Truss Calculator

How to Use the Truss Calculator

This roof truss calculator helps you determine the dimensions and specifications for common residential roof trusses. Enter your building span (the width from outside wall to outside wall), select your desired roof pitch, choose the truss type, and specify the spacing between trusses. Input your local snow load and overhang length. The calculator computes top and bottom chord lengths, ridge height, load per truss, and the total number of trusses needed for your building. Use these specifications when ordering prefabricated trusses or planning site-built trusses.

Advantages of Roof Trusses

Roof trusses offer significant advantages over traditional rafter framing. Trusses can span longer distances without interior load-bearing walls, creating open floor plans. They distribute loads more efficiently through triangulated geometry, using smaller lumber members than rafters would require. Prefabricated trusses arrive ready to install, dramatically reducing on-site labor and construction time. Quality control in factory settings ensures consistent dimensions and proper engineering. Trusses are typically more economical for spans over 20 feet, and their engineered design provides predictable performance. The open web design of most trusses also facilitates running HVAC ducts and utilities.

Common Truss Types and Applications

King Post trusses are the simplest design, featuring a single vertical web member from the peak to the center of the bottom chord. They're suitable for smaller spans up to 20-25 feet. Queen Post trusses add two vertical posts positioned away from center, providing better support for spans up to 30 feet. Fink trusses (W-trusses) are the most common residential truss, using diagonal web members in a W pattern to efficiently handle spans up to 40 feet. Howe trusses feature vertical and diagonal members, excelling at longer spans. Scissors trusses create vaulted ceilings, while attic trusses provide usable attic space with room for stairs.

Truss Spacing Considerations

Truss spacing affects both structural performance and material costs. Standard spacing is 24 inches on center (OC), which balances truss count against roof sheathing requirements. This spacing works well with 4x8 plywood or OSB panels. Closer spacing like 16 inches OC increases truss count but may allow thinner sheathing and reduces potential for roof deflection between trusses. Wider spacing of 32 inches OC uses fewer trusses but requires thicker sheathing (typically 5/8" or 3/4" panels) and may be limited by snow load capacity. The choice depends on your specific loads, sheathing type, and local building codes.

Understanding Truss Loads

Roof trusses must support dead loads (the weight of roofing materials, sheathing, and the truss itself) plus live loads (primarily snow in most regions, plus wind). Dead loads typically run 10-15 psf for asphalt shingle roofs, or up to 25 psf for tile roofs. Snow loads vary dramatically by location, from zero in warm climates to 50+ psf in heavy snow areas. Wind loads create both uplift and lateral forces. Design loads are applied to the top chord members and transferred through the web members to the bottom chord and ultimately to the support walls. Truss spacing directly affects the load each truss carries - closer spacing means each truss supports less roof area.

Top Chord and Bottom Chord

The top chord forms the sloped upper members of the truss that support the roof sheathing. Top chords are typically in compression and must resist buckling. They're usually 2x4 or 2x6 lumber depending on span and loads. The bottom chord runs horizontally at the base of the truss, forming the ceiling plane. Bottom chords are in tension and support ceiling materials. In conventional trusses, the bottom chord doesn't carry any floor load - it's only a ceiling member. For attic trusses designed for storage or living space, the bottom chord must be sized as a floor joist, typically requiring 2x6 or larger members with appropriate spacing.

Web Members and Connections

Web members connect the top and bottom chords, creating the triangulated structure that gives trusses their strength. In Fink trusses, diagonal web members transfer compression forces, while vertical members carry tension. Web members are typically 2x4s for residential trusses. The critical element is the connections - metal truss plates (gang-nails) are pressed into each joint, providing the necessary strength to transfer forces between members. Proper plate size and placement is engineered for each connection based on the forces involved. This is why prefabricated trusses are preferred - the engineering and precise fabrication ensure proper connections that are difficult to achieve with site-built trusses.

Truss Installation and Bracing

While this calculator helps size trusses, proper installation is critical. Trusses must be installed plumb and at the correct spacing. Temporary bracing prevents trusses from tipping during construction - lateral bracing connects trusses together, while diagonal bracing prevents lateral movement. Permanent bracing specified by the truss engineer must be installed according to plans. The bottom chord typically requires lateral restraint from ceiling framing. Trusses must bear fully on the exterior walls with proper connection hardware to resist uplift from wind. Never cut or modify truss members without engineering approval - even small cuts can compromise structural integrity. Many truss failures result from field modifications rather than design problems.