Conventional roof rafters and ceiling joists are less often used in new home construction these days. In fact, nearly 80% of homes built today use pre-manufactured roof trusses instead of traditional rafters to support the roof!
Roof trusses are pre-fabricated, triangulated wood structures which are built in a factory and carefully designed to carry the load of a home’s roof to the outside walls. They are then shipped to the construction site and installed using a crane after the home’s walls have been framed.
Ask most builders today and they will tell you that engineered roof trusses are the only way to go and are far better than the old roof frames. The primary benefits of using pre-fabricated roof trusses are cost savings and construction speed.
The flexibility in the roof design and complexity that roof trusses enable have also made them increasingly popular. With today’s home styles, more complex roof designs, angles, cross gables and other features have added cost, which can be at least partially offset by using pre-manufactured roof trusses rather than building a roof frame on site.
The Advantages of Pre-manufactured Roof Trusses
While the basic advantages have been mentioned above, a more detailed list of the advantages of roof trusses in building a home include:
- Professional design and fabrication techniques; in conventional roof building, the carpenters build a roof frame, leaving much to chance and the builder’s opinion in terms of the roof’s strength. By comparison, a trussed roof system is designed by engineers to accommodate the specific roof design and meet building codes and enabling a more uniform size and roof pitch.
- Higher quality materials and strength; roof trusses are fabricated inside a shop, where the materials are not exposed to inclement weather or moisture conditions.
- Trusses can typically be installed in a single day, speeding the home construction process and getting the structure closed up sooner, which helps prevent moisture and other weather elements from getting inside.
- Trusses are cheaper than conventional roof framing due to labor savings and since they are built from shorter lengths of 2×4 lumber rather than the large size lumber required in building rafters and ceiling joists.
- Roof truss systems allow for almost any custom roof design and shape imaginable, allowing features such as cross gables and cathedral ceilings without being prohibitively expensive.
- Roof trusses can span much longer distances without the need for load bearing interior walls
The only real disadvantage of roof trusses is that the homeowner ends up with less usable space in the attic area; a very small price to pay!
Types of Roof Trusses Used in Building Homes
Two basic types of roof truss designs are used in home building; the pitched or common truss and the parallel chord or flat truss.
A common truss is recognizable by its triangular shape and is most often used in roof construction. Most often, variations of the common truss are named for their web configuration, such as the King Post, Fan, Fink and Howe trusses, with the chord size and web configuration typically being determined by the load, span and spacing.
A parallel chord or flat truss is so named for its parallel top and bottom chords and is often used to construct floors.
There are many roof truss types and variations including Arch Frame, Belgian, Bowstring, Dropped Chord, Gambrel, Hip Step-down, Howe, Parallel Chord, Pratt Bottom Chord, Pratt Top Chord, Raised Heel, Scissor, Semi Howe, Slope Frame and Warren.
Each type of roof truss offers pros and cons and is designed to suit specific structural and aesthetic purposes. It is important to understand the different types of roof trusses when designing a new home, doing major remodeling or a room addition. This will help match the look and function of your home’s roof to the correct roof truss type and design.
Both aesthetic and functional considerations are required in choosing the best type of truss to use for a given roof shape, size and design complexity. You will want to discuss all of these factors with your architect and roof truss engineer before ordering roof trusses for any new home or major remodeling project.
Climate is another consideration, since each type of roof truss has different characteristics that make it more or less desirable in terms of the insulation and air-vapor barrier.
We have expanded on the advantages and disadvantages of several of the major types of roof trusses:
A Raised Heel Truss is designed to span an area and provide adequate space for full depth attic insulation. While enabling greater energy efficiency, Raised Heel Trusses also enable an air tight vapor barrier to reduce problems arising due to condensation, dry rot and mold. It is also more expensive than other types of roof trusses due to the need for soffit siding, higher manufacturing cost and the additional insulation required.
A Dropped Chord Truss has two segments; a convention truss, with a secondary chord truss suspended below to help reduce truss uplift, which is when an interior ceiling may “lift upward” and result in ceiling and wall damage. A Dropped Chord truss design enables a vapor barrier and full depth attic insulation, as with a Raised Heel design. Taller studs and additional blocking and siding are required where walls and ceilings intersect in order to accommodate the air-vapor barrier, which adds to the construction cost.
A Scissor Truss also uses lower chords, but instead of being horizontal, the lower chords are sloped inward to form a shape that looks somewhat like a hang glider. Cathedral ceilings many times require Scissor Trusses and eliminate the need to use a bearing beam and wall. Insulating the attic area is more difficult with this type of truss and manufacturing costs are typically higher than other roof truss types.
A Parallel Chord Truss may also be used with cathedral ceilings, but allows for fuller and easier installation of insulation of the attic area. Because it requires steel braces and several wood products to manufacture, the cost is higher and thermal bridging caused by the steel braces can decrease energy efficiency.
Ordering Roof Trusses For Home Construction
It is very important that you specify the correct type, measurements and other factors when ordering roof trusses. Common mistakes include inaccurate specifications and failing to inspect the roof trusses when they are delivered to the construction site.
A list of the basic specification required when ordering roof trusses includes:
- Truss Span – specifies how long the bottom chord needs to be.
- Overhang Length – specifies the horizontal distance between the end of the bottom chord and the bottom edge of the top chord.
- Number of Trusses – specifies how many trusses are required. The uniform spacing of roof trusses (usually either 24 inches or 48 inches on center) makes this relatively easy to calculate.
- Design Load – specifies both the live and dead loads of the top and bottom chords, as well as wind and any other loads to which the trusses will be subjected.
- End Cut – specifies the plumb or square cut and any custom specifications
- Heel Height – specifies the vertical distance between the bottom of the bottom chord and the top of the top chord.
- Roof Slope – specifies the vertical rise in inches per each 12 inch horizontal run.
- Bearing Width – specifies the requirements for the truss bearing.
- Type of Truss – specifies the type of truss required.
- Overall Height – specifies the total vertical distance from highest point of top chord or peak to the bottom edge of the lowest bottom chord, not including the overhang.