Roof Truss Calculator

Understanding Truss Geometry and Load Distribution

Roof trusses transfer weight through a network of triangles, the most stable geometric shape. The top chords (rafters) slope from the peak to the eave, the bottom chord spans horizontally between walls, and internal web members connect them. This configuration distributes roof load—shingles, snow, wind—across the entire structure rather than concentrating it on a few long spans.

Engineers design each truss to match specific span, pitch, and load requirements. A 30-foot span with a 6:12 pitch uses different web patterns and lumber grades than a 30-foot span with a 10:12 pitch, even though the span stays constant. Steeper pitches create taller trusses with longer top chords, increasing material cost but improving attic space and snow shedding.

Truss plates—galvanized steel connectors pressed into lumber joints—hold everything together. Each plate has dozens of teeth that embed into the wood grain, creating a connection stronger than nails or screws. These plates are the reason you can't modify a truss after manufacturing: removing or cutting a plate compromises the entire assembly's structural integrity.

Truss Spacing, Span Limits, and Building Codes

Standard truss spacing of 24 inches on center works for most residential roofs supporting asphalt shingles or metal panels. The spacing determines how much weight each truss carries—closer spacing means each truss supports a narrower slice of roof, reducing individual truss load. Heavy roofing materials like concrete tile or slate often require 16-inch spacing or larger truss members.

Maximum truss span without interior bearing walls typically tops out around 60 feet for residential applications, though engineered lumber and custom designs push that limit higher. Spans beyond 40 feet often require raised-heel trusses to provide adequate attic insulation depth at the eave, where standard trusses pinch down to a few inches of clearance.

Building codes govern truss design through snow load, wind speed, and seismic ratings specific to your location. A truss engineered for a 20-psf snow load in Georgia fails catastrophically under a 50-psf load in Vermont. Always provide your truss supplier with your exact building location, roof pitch, and local code requirements. They submit designs to a professional engineer who stamps and certifies the truss package.

Ordering, Delivery, and Installation Considerations

Manufactured trusses require 2-6 weeks of lead time from order to delivery, depending on the supplier's backlog. Complex custom shapes or unusual lumber sizes extend that timeline. Order early in your project schedule—trusses often arrive on the critical path, delaying the entire job if they're late.

Delivery presents logistical challenges. A 40-foot truss requires a flatbed truck and enough clear space for a crane or forklift to offload. Many suburban lots lack room for large truck access, forcing hand-carry or smaller truck shuttles that add cost. Plan delivery for when your wall framing is complete so trusses can be set directly on the walls rather than stockpiled on the ground.

Installation requires careful bracing to prevent collapse. Trusses are strong vertically but fragile horizontally until sheathing and bracing lock them into position. Set trusses at the marked spacing, install permanent lateral bracing per the engineer's plan, then add temporary diagonal braces before releasing the crane. Many truss failures happen during installation when a gust of wind topples unbraced units like dominoes.