The main differences between rafters and trusses in roof framing lie in their construction, support systems, and structural behavior. Rafters are traditionally built on-site piece by piece, while trusses are prefabricated in a factory.
Here's a breakdown of the key distinctions:
Construction and Assembly
- Rafters: Constructed on-site, typically from individual pieces of lumber. This allows for more customization but requires significant carpentry skill and time.
- Trusses: Prefabricated in a factory setting to precise specifications. This results in consistent quality, faster installation, and often lower labor costs. Trusses are delivered ready to install.
Support and Span
- Rafters: Rely on load-bearing walls and potentially interior support beams for stability. They transfer the roof load directly to these supports. Longer spans require larger rafters and more support.
- Trusses: Designed as a self-supporting structure. Their triangular webbing distributes weight efficiently, allowing for longer spans with less reliance on interior load-bearing walls. This can create more open floor plans. They tie the outside walls of the home together.
Design and Engineering
- Rafters: Design is often simpler, but requires careful consideration of local building codes, snow loads, and wind conditions. On-site adjustments are possible but must be done correctly.
- Trusses: Engineered for specific loads and spans. Changes to a truss design are not recommended without consulting a structural engineer, as altering the design can compromise its structural integrity.
Cost and Installation
- Rafters: Material costs may be lower, but labor costs can be higher due to the time-consuming on-site construction.
- Trusses: Higher upfront material costs (including engineering and fabrication) but lower labor costs due to faster installation.
Attic Space
- Rafters: Typically provide more usable attic space because the framing members aren't internally webbing the same way as trusses.
- Trusses: Generally result in less usable attic space due to the internal bracing and webbing inherent in their design. Some truss designs, however, like attic trusses, are created with space in mind.
Summary Table
| Feature | Rafters | Trusses |
|---|---|---|
| Construction | On-site, piece by piece | Prefabricated |
| Support | Load-bearing walls, interior supports | Self-supporting, less reliant on interior walls |
| Span | Shorter spans unless heavily supported | Longer spans possible |
| Design | Simpler, requires carpentry skill | Engineered, precise specifications |
| Cost (generally) | Lower material, higher labor (often overall higher) | Higher material, lower labor (often overall lower) |
| Attic Space | More usable space | Less usable space (unless designed otherwise) |
