A wood truss is a prefabricated structural frame made of lumber pieces arranged in triangles, designed to support a roof or floor by spreading weight across a wide span without needing interior walls for support. Most residential homes built in the last several decades use wood trusses rather than traditional stick framing, and they can clear spans of 36 feet or more in standard residential applications.
How a Wood Truss Works
The core principle behind every truss is triangulation. A triangle is the only geometric shape that can’t be deformed without breaking one of its sides, which makes it inherently rigid. When weight presses down on a triangular frame, that force gets converted into tension (pulling) along some members and compression (pushing) along others. This is far more efficient than a single beam trying to resist bending across the same distance.
A wood truss takes this idea and multiplies it. Instead of one large triangle, a truss contains a series of smaller triangles formed by interior pieces, distributing the load across every member in the frame. The result is a structure that can span an entire room or building width using relatively small pieces of lumber, typically 2x4s or 2x6s, that would be far too weak on their own.
Parts of a Wood Truss
Every wood truss has three types of components:
- Top chords form the upper edges. In a roof truss, these are the sloped pieces that follow the roof pitch and carry the roofing material.
- Bottom chord is the horizontal piece along the base, running the full span of the truss. In a roof truss, this is what your ceiling hangs from.
- Webs are the interior pieces connecting the top and bottom chords, forming the triangular patterns that give the truss its strength.
These pieces are joined together with metal connector plates, which are steel plates with rows of small teeth stamped into them. The teeth get pressed into the wood at each joint during factory manufacturing, locking everything together. This connection method, governed by the national design standard ANSI/TPI 1, replaced the older practice of using nails or bolts and allows trusses to be produced quickly with consistent quality.
Common Truss Configurations
The specific arrangement of webs inside a truss changes depending on what it needs to do. The chord size and web pattern are determined by the span, the expected load, and the spacing between trusses. Here are the configurations you’ll encounter most often in residential construction.
A Fink truss is the most common type for simple gable roofs. Its webs form a “W” pattern inside the triangle, making it efficient for moderate spans. A king post truss is the simplest design, with just a single vertical web in the center connecting the peak to the bottom chord. A queen post truss adds two vertical webs, creating a wider open area in the center.
A scissors truss is used when you want a vaulted or cathedral ceiling inside the room. Instead of a flat bottom chord, both the top and bottom chords are sloped, with the bottom chord pitch varying depending on the top chord pitch, span, and the height where the truss sits on the wall. The tradeoff is a reduced span capacity compared to a standard truss of the same size.
A parallel chord truss (also called a flat truss) has top and bottom chords running parallel to each other. These are commonly used as floor trusses rather than roof trusses.
Roof Trusses vs. Floor Trusses
When most people picture a wood truss, they’re thinking of a roof truss with its characteristic triangular profile. But floor trusses are equally important in modern construction. These are the flat, parallel-chord trusses that replace traditional floor joists.
The biggest advantage of open-web floor trusses is the gaps between the chords. Plumbing pipes, HVAC ductwork, and electrical wiring can all be routed through these openings without anyone needing to drill or cut holes through solid lumber. This saves significant time during construction and avoids weakening the structural members. In a home with traditional solid joists, every pipe and duct that needs to cross the framing requires a hole, and each hole reduces the joist’s load-carrying capacity.
Trusses vs. Traditional Stick Framing
Before trusses became standard, roofs were framed with individual rafters, each one measured, cut, and nailed into place on the job site by skilled carpenters. A typical rafter-framed roof takes two to five days to complete. Trusses arrive from the factory pre-built and can be lifted into place and secured in one to two days, cutting framing time by 60 to 70 percent.
The cost difference is substantial. Truss installation runs roughly $7 to $9 per square foot, while rafter framing costs $10 to $14 per square foot including labor. That gap exists because trusses use less lumber (the engineered triangulation achieves the same strength with smaller, shorter pieces) and require far less skilled on-site labor. Regional variation matters: in the South, trusses can run as low as $6 to $8 per square foot, while in the Northeast they trend toward $8 to $10.
Rafters still have their place. If you want to use your attic as living space, traditional rafter framing leaves the area open. A standard truss fills the attic with webs, making the space unusable for anything but light storage. Scissors trusses and other specialty configurations can partially address this, but they add cost and complexity.
Span Capabilities
One of the main reasons trusses dominate residential construction is their ability to span wide distances without intermediate support. Standard residential roof trusses can span up to 36 feet under normal snow and wind loads. That’s wide enough to cover most single-family homes without a load-bearing wall in the middle of the house, giving architects and homeowners flexibility in floor plan design.
Trusses can span even further in commercial and industrial applications. Once a truss spans 60 feet or more, building codes require a registered design professional to engineer the bracing, and special inspections become mandatory during installation. For typical homes, though, the 36-foot limit covers the vast majority of designs.
Installation and Bracing
A wood truss is extremely strong once it’s fully installed, sheathed, and braced as part of a complete roof or floor system. During installation, however, individual trusses are surprisingly vulnerable. A single truss standing on its own can topple sideways, and truss collapses during construction are a known safety hazard.
Temporary bracing is critical. Each truss needs to be braced to the one before it as it’s erected, either with reusable metal bracing devices or with wood bracing nailed between trusses. Some systems allow workers to attach one end of a brace to a truss while it’s still on the ground, then use a rope from floor level to lock the other end onto the adjacent truss after it’s raised. This keeps workers off the trusses during the most dangerous phase. Once all trusses are braced and the roof sheathing (plywood or oriented strand board) is nailed down, the system becomes a rigid unit.
Permanent bracing also matters. The truss design drawings specify which members need continuous lateral restraint, meaning long boards running perpendicular to the trusses to prevent individual webs or chords from buckling under load. Skipping or misplacing this permanent bracing is one of the more common construction errors, and it can lead to cracked drywall, sagging, or in extreme cases, structural failure.
What the Design Drawings Tell You
Every set of wood trusses comes with engineering drawings created by the truss manufacturer’s design team. These drawings specify the slope, span, spacing, lumber grades, connector plate sizes, load ratings, and bracing requirements for each truss in the project. Building inspectors check the actual installation against these drawings, and they’re a required part of the permit process in virtually every jurisdiction in the United States. The governing standard, ANSI/TPI 1-2022, is referenced directly in the 2024 International Building Codes, making compliance a legal requirement rather than a suggestion.
If you’re building or renovating, the truss design drawings are worth understanding. They tell you exactly how much weight the truss can handle (including dead loads from roofing materials and live loads from snow, wind, or workers), where bracing must be installed, and whether the truss bears on the top chord or the bottom chord. Altering a truss after manufacturing, such as cutting a web to make room for a duct, compromises the engineering and typically voids any warranty or certification.