A truss is a structural framework made of straight members connected at joints to form a series of triangles. This triangular geometry is what makes trusses so effective: unlike a rectangle, a triangle can’t be deformed without changing the length of one of its sides. That rigidity allows trusses to span long distances while using far less material than a solid beam. You’ll find them holding up roofs, bridges, towers, and stadium canopies across virtually every type of construction.
Why Triangles Make Trusses Work
Every truss relies on a principle called triangulation. When a load pushes down on a triangular frame, the forces travel along each member as either pure tension (pulling apart) or pure compression (pushing together). The members themselves stay straight and don’t bend, which means the material is used as efficiently as possible. Engineers arrange just enough triangular cells within the truss to keep the whole system geometrically stable.
This is fundamentally different from a single beam, which resists loads by bending. Bending creates stress concentrations and requires more material to handle the same weight. A truss distributes that same load across many lightweight members, each handling only tension or compression along its length. The result is a structure that can be surprisingly light for the distances it spans.
Parts of a Truss
Despite the many shapes trusses come in, they all share the same basic anatomy:
- Top chord: The upper edge of the truss. In a roof truss, this is the sloped member that follows the roofline and typically handles compression.
- Bottom chord: The lower horizontal edge, which usually carries tension as the load tries to spread the truss apart.
- Web members: The interior diagonal and vertical pieces that connect the top and bottom chords. These form the triangles that give the truss its strength, with each web member carrying either tension or compression depending on its orientation and the truss design.
- Panel points (nodes): The joints where members meet. In wood trusses, these connections are typically made with metal connector plates pressed into the wood, or with plywood gusset plates nailed or bolted into place.
The connections at panel points are critical. A truss is only as strong as its joints, which is why metal plate connected wood trusses are manufactured under the ANSI/TPI 1 standard, a national design specification maintained by the Truss Plate Institute.
Common Roof Truss Types
Most residential and light commercial buildings use one of a few standard wood truss configurations, each suited to different spans and roof shapes.
The king post truss is the simplest design: two top chords meeting at a peak, a horizontal bottom chord, and a single vertical web member in the center. It’s cost-effective and works well for shorter spans in small residential structures. The queen post truss extends this concept by adding two vertical members instead of one, creating a wider flat section at the top. That extra width lets it support longer spans, making it a better fit for commercial buildings or large halls.
The Fink truss is one of the most common in residential construction. Its web members form a W-shaped pattern that distributes loads efficiently across moderate spans. If you look up into an unfinished attic in a typical house, you’re probably looking at Fink trusses.
Bridge and Industrial Truss Designs
Larger structures use truss configurations where the direction of the diagonal members matters a great deal, because it determines which pieces are in tension and which are in compression.
In a Pratt truss, the diagonal web members angle away from the center of the span. This puts the diagonals in tension and the vertical members in compression. Since longer members handle tension better than compression (compression can cause buckling), this arrangement is efficient for many bridge applications. The Howe truss flips this relationship: its diagonals angle toward the center, putting them in compression and the verticals in tension. Early Howe trusses used wood for the compression members and iron rods for the tension members, which was a practical combination in the 19th century.
The Warren truss takes a different approach entirely. Its original form was simply a series of equilateral triangles with no vertical members at all, making it one of the earliest and simplest truss types. Later versions added verticals or extra diagonals to handle heavier or more complex loading.
How Far a Truss Can Span
One of the biggest advantages of trusses is their ability to clear long distances without intermediate supports. Traditional stick-framed rafters work well up to about 30 feet, and anything longer requires additional vertical beams, posts, or columns beneath them. Trusses, with their triangulated webbing, handle longer spans and heavier loads with far fewer interruptions to the open space below.
For residential wood trusses, building codes set specific limits. Under simplified residential provisions, truss spans top out at 36 feet, with buildings no wider than 60 feet perpendicular to the span. Roof slopes must fall between 3:12 and 12:12, and design wind speeds can’t exceed 140 miles per hour. Structures that fall outside these limits require full engineered design under the International Building Code.
Trusses spanning 60 feet or more enter a different category entirely. At that length, a registered design professional must design both the temporary bracing used during installation and the permanent bracing that restrains individual members. Special inspections are also required.
Trusses vs. Traditional Rafters
For roof framing, builders choose between prefabricated trusses and site-built rafters. Trusses win on cost and speed in most situations. They arrive at the job site already assembled to match the roof design, so a crew can set an entire roof structure in a day. Rafters require onsite measuring, cutting, and fitting of each individual piece, which takes significantly more labor and time, especially on complex roof shapes.
The tradeoff is flexibility. Rafters leave the full attic space open, which is why they’re preferred when the homeowner wants usable attic rooms. Standard trusses fill that space with web members, making it impractical for storage or living area. Some truss designs, like scissor trusses or attic trusses, work around this by reshaping the interior geometry, but they cost more than a basic Fink.
Wood vs. Steel Trusses
Wood trusses dominate residential construction because they’re inexpensive, lightweight, and easy to manufacture in large quantities. Steel trusses show up in commercial, industrial, and institutional buildings where longer spans, higher loads, or fire resistance requirements push beyond what wood can handle. Steel is stronger per unit of weight, so a steel truss can span farther without growing as deep. However, research from MIT notes that timber trusses carry a significant weight penalty compared to equivalent steel designs. For projects where reducing the carbon footprint matters, timber can still be the better environmental choice despite the added bulk.
How Trusses Are Installed
Truss installation follows a carefully sequenced process, largely because an individual truss standing on its own is unstable. It’s a two-dimensional frame trying to exist in three-dimensional space, and until it’s braced to neighboring trusses, it can topple sideways with very little force.
The process starts with ground bracing: stable vertical supports anchored to the building’s structure. The first truss or gable end frame gets fastened to these supports and checked for plumb, since it sets the alignment template for every truss that follows. The next four trusses are set at the correct spacing, and then top chord diagonal bracing is installed across all five to lock them together. Web member diagonal bracing and bottom chord lateral restraints follow before the crew moves on to the next group of four.
Temporary bracing spacing depends on the truss span. For trusses up to 30 feet, lateral restraints along the top chord go every 10 feet. That tightens to 8 feet for spans of 30 to 45 feet, 6 feet for 45 to 60 feet, and 4 feet for spans of 60 to 80 feet. These numbers come from the BCSI guide, the industry standard reference for safe truss handling and installation. Skipping or under-installing bracing is one of the leading causes of truss collapses during construction.