What is a Roof Truss?
A roof truss is a structural framework of timbers or steel members designed to bridge the space above a room and to support the roof. Trusses are manufactured off-site as pre-engineered components and delivered to the construction site ready for installation. They consist of top chords (rafters), a bottom chord (ceiling joist), and web members that connect them in a triangulated pattern for maximum strength.
Trusses offer several advantages over traditional stick-built rafter systems: they span greater distances without interior load-bearing walls, are faster to install, and are engineered for specific load requirements. Understanding the dimensions and count of trusses needed is essential for accurate budgeting and project planning.
Truss Calculation Formulas
Types of Roof Trusses
| Type | Max Span | Best For |
|---|---|---|
| King Post | Up to 24 ft | Small spans, simple construction |
| Queen Post | Up to 36 ft | Medium spans, residential |
| Fink (W) | Up to 40 ft | Most common residential truss |
| Howe | Up to 60 ft | Heavy load requirements |
| Scissor | Up to 40 ft | Vaulted ceilings |
| Attic | Up to 36 ft | Usable attic space |
Truss Spacing Guidelines
- 16" O.C.: Standard for heavy load areas, required in high snow load regions and for heavy roofing materials like tile or slate.
- 24" O.C.: Most common for residential construction with asphalt shingles and moderate snow loads.
- 48" O.C.: Sometimes used in agricultural buildings with metal roofing panels.
Worked Example
A building 40 ft long with a 30 ft span, 6/12 pitch, and 24" O.C. spacing:
Rafter Length = √(15² + 7.5²) = 16.77 ft
Trusses = (40 × 12 / 24) + 1 = 21 trusses
Frequently Asked Questions
How far apart should roof trusses be?
Standard truss spacing is 24 inches on center (O.C.) for residential construction. In areas with heavy snow loads or when using heavy roofing materials, 16 inches O.C. may be required. Always consult local building codes and engineering specifications.
Can I cut or modify a roof truss?
Never cut, drill, or modify a manufactured truss without approval from a structural engineer. Each member of a truss carries specific loads, and altering any member can compromise the entire structural integrity of the roof system.
How much do roof trusses cost?
Simple residential trusses typically cost $3–$5 per linear foot of span. A 30-foot span truss might cost $90–$150 each. Complex configurations, larger spans, and special requirements increase the price. Delivery and crane costs are additional.
What is the maximum span for a roof truss?
Standard wood trusses can span up to about 60–80 feet, though 24–40 feet is most common for residential construction. Longer spans require deeper trusses, heavier lumber, or steel components. Spans over 60 feet typically use steel or engineered hybrid trusses.