Beginners Guide to Truss Anatomy
Roof trusses are the backbone of any building's roofing system, providing both structural support and shape to the roof. Understanding the different components of a roof truss is essential for builders, architects, and even homeowners looking to renovate or build from scratch. From the top chord, which defines the slope, to the heel, which plays a critical role in energy efficiency, each part of the truss works together to create a strong and durable framework. In this article, we’ll break down the key components of a roof truss and explain why features like pitch and overhangs are crucial to a well-functioning roof structure.
Key Components of a Roof Truss
Top Chord: The Roof’s Primary Support
The top chord is the top defining member of a truss. This member defines the truss’s slope and supports roof sheathing. In a common truss, the top chord consists of two pieces of wood, commonly a 2x4, that are connected by a metal plate. In areas with snow loads over 45 psi, truss designers may opt to use 2x6 lumber in order to prevent sagging over time.
Bottom Chord: The Ceiling’s Foundation
The bottom chord is the bottom member of a truss that serves as the base. Your sheet rock goes over this area of the truss, which you see as your ceiling instead of the wood members.
Webs: The Reinforcement System
Webs are members that run between the top and bottom chords on the truss. They reinforce the shape of the truss and transfer forces between the top and the bottom chords.
Bearing Plate: Where the Load Transfers
The bearing plate is the horizontal top member of the wall. It usually consists of two 2x4 or two 2x6 members. They are usually drawn on the truss profile as two rectangles with an X drawn in the middle of each. The load on a truss comes down from the top chords, through the webs and the bottom chord to the bearing plates.
Bringing Everything Together
Joints are the connecting points between the webs and the bottom or top chords. Each of the spaces between the joints on the top and bottom chords are referred to as panels. The longer a panel is, the more prone it is to sagging over time. To prevent sagging when using 2x4 lumber, the panels on the top chord are limited to an 8’ span and the bottom chord panels are limited to a 10’ span.
The Heel: Where the Truss Meets the Wall
The heel is a special joint that refers to where the truss sits on the exterior wall. It is typically the end of the truss. These heels are important because they are a fixed point where all the trusses need to line up. In a renovation, it is critical that the heel has the same measurements as the existing framing to ensure a seamless transition between the existing structure and the new structure.
Heel Height for Energy Efficiency
The heel design has grown even more important in Washington due to new energy codes. Truss Components of WA recommends builders to reach out to their architect or insulation contractor to find out what the heel height needs to be to allow adequate space for insulation and air flow.
Understanding Roof Pitch
The pitch represents the slope of the top chord of your truss. Pitch height is defined as how many inches up your truss goes in 12 inches of horizontal run. So in a 6:12 pitch, there is 6” of height gained in 12” of run. The pitch is equally as important as the heel height in renovations or additions. The pitch must be carefully calculated to ensure a seamless transition from the existing framing to the new trusses. For new buildings, a steeper pitch will have a high sloping roof, which gives an elegant look. A lower pitch is practical and provides a great looking roof. When builders decide on an extremely low pitch they may need to take into account extra waterproofing measures in order to prevent water intrusion.
Overhangs: Why They Matter
An overhang is the part of the truss that extends past the bearing wall of the structure. If you experience heavy rainfall and your gutter spills over, the overhang ensures that the water falls away from your foundation. This prevents the footing of your foundation from eroding and keeps water from running down the side of the wall.
Conclusion
A well-designed roof truss system is essential for the longevity and efficiency of a building. By understanding the top and bottom chords, webs, bearing plates, and heel height, builders can order trusses that not only support the structure but also enhance insulation and energy efficiency. Choosing the roof pitch affects both the appearance and functionality of a building’s roofline. Additionally, incorporating overhangs helps protect the foundation and improves the durability of the entire structure. Whether you're working on new construction or a renovation, having a solid grasp of roof truss anatomy ensures a stronger, safer, and more efficient building.
