The Modern Method of Roof Framing
For hundreds of years, carpenters built roofs by hand, measuring and cutting individual rafters on the job site. Today, nearly 80% of all new residential construction uses Prefabricated Roof Trusses.
A roof truss is an engineered, triangular wooden structure built in an off-site factory. It uses advanced computer modeling to maximize structural strength while minimizing the amount of lumber required. The wooden web members are permanently pressed together using spiked steel "gusset plates."
Trusses are delivered to the job site on a flatbed truck and hoisted onto the walls with a crane. What used to take a framing crew a full week to build by hand can now be assembled in a single afternoon.
The Advantages of Trusses
- Speed: A crane can set a new truss every 5 minutes.
- Massive Spans: Because the internal webbing creates a rigid, self-supporting bridge, trusses can easily span 30, 40, or even 50 feet without needing any interior load-bearing walls underneath them. This allows for massive, open-concept floor plans.
- Cost: Despite the engineering, factory-built trusses use significantly less lumber than traditional rafters, making them cheaper overall.
How to Calculate Roof Trusses
Calculating the number of trusses you need is a linear math equation based on the length of the building and the required spacing between the trusses.
The Formula
- Measure the total horizontal Length of the Roof (the length of the building from end to end).
- Determine your required Truss Spacing. (The universal industry standard is 24 inches on center, or 2 feet).
- Divide the Roof Length by the Spacing.
- Add 1 for the final truss at the end of the building. (When you start at 0, you need a truss at the very beginning and the very end).
Total Trusses = (Roof Length ÷ Truss Spacing) + 1
(Note: The 'Span' of the truss—how wide the building is—dictates the size and cost of the truss, but it does not change how many you need).
Example Calculation
You are building a massive rectangular barn that is 60 feet long. The building is 40 feet wide (the span). Your blueprints call for the standard 24-inch (2-foot) spacing.
- Convert spacing to feet: 2 feet.
- Divide Roof Length by Spacing:
60 ft ÷ 2 ft = 30 spaces - Add 1 for the end cap:
30 + 1 = 31
You need to order exactly 31 roof trusses for this building.
Gable End Trusses vs. Common Trusses
When you place your order, you cannot just order 31 identical trusses. You must specify that the two outside trusses (the ones sitting flush on the exterior end walls) are Gable End Trusses.
Standard "Common Trusses" have diagonal webbing designed to support weight from above. Gable End Trusses have vertical vertical webbing (studs spaced every 16 inches) designed so carpenters can easily nail exterior siding directly to the face of the truss.