What is a Roof Truss?
A roof truss is a prefabricated structural framework designed to support the roof load of a building and transfer that load to the exterior walls. Unlike traditional stick-framed roofs built rafter by rafter on-site, trusses are engineered and manufactured in a controlled factory environment, then delivered to the job site ready to install. This approach saves significant labor time and produces a stronger, more consistent result.
Trusses consist of three primary components: the top chords (the rafters), the bottom chord (the ceiling joist), and the web members that connect them in triangulated patterns. The triangle is the strongest geometric shape, which is why truss designs rely on it. Common truss profiles include the King Post, Queen Post, Fink (W-truss), Howe, Pratt, and Scissors truss for vaulted ceilings.
Roof trusses are used in the vast majority of new residential construction in North America because they are cost-effective, structurally superior, and allow for wide open interior spans without load-bearing walls. They are designed by licensed structural engineers to meet local building codes, wind loads, snow loads, and seismic requirements.
How to Calculate Roof Trusses
Calculating roof trusses involves several key measurements. Our roof truss calculator automates these formulas, but understanding the math helps you verify results and communicate with contractors.
Rafter Length: The rafter (top chord) length is calculated using the Pythagorean theorem. With a building half-span of run and a rise defined by the pitch, the rafter length equals run / cos(arctan(rise/12)). For a 30 ft span at 6:12 pitch, the run is 15 ft and the rafter length is approximately 16.77 ft per side.
Ridge Height: The peak height above the top plate equals run × (rise / 12). For our 30 ft span at 6:12, that is 15 × 0.5 = 7.5 ft of ridge height.
Truss Count: The number of trusses needed is ceil((building length in inches / spacing in inches)) + 1. The +1 accounts for the end truss. A 40 ft building at 24" OC requires ceil(480/24) + 1 = 21 trusses.
Total Linear Footage: Multiply the truss count by the building span to get the total linear footage of truss material. This figure is used by suppliers to quote material costs.
Pitch Angle: The angle of the roof slope in degrees is arctan(rise / 12) × (180 / π). A 6:12 pitch equals approximately 26.6 degrees.
Common Truss Spacing
Truss spacing determines how many trusses are needed and directly affects material cost, sheathing requirements, and structural performance. The three standard spacings used in residential and light commercial construction are:
- 24 inches OC (on center) – The most common spacing for residential roofs. Works perfectly with standard 4×8 plywood or OSB sheathing panels and keeps truss counts and costs lower. Suitable for most standard roofing materials including asphalt shingles, metal panels, and lightweight tile.
- 16 inches OC – Used when additional rigidity is required, such as for heavier roofing materials like concrete tile or slate, or in high-snow-load regions. Increases truss count by 50% compared to 24" spacing.
- 12 inches OC – Rarely used in standard residential construction. Specified for very heavy loads, structural insulated panel (SIP) roofs, or specialty applications. Significantly increases material and labor costs.
Always verify spacing requirements with your local building department and structural engineer, as regional codes and load conditions vary significantly across the country.
Frequently Asked Questions
How far can a roof truss span without support?
Standard prefabricated roof trusses can span 20 to 40 feet without intermediate support, which is one of their greatest advantages over stick framing. This allows for completely open floor plans without load-bearing interior walls. For larger structures, engineered trusses can span 60 feet or more, though these require specialized design and are typically used in commercial or agricultural buildings. The maximum span depends on the truss design, lumber species and grade, load requirements (dead load, live load, snow load, wind), and local building codes. For any span exceeding 40 feet, consult a licensed structural engineer.
How many trusses do I need for a 40 ft building?
For a 40-foot-long building, the number of trusses depends on your chosen spacing. At the most common spacing of 24 inches OC, you need 21 trusses. At 16" OC you need 31 trusses, and at 12" OC you need 41 trusses. The formula is: trusses = ceil((length × 12) / spacing) + 1. The extra truss accounts for the starting end of the building. Always order one or two extra trusses as contingency for damaged units during delivery or installation.
What is the standard roof truss spacing?
24 inches on center (OC) is the standard roof truss spacing for the vast majority of residential construction in North America. This spacing aligns perfectly with standard 4×8 sheet goods (plywood or OSB sheathing), minimizes the number of trusses needed, and reduces overall material costs. It is approved by most building codes for standard residential loads with asphalt shingles. If you are using heavier roofing materials, building in a high snow-load zone, or need extra rigidity, 16" OC may be required by your structural engineer or local code.
Related Roofing Calculators
Use our suite of free roofing calculators to plan your entire project:
- Roof Square Footage Calculator – Find your total roof area and roofing squares.
- Roof Pitch Calculator – Convert between pitch, angle, and slope multiplier.
- Roof Cost Calculator – Estimate total roofing project costs by material and labor.
- Shingle Calculator – Calculate how many shingles you need for your roof area.
- Roof Sheathing Calculator – Determine how many plywood or OSB panels you need.
- Metal Roof Calculator – Estimate panels, screws, and cost for metal roofing.