Truss Calculator

How to Calculate the Number of Roof Trusses?

Divide the building length by the truss spacing and add one. A 40-foot building with trusses at 24-inch on-center spacing: 40 feet = 480 inches / 24 = 20, plus 1 = 21 common trusses. Add 2 gable end trusses (one for each end). Total: 23 trusses. The calculator above computes the truss count, gable trusses, ridge height, and top chord length based on your building span, length, pitch, and spacing selections.

What Is a Roof Truss?

A roof truss is a pre-engineered, factory-built triangular frame that supports the roof structure. It consists of a top chord (the sloped member forming the roof surface), a bottom chord (the horizontal member forming the ceiling), and web members (diagonal and vertical pieces connecting the top and bottom chords). Trusses are designed by specialized engineering software, built in a controlled factory environment using gang-nail plates at every joint, and delivered to the job site ready to install. They span the full building width without interior load-bearing walls, giving builders flexibility in floor plan layout.

What Truss Spacing Is Standard?

24-inch on-center is the standard truss spacing for residential construction. This wider spacing (compared to 16-inch rafter spacing) is possible because trusses are engineered as complete structural units with higher load capacity than individual rafters. 16-inch spacing is used when the roof sheathing or ceiling material requires closer support, or in heavy snow load areas where the engineer specifies tighter spacing. Some commercial and agricultural buildings use 48-inch spacing with heavy-duty trusses and purlins spanning between them. The truss manufacturer specifies the maximum spacing based on the truss design, span, and local load requirements.

What Types of Trusses Exist?

Common (Fink) trusses are the standard triangular shape with W-pattern web members, used for most gable roofs. Scissor trusses have a raised bottom chord that creates a vaulted ceiling inside. Attic trusses include a rectangular open space in the center for storage or a room. Hip trusses combine with hip jack trusses to create a hipped roof (slopes on all four sides). Mono trusses slope in one direction for shed roofs and lean-to additions. Flat trusses span horizontal distances for commercial flat roofs. Gambrel trusses create the barn-style double-slope profile. Each type is engineered specifically for its geometry and intended load conditions.

How Much Do Roof Trusses Cost?

Standard common trusses for a 24-30 foot residential span cost $75-$200 each, depending on span, pitch, and lumber prices. Specialty trusses (scissor, attic, hip) cost $150-$400+ each due to additional material and engineering complexity. Delivery charges add $200-$500 per load, often requiring a crane or boom truck for unloading. A 40-foot building needing 23 trusses at $150 each runs approximately $3,450 for the trusses plus delivery. The truss manufacturer provides a complete engineered package including a sealed truss drawing, bearing details, and bracing plan at no additional charge with the order.

How Are Trusses Installed?

Trusses are typically set by a crane or boom truck that lifts each truss from the delivery stack and places it on the bearing walls. A crew of 3-4 workers guides each truss into position, secures it with temporary bracing, and nails it to the wall top plate through a hurricane tie or truss clip. Temporary lateral bracing (long 2x4s nailed diagonally across the top chords) holds the trusses upright and straight until the roof sheathing is installed. Permanent lateral bracing follows the truss engineer bracing plan and includes continuous lateral restraint of web members at specified locations. Never cut, drill, or modify a truss without consulting the truss engineer, as removing any member compromises the structural design.

Trusses vs Rafters: Which System Is Better?

Trusses install faster (a crew can set an entire roof in one day), cost less per square foot for standard designs, and provide longer clear spans without interior bearing walls. They are engineered and carry a manufacturers structural warranty. Rafters allow usable attic space, cathedral ceilings, and custom roof shapes that trusses cannot easily accommodate. Rafter systems are built piece by piece on-site, requiring more carpentry skill and labor. For production housing and straightforward roof designs, trusses dominate because of speed and cost advantages. For custom homes, complex roof lines, and finished attic spaces, rafters remain the preferred choice.

Ordering and Lead Time for Trusses

Trusses are custom-engineered for each building. Allow 2-4 weeks from order to delivery for standard residential trusses, longer during peak building season. The order process starts with a truss layout drawing provided by the builder or architect. The truss manufacturer designs each truss, produces a sealed engineering package, and schedules production. Verify the delivery date and site access requirements before the trusses arrive. Trusses should be stored on-site on flat blocking, covered with tarps if rain is expected. Do not store trusses flat on the ground, as ground moisture and mud damage the lumber and plate connections. Set trusses within a few days of delivery to minimize site storage risks.

Frequently asked questions