structure, may require no interior support at all. This feature makes large, open spaces available that would be impossible for structures using only rafters.
Trusses also allow the building of larger structures not spanable with rafters. For example, some home designs will involve a “piggyback” truss in addition to a regular truss. Piggyback trusses are small trusses that sit atop a normal truss to increase its height. This is typically required because if the truss were all one unit, it would be too large to transport to the job site on public roads. This gives the use of trusses a huge advantage in large projects because rafter construction would require extremely large lumber to span the same distance.
What Are the Disadvantages Of Using Trusses?
As mentioned previously, one disadvantage of trusses is that they cannot be modified on site. Trusses depend entirely on their design, so if an installer were to trim or otherwise modify a truss it would no longer support its weight, nor pass most building inspections. Even removing ½” of material from a truss would affect its ability to support itself. Therefore, trusses are most advantageous when they can be used as delivered and no modification will be necessary.
Trusses are also more expensive to purchase and install than standard lumber. The extra cost of trusses includes the services of a crane, delivery, design engineers, and the resources needed to actually build the truss. However, most professional builders still use them because the time and labor expense saved usually offsets these costs.
Another disadvantage of trusses is scheduling. Because stick built roofs are constructed one piece at a time, there are usually no complex scheduling requirements. As long as the carpenters have the lumber they need, they can work at their own pace. With trusses however, there is a greater risk of delays, which often translates to lost money.
Truss installation usually requires that the trusses, labor, and crane converge on the building site at the same time. If a delivery is late, the crane is unavailable, or labor doesn’t show up, the project will often be at a very expensive standstill.
How Do I Decide If I Should Use Rafters Or Trusses?
Generally speaking, the decision to use trusses or rafters comes down to time and cost. As mentioned earlier, trusses are great for new construction, but maybe not so much for remodels. This does not apply universally, as many older homes are still in very good condition and will accept trusses without issue. For the most part, however, trusses will be used in new construction if the budget allows for it. This is especially true if the home is larger, because the economy of scale increases.
Rafters on the other hand can solve a litany of problems when a home is in disrepair, or the building site will not allow for a crane.
In many instances, homes are constructed on remote building lots a crane cannot access, or the lots are so narrow there is no physical space for a crane. In these situations, both professional builders and most building inspectors prefer rafters because they can be modified as needed for the application.
Can I Use Rafters and Trusses Together?
Absolutely. In fact, unless the structure is quite simple, many times trusses and rafters will be used together for strength. Design elements such as vaulted ceilings, cathedral ceilings, tray ceilings, and other upscale features are often built using both trusses and rafters. If the design involves complex rooflines, such as a bay window or several hips and valleys, rafters (and joists) are often integrated with trusses to provide the support for drywall.
The Best Of Both Worlds
In everyday construction, professionals will integrate both trusses and rafters into the project, depending on the support needed. There are no hard and fast rules governing where trusses or rafters must be used. Either solution is perfectly acceptable and both will provide the structural support required. Therefore, in most situations, the decision will come down to location, experience, resources, and time.