Trusses and rafters are the two methods for framing the roof of a home or other building with a pitched roof. Rafters have been the more traditional approach since men first started building permanent dwellings. Trusses aren’t quite so ancient, though they were first used about 2500 BC. Modern trusses, used in home construction, were first introduced in 1952 and patented then.
Today, over 80% of new homes are built with trusses rather than rafters. The main reason for this is cost. Trusses are 40% to 60% cheaper in raw material alone than making a roof using a rafter or “stick” construction. There’s also a much lower labor cost, as a truss roof can be installed in a day, while a stick-constructed roof can take a week, and that week will be much more skilled carpenters’ time than that needed to install trusses.
Nevertheless, there is still a place for rafters in modern homes, as they allow for some capabilities that trusses can’t match. Even in cases where trusses are modified to allow for these capabilities, they don’t do it as well as rafters do. Specifically, I’m talking about times when a high vaulted cathedral ceiling is desired or when it is desirable to put a room in the attic. Rafters are also helpful in cases where complex roof structures are to be built, which would be challenging to build trusses for.
What are Trusses?
Trusses are wood roof structures that take advantage of the natural strength of the triangle. Squares and rectangles are weak structures where a triangle is intense. You can easily see this weakness whenever you try making a cabinet frame or shelving unit out of wood. It tends to collapse to one side without a back or other bracing to hold the case square. But if you make a triangle out of three pieces of plywood and stand it up like that case, without a back, it will stand on its own.
This is because, in a triangle, nothing can move; the sides and angles must stay the same. On the other hand, in a square or rectangle, the corners can change, turning into a parallelogram without the sides changing at all.
This is why many bridge structures are made of trusses. Surprisingly, trusses are even more potent than massive I-beams, although we also see those I-beams used extensively in building bridges. While steel or precast I-beams may be used for overpasses and approaches to bridges, the longer spans and largest bridges will almost always be made with trusses.
In the diagram above, we see the force of gravity and the weight of whatever load is on the truss pressing down on it. This tries to deform the truss, causing the center to bend down, as the shadow image shows (exaggerated). The upper stringer of the truss is put under compression, and the lower stringer is placed under tension (pulling on it). These two opposing forces are transferred from one stringer to the other by the diagonal braces making up the web in the interior of the truss, ultimately preventing the truss from deflecting downwards like the gravitational and weight forces are trying to force it to deflect.
The same thing happens in an I-beam. So why are trusses preferred over I-beams for long spans? Because trusses accomplish the same thing by using much less material, lowering costs, and having a much lighter structure. So, the structure can ultimately support more weight because it doesn’t have to work so hard to support itself.
While I-beams can be made of steel or precast concrete, Concrete (like any stone) is only strong under compression, not tension. The rebar’s strength depends on its ability to provide it under tension.
Applying this to Roof Structures
Although a roof structure is shaped differently than a bridge truss, the principles we discussed apply similarly. By using trusses rather than rafters, roof structures can be made out of 2”x 4” dimensional lumber rather than 2”x 10” or 2”x 12”s. This dramatically reduces the material cost while keeping the roof’s load-bearing strength nearly identical.
Roof trusses can also be made to span a much greater distance than a stick-built roof structure can. With stick-built roofs, you are limited by the length of the material you can find for the bottom stringer, which is the ceiling joist. This causes the use of a supporting wall down the center of most homes to support the overlap of two pieces of lumber that are making this joist. No matter what you do, you are limited to an overall span of 30 feet, limiting the home’s design.
On the other hand, trusses can be made that span as far as 60 feet without the need for any supporting wall or columns. This dramatically affects the design of the building, allowing for much larger rooms and overall building size.
Trusses can be made in many different styles, the three below being only a few of the most common. Truss styles can also be combined so that one side is a pair of scissors, allowing a cathedral ceiling over the main living area while an entire truss covers the bedrooms. It all depends on what the architect selects when he designs the home.
Although the drawings below do not show it, all joints in trusses are covered by plates, which are nailed through to connect the different parts of the truss. In many commercially manufactured trusses, these plates have nails built right into them, so a press can drive the nails into the wood, making the joint.
Making Your Trusses
While most trusses are manufactured by small, local manufacturing companies for lumberyards and home improvement centers, you can build your trusses on-site. It’s a good idea to do this before beginning the home framing to have the full slab available to lay out and assemble the trusses. If you are building a house with a basement, assemble the trusses on the floor before the interior walls are framed.
Making your trusses is still considerably faster and easier than building a stick roof if your home’s design allows it. If you make your trusses in advance, as I am recommending, store them somewhere to keep them off the ground and cover them with a tarp to keep them from getting wet.
To make your trusses, lay out the design on the slab or floor and possibly attach blocking to help you locate the individual pieces. Then, make a first truss, calculating the angles for your cuts and the overall dimensions for each part. Then, you can cut the material in mass, saving time.
The joining plates or “gussets” can be made from one-half-inch CDX plywood. Then, using 6D-coated box nails, nail them to the 2”x 4” pieces used to make the trusses. Please don’t be stingy with the nails, as they hold the trusses together.
Installing Trusses
The one difficulty in using trusses for a roof structure is getting them up to the roof. While I have seen carpenters physically lift trusses from the ground to the top of a second-story home, I wouldn’t want to try it myself. It is considerably safer to rent a small crane or boom for the day, lifting the trusses to the rooftop level.
Trusses are mounted directly to the top plate of the wall. Some places require truss clips for attachment. These bent metal plates attach to the wall structure and the truss, holding them together in high winds like hurricanes. Even if your municipal building code does not require this, it is still a good idea for security.
Temporary cross-braces must be added to the trusses on the underside of the top stringers to maintain the spacing between them until the roof sheathing is installed. You will also need cross-bracing to make the end trusses precisely vertical. The sheathing will then work to maintain the spacing between the trusses.
Rafter Construction
Unlike trusses, which can be prefabricated, rafters need to be assembled in place. Just as with trusses, you’ve got the problem of getting the material up to the roof level to use it. Since these are more significant pieces of dimensional lumber, they can be heavy. However, they are not as heavy as the weight of a truss.
Precutting as much material as possible at ground level is constructive and considerably safer, ensuring consistency in size. This leaves only the assembly process at the roof’s level.
Starting from one end, the two rafter pieces are attached to the tops of the wall, and the ridge board is captured between them. Additional rafters are added, working one way down the roof, with ceiling joists and collar ties added.
Notice that the after-construction of a roof still provides the triangular shape that gives trusses their strength. The big difference is that it is one large triangle, whereas trusses break the shape into several smaller triangles. It is the larger span between supports, since rafters don’t have the web of supports that trusses do, that makes it necessary to use larger dimension lumber for making a roof with rafters, instead of trusses.
The ceiling joist in the diagram above obviously provides for the attachment of drywall for the ceilings of the rooms below. It also includes floor joists for the attic space, allowing you to use it for a room or storage. The collar tie (which is not used in all places) provides tension, keeping the rafters and walls from spreading apart from their own weight and the weight of snow piling on the roof.
It is unnecessary to install temporary stringers to hold the rafters parallel to each other until the roof sheathing is installed, as the ridge board will do that. However, it might be necessary to provide some temporary support for the ridge board’s far end while the first few rafters are being started.
While truss clips were initially designed for use with trusses, they are just as helpful in attaching rafters to the wall as they are for trusses. They will perform the same function, adding safety to the roof in the case of a hurricane or other gale-force winds.
Pros and Cons of Trusses and Rafters
Both trusses and rafters are still widely used today, with trusses being the clear favorite in most cases due to the lower cost of building with trusses. However, there are still homes built with rafters due to their advantages.
Advantages of Trusses:
- Lower cost – Building with roof trusses is 30% to 50% cheaper than building a stick roof.
- Accuracy—Because the trusses are manufactured under controlled conditions, dimensional errors and inconsistencies are less likely, reducing the risk of a wavy roof.
- Time and labor: A truss roof can be built in about a day, whereas a rafter roof can take a week.
- Weather risk – Because the truss roof can be built quickly, the risk of weather damage due to rain is minimized.
- Span – Trusses can handle long spans over open areas better than rafters.
- Suitable for DIY – Because of the ease of installing trusses, they are much easier for the do-it-yourselfer to build with.
Disadvantages of Trusses:
- Weight and size—Due to their weight and size, trusses are often transported by semi-truck and crane and placed in position for installation, especially on two-story homes.
- Location – Due to transportation issues, getting trusses into remote locations can be challenging.
- Ceiling height—Cathedral ceilings are limited so that a truss can be built and webbing can be accommodated. They also limit the ability to make sculptured ceilings with insets.
Advantages of Rafters:
- Dramatic ceilings – Rafters are ideal when trying to make soaring cathedral ceilings.
- Transportation – There is no transportation issue in getting the materials for rafters to a job site, as there can be trusses.
- Complex roofs – Rafters allow you to make complex roof designs more efficiently, as you aren’t trying to fit the roof’s design into standard truss frameworks.
- Ready now. The lead time for buying materials to make rafters is as long as it takes you to drive to the lumber yard.
- Go small. Rafters are much more convenient for small projects, such as adding a room to a home or building a shed.
- More space—Since there is no internal truss webbing, the entire attic can be converted into a room or used for storage.
Disadvantages of Rafters:
- Cost—Due to the larger dimensions of the material used, rafters cost considerably more to build than trusses.
- Expertise – Stick construction is more complex, requiring more expertise from the construction crew.
- Time – It takes longer to build a roof structure of rafters than to install trusses.