A truss is a structural framework made of straight members arranged in triangles, designed to support heavy loads across open spaces. You’ll find trusses holding up roofs, bridges, towers, and stadium canopies. The triangle is the key to the whole concept: unlike a rectangle, which can be pushed into a parallelogram, a triangle holds its shape under force. That geometric rigidity is what makes trusses one of the most efficient ways to span long distances without columns or walls in between.
Why Triangles Make Trusses Work
Every truss, no matter how complex it looks, is built from connected triangles. Each member in a truss carries force along its length in one of two ways: tension (being pulled apart) or compression (being pushed together). There’s no bending involved, which is what makes trusses so material-efficient compared to solid beams.
Think of it this way. A horizontal beam spanning a gap has to resist bending in its middle, which requires a lot of material to handle. A truss breaks that same job into a network of smaller members, each one simply being pulled or pushed. The members provide a “load path” that channels weight from wherever it’s applied down to the supports at each end. This is why a lightweight truss can replace a much heavier solid beam and carry the same load.
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.
- Bottom chord: The lower edge, running horizontally. It acts as a tie holding the two ends together and is typically in tension.
- Web members: The interior pieces connecting the top and bottom chords, forming the triangular patterns. These can be vertical, diagonal, or both, and they carry either tension or compression depending on the truss design.
- Joints (or nodes): The points where members meet. In engineering terms, truss joints are treated as pins, meaning forces are applied at these connection points rather than along the length of a member.
In wood trusses, the joints are often held together with metal connector plates pressed into the wood. Steel trusses use welded or bolted gusset plates. Some modern designs pair wooden chords with tubular steel web members, connected by pins.
Common Roof Truss Types
Most residential and commercial roofs today use prefabricated trusses rather than traditional stick-built rafters. Here are the designs you’re most likely to encounter:
The King Post is the simplest truss. A single vertical post connects the peak to the base, with two angled rafters on either side. It works well for short spans up to about 8 meters (roughly 26 feet) and uses minimal material, making it the most economical choice for small buildings like garages and sheds.
The Queen Post adds a second vertical post, creating a rectangular section in the center. This opens up slightly longer spans, around 10 meters, and is a common choice for medium-sized residential roofs.
The Fink truss is one of the most popular designs in modern homebuilding. Its web members form a W-shaped pattern that distributes weight efficiently across the whole structure. Fink trusses use less timber than many alternatives and can span up to 14 meters (about 46 feet).
The Scissor truss is what you want if you’re after vaulted ceilings. Its bottom chords cross each other like an open pair of scissors, angling upward instead of running flat. This creates that dramatic cathedral ceiling effect from the inside. Scissor trusses handle spans up to 15 meters, though they require more material because of the more complex geometry.
Bridge Truss Designs
Bridge trusses follow the same physics as roof trusses but are optimized for horizontal loads and traffic. Three designs have dominated bridge engineering for over a century, and they’re distinguished by how their diagonal members handle force.
In a Pratt truss, the diagonal members angle toward the center and carry tension, while the vertical members handle compression. Since steel performs well in tension, this layout became extremely popular once steel replaced iron in bridge construction.
The Howe truss is essentially the reverse. Its diagonals carry compression and its verticals carry tension. Historically, Howe trusses were built with wooden diagonals and metal vertical rods, a practical combination for the materials available in the 19th century.
A Warren truss uses a zigzag of equilateral triangles with no vertical members at all. Each diagonal alternates between tension and compression. Adding vertical members to a Warren truss creates extra bracing, which is common in longer spans. The clean triangular look makes Warren trusses a frequent choice in modern pedestrian bridges and overpasses.
Wood vs. Steel Trusses
Wood trusses are the standard for residential construction. They’re cheaper upfront, widely available as prefabricated units, and fast to install. The tradeoffs are durability related: wood is vulnerable to warping, rot, and insect damage over time, and it performs worse in extreme weather. These risks can mean ongoing maintenance costs, but for most homes, timber trusses remain the most economical option by a wide margin.
Steel trusses cost more initially but last significantly longer with almost no maintenance. They won’t rot, warp, or attract termites, and they don’t need chemical treatments. Steel is also lighter than wood for equivalent strength, which can simplify installation. At the end of a building’s life, steel trusses are fully recyclable. Commercial buildings, industrial facilities, and larger structures tend to favor steel for these reasons.
How Prefabricated Trusses Get Installed
Most trusses arrive on a job site already assembled at a factory, built to match engineered drawings specific to that building. The installation follows a straightforward sequence. First, the wall plates (the top surfaces of the exterior walls) are prepared and marked with the truss spacing layout. A crane lifts the first truss into position, where it’s plumbed vertical and held in place with temporary bracing, usually 2x4s or 2x6s nailed between the truss and a fixed point.
The remaining trusses go up one at a time, each braced to the previous one to prevent the whole line from toppling like dominoes. Spacing is typically 24 inches on center for residential work. Once all trusses are standing, permanent bracing is installed, including lateral bracing along the bottom chords (often at 10-foot intervals) with cross bracing between them. Roof sheathing goes on last, locking everything into a rigid system. A building inspector signs off on the work before anything else goes on top.
The engineered drawings that ship with each truss package are the single most important document on the job site. They specify exact placement, spacing, and bracing requirements for that particular structure. Deviating from them compromises the load path the engineer designed, which can lead to structural failure under heavy snow, wind, or even just the weight of the roofing materials themselves.