Roof trusses and rafters are the two main ways to frame a roof. Rafters are individual boards cut and built piece-by-piece on-site, while trusses are pre-engineered, factory-built triangular units delivered ready to install. Trusses cost less and span further, but rafters allow for usable attic space. [1, 2, 3, 4]
Design and Construction
- Rafters: Built "stick-by-stick" by carpenters on the job site using individual dimensional lumber leaning against a central ridge board.
- Trusses: Manufactured off-site in climate-controlled factories using smaller lumber pieces joined tightly together with metal connector plates. [2, 3, 5]
Cost and Labor
- Rafters: Higher overall cost due to intensive on-site labor and longer build times.
- Trusses: Lower overall cost; prefabricated design cuts down installation labor significantly, though they often require a crane to lift into place. [2, 6, 7, 8]
Span and Strength
- Rafters: Shorter span capacity (typically up to 24–30 feet) before needing interior load-bearing support walls.
- Trusses: Excellent engineering allows long spans (up to 60 feet) without intermediate interior supports, giving more layout freedom below. [1, 6, 7]
Attic Space and Flexibility
- Rafters: Leave the attic completely open and free of obstructions, making it ideal for future loft conversions, extra storage, or bonus rooms.
- Trusses: Internal web bracing fills the attic space completely, leaving little to no functional headroom or storage room. [1, 4]
A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. The space between each truss is known as a bay.[1]
A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck, roof covering and its associated loads.[2]
Rafters VS Trusses: What Are The Pros And Cons Of Each? - YouTube
Trusses Vs. Rafters | How The Truss Ruined American Houses - YouTube
This video explores the evolution of American roof construction, arguing that while roof trusses have become the industry standard for their speed and cost-effectiveness, they have negatively impacted the design, usability, and long-term health of modern homes.
Roof Trusses vs. Rafters
- Roof Trusses (0:00 - 4:23): Developed in the 1950s using metal plates and hydraulic presses, trusses allow for fast assembly and wider spans without needing internal bearing walls. However, the complex interior 'webs' of a truss typically render attic space unusable and create 'out of sight, out of mind' areas prone to undetected issues like condensation and mold.
- Traditional Rafters (4:24 - 9:30): Hand-framed rafters require more time and skilled labor but offer significant advantages in design flexibility and usable living space (9:30 - 10:07). Without the dense web structure of a truss, attics can be converted into functional lofts, storage, or additional rooms.
The Impact on Housing
- Wasted Space: The speaker argues that approximately 75% of modern American homes use trusses, leading to massive, tall roof structures that contain entirely empty, wasted space (10:07 - 11:20).
- Proportions and Design: Trusses have contributed to modern homes having less desirable proportions compared to older homes that utilized attic space through steep pitches or dormers (11:20 - 13:25).
- Health and Maintenance: The speaker advocates for designs that allow for easier inspection of the home's structure (like vaulted ceilings), suggesting that closed-off, unused attics are a recipe for mold and maintenance neglect (14:15 - 15:26).
When using Insulated Sandwiched Panels (IMPs) or Structural Insulated Panels (SIPs), the panel itself acts as the roof framing and decking combined into one solid unit. Because these panels are self-supporting, thick, and highly rigid, traditional rafters or trusses are usually completely eliminated for standard spans. [1, 2]
Instead, the roof framing shifts from a complex network of skeleton wood or steel to a minimalist system of perimeter and structural support beams.
The Framing Layout with Panels
- Perimeter Supports: Panels rest directly on top of your exterior structural walls, requiring a continuous top plate or structural rim track. [3, 4]
- Ridge Beam: A heavy, continuous wood or steel beam running down the peak of the roof to support the upper ends of the panels. [5, 6, 7, 8]
- Purlins or Purlin Beams: Large horizontal beams spaced several feet apart, running perpendicular to the panels to catch and support them on very wide or steep spans. [9, 10, 11, 12]
- Splines: Internal connection strips (made of wood, composite, or thin metal) inserted inside the panel joints to lock adjacent panels together structurally.
How It Works (No Rafters Needed)
- Long Spans: Standard SIPs or IMPs can easily span 8 to 16 feet between support beams without sagging or needing a single rafter underneath.
- Vaulted Ceilings: Eliminating trusses automatically creates an open, vaulted ceiling inside the building with no extra framing effort.
- Direct Fastening: Panels are lifted into place via crane and screwed down directly into the ridge beam and exterior walls using long, heavy-duty panel screws. [13, 14, 15]
[3] https://up.codes
Here are the official Wikipedia resources for the different roof framing and panel systems discussed:
Traditional Framing
- Rafters: Learn about traditional "stick" framing, maximum length constraints, and the use of engineered lumber.
- Trusses: Explore the engineering principles behind planar trusses, structural depth efficiency, and overall weight distribution.
- Timber Roof Trusses: Dive deeper into wood-specific truss designs, historical variations, and open vs. closed configurations. [1, 2, 3, 4]
Panel Systems & Components
- Structural Insulated Panels (SIPs): Read about the composite sandwich design utilizing OSB boards and rigid foam insulation cores.
- Purlins: Review how longitudinal, horizontal beams support roofs over wide spans, which is critical when layout out structural panels. [5, 6, 7, 8]
(Note: Insulated Metal Panels (IMPs) fall under the broader technical definition of a Sandwich Panel on Wikipedia). [9]
Would you like information on how the building codes differ between traditional framing and panel-based roof installations?
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