Roof Truss Calculator

Frequently Asked Questions

How many trusses for a 40-foot building at 24 OC?

ceil(40 / (24/12)) + 1 = ceil(20) + 1 = 21 trusses.

How long is the top chord for a 28-foot wide building at 6:12 pitch?

Top chord per side = (28/2) / cos(atan(6/12)) + 1 = 14 / cos(26.6 deg) + 1 = 14 / 0.8944 + 1 = 15.65 + 1 = 16.65 ft.

What is the standard roof truss spacing?

24 inches on center is the standard for engineered roof trusses. Some designs use 16 OC for heavy snow loads or special sheathing requirements.

Should I buy trusses or frame with rafters?

Trusses are faster to erect, use less lumber overall, and are structurally engineered. Conventional rafters allow attic access and are preferred when the attic will be finished living space.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A gable roof truss consists of two top chords meeting at the peak, one bottom chord spanning the building width, and internal web members. This calculator estimates the count and chord lengths needed for a project.

Truss count = ceil(building_length_ft / (spacing_in / 12)) + 1. Top chord length per side = (building_width / 2) / cos(atan(pitch/12)) + 1 ft overhang. Bottom chord = building_width + 2 ft (1 ft overhang each side). Total lumber per truss = 2 x top_chord + bottom_chord. Total project lumber = total_per_truss x truss_count.

Common Truss Types

Manufactured roof trusses are engineered by a truss fabricator using metal connector plates. They arrive on-site pre-built and can be set in a single day with a crane. Common configurations:

Fink (W-truss): most common for residential, efficient for spans up to 40 ft

Scissors truss: creates a vaulted ceiling below the top chord

Attic truss: includes a room-in-attic space within the bottom chord

Mono truss: single slope, used for shed roofs or additions

Hip truss: corner and transition trusses for hip roof systems

This calculator focuses on standard gable (Fink) trusses and estimates chord material only. Web members add approximately 30-40% more lumber to the total per-truss estimate.

Ordering Trusses

Trusses are typically ordered from a fabricator with a lead time of 2-4 weeks. The fabricator needs: building width, heel height, pitch, overhang, spacing, design loads (roof dead load, snow load, ceiling dead load), and bearing conditions. Always obtain a sealed truss design drawing from the fabricator; it forms part of the construction documents.

Stock trusses may be available for common spans at some lumber yards, but custom fabrication is normal for any unusual pitch, span, or load. The lumber estimate in this calculator covers rough material planning; actual fabricated truss prices include labor, plates, and delivery.

FAQ

Why is 24 OC standard for trusses?

Trusses are engineered as complete structural systems and can be spaced at 24 OC because the truss depth provides enough stiffness. Conventional rafter framing typically uses 16 OC because individual 2x lumber at 24 OC would be undersized for most spans.

What overhang should I use?

This calculator adds 1 ft of overhang to each chord for a total 2-ft overhang. Standard residential overhangs range from 12 to 24 inches. Adjust with your fabricator for your specific design.

Do I order one truss per position or per pair?

One truss per position. A gable roof 40 ft long at 24 OC needs ceil(40/2) + 1 = 21 trusses, not 21 pairs. Each truss is the full width of the building.

What does the lumber estimate include?

This calculator shows top and bottom chord linear footage only. Web members, metal connector plates, gable-end framing, and ridge blocking are not included. Expect the fabricated truss to cost more than raw lumber cost would imply.

Roof Truss Calculator

Frequently Asked Questions

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