Roof Truss Calculator
Calculate the number of roof trusses, chord lengths, and total board feet for your roofing project. Also see our Rafter Length Calculator, Roof Pitch Calculator, and Roofing Calculator.
Truss Dimensions
How to Use the Roof Truss Calculator
Enter the building span (distance between exterior walls), roof pitch expressed as rise per 12 inches of run, truss spacing (typically 24" on center for residential), overhang length, and total roof length. The calculator determines the number of trusses needed, individual chord lengths, web member count, total lumber in board feet, and estimated assembly weight. This helps you plan material orders and crane requirements for truss installation.
Formula
Truss Height = (Span / 2) × (Pitch / 12)
Top Chord = 2 × √((Span/2)² + Height²) + 2 × Overhang
Bottom Chord = Span + 2 × Overhang
Number of Trusses = (Roof Length × 12 / Spacing) + 1
Weight ≈ 2.5 lbs per foot of span × number of trusses
Example Calculation
Span = 24 ft, Pitch = 6:12, Spacing = 24" OC, Overhang = 12", Length = 40 ft
Truss Height = (24/2) × (6/12) = 6 ft
Top Chord = 2 × √(12² + 6²) + 2 = 2 × 13.42 + 2 = 28.8 ft
Bottom Chord = 24 + 2 = 26 ft
Number of Trusses = (40 × 12 / 24) + 1 = 21 trusses
Estimated Weight = 21 × (2.5 × 24) = 1,260 lbs total
Reference Table — Truss Types and Maximum Spans
| Truss Type | Max Span (ft) | Best For | Web Count |
|---|---|---|---|
| King Post | 20 | Small sheds, garages | 1 |
| Queen Post | 30 | Residential, moderate spans | 2 |
| Fink (W) | 40 | Most residential roofs | 5 |
| Howe | 50 | Commercial, heavy loads | 5-7 |
| Pratt | 60 | Long spans, light loads | 5-9 |
| Scissor | 35 | Vaulted ceilings | 4-6 |
| Attic | 30 | Livable attic space | 6-8 |
Frequently Asked Questions
What is the standard spacing for roof trusses?
Residential roof trusses are typically spaced 24 inches on center (OC). For heavier loads such as tile roofing or areas with high snow loads, 16-inch spacing may be required. Commercial buildings may use 48-inch spacing with heavier trusses.
How much does a roof truss weigh?
A standard residential truss weighing approximately 2.5 lbs per foot of span. A 24-foot span truss weighs about 60 lbs, while a 32-foot span truss weighs approximately 80 lbs. Larger commercial trusses can weigh 200+ lbs.
What is the most common truss type for homes?
The Fink truss (also called a W-truss) is the most common for residential construction. It handles spans up to 40 feet efficiently and uses less lumber than other designs while providing excellent load distribution.
Can I build my own roof trusses?
While it's possible to build trusses on-site, most building codes require engineered trusses with metal connector plates for structural integrity. Factory-built trusses come with engineering stamps and are typically more cost-effective than site-built alternatives.
What pitch is best for roof trusses?
A 4:12 to 6:12 pitch is most common for residential roofs. Steeper pitches (8:12+) shed snow and rain better but require more material. Low-slope roofs (2:12 to 4:12) are more economical but need special waterproofing considerations.
How far can a roof truss span without support?
Standard residential Fink trusses can span up to 40 feet without intermediate support. For spans exceeding 40 feet, special engineering, heavier lumber, or intermediate bearing walls may be required. Always consult a structural engineer for spans over 30 feet.