Ever looked up at a roof and thought, “What’s holding that thing up?” Well, let’s take a peek under the hood of roof trusses, the unsung heroes of construction. These triangular marvels are more than just pretty shapes; they’re the backbone of your roof, bravely battling snow, rain, and the occasional squirrel invasion.
1. Structure of a Basic Roof Truss
A roof truss consists of several key components that work together to form a stable and efficient structure. Here are the primary parts:
- Top Chord: This is the upper edge of the truss, typically experiencing compressive forces. It connects the apex of the triangle to the ends of the truss.
- Bottom Chord: This member runs parallel to the top chord and usually faces tensile forces. It provides support for the ceiling and maintains the truss’s overall shape.
- Web Members: These include the structural elements that form the triangles between the top and bottom chords. They can be in tension or compression, depending on their position and load direction.
- Panel Points: These points represent the intersections of truss members. They are key for distributing loads and transferring forces between members.
- Ridge Line: The topmost point of the truss where two top chords meet, serving as the peak of the roof.
- Eaves: This refers to the lower edge of the roof overhanging beyond the exterior walls. It helps in directing water away from the building.
- Purlins: Horizontal members that support roof covering materials and provide additional stability to the structure. They can sit atop the truss or be part of the overall design.
- Rafters: While not always included in truss designs, rafters can work alongside trusses in certain applications to provide extra support.
- Struts and Ties: Struts handle compressive forces, while ties manage tensile forces within the truss structure, ensuring that loads are effectively carried across the entire unit.
Understanding these components helps in recognizing how a roof truss supports the entire roof system, ensuring durability against various environmental factors.
2. Structure of a Traditional King Post Roof Truss
A King Post Roof Truss stands out for its simple yet effective design. It’s primarily composed of a central vertical post, known as the king post, along with two rafters that meet at the apex and a horizontal tie beam that forms the base.
a. What Is a King Post Roof Truss?
A King Post Roof Truss is the most straightforward type of truss, making it a popular choice for various structures. This truss comprises a king post, two inclined rafters, and a tie beam that forms a triangular shape. Commonly found in smaller buildings like sheds, porches, and garages, its uncomplicated layout provides ample support for roofing materials while maintaining structural integrity.
- Ridge Board: A horizontal member that helps align rafters; it’s non-structural but essential for overall stability.
- Common Rafter: Also known as a minor rafter, it’s shorter than the principal rafter, providing additional support.
- Principal Rafter: The main, larger rafter that extends from the tie beam, capable of bearing significant load and supporting purlins.
- Wall Plate: A horizontal piece of lumber secured to the wall that supports the base of the rafters.
- Supporting Wall: A load-bearing wall that provides vital structural support to the entire truss system.
- King Post: The vertical element at the truss’s center, which transfers loads from the roof to the tie beam.
- Strut: A diagonal member under compression, enhancing the truss’s rigidity and overall strength.
- Tie Beam: The horizontal beam that connects the lower ends of the rafters, key for holding them in place.
- Purlin Cleat: A strip of iron affixed to purlins, securing them in place.
- Purlin: An important horizontal beam that rests on the principal rafter, supporting the common rafters. It comes in three types in wood construction: purlin plates (under purlin), principal purlins, and common purlins.
3. Components of a Traditional Queen Post Roof Truss
What Is a Queen Post Roof Truss?
The Queen Post Roof Truss is a variation of the classic King Post Truss. Its primary distinction lies in the inclusion of two vertical posts, known as queen posts, instead of a single king post. This design offers a lighter structure while accommodating larger spans, making it an excellent choice for extended areas.
Key Elements of the Queen Post Roof Truss
- Ridge Board: A central, non-load-bearing beam that provides support for connecting rafters at the apex.
- Straining Beam: A short timber piece that stabilizes the ends of the struts and rafters, maintaining their alignment.
- Common Rafter: Also referred to as a minor rafter, this component is smaller than the principal rafter and aids in supporting the roof structure.
- Purlin Cleat: An iron strip affixed to the purlin, adding strength and stability to the truss system.
- Wall Plate: A horizontal timber beam positioned atop the walls to support the rafters securely.
- Supporting Wall: This structural element bears the roof load and serves as a critical foundation for the truss system.