When designing or building a structure, one of the key elements to consider is the roof structure. The choice of truss system plays a vital role in deciding the strength, durability, and appearance of the roof. Among the various roof trusses available, the queen truss is one of the oldest and most widely used. Known for its simplicity and efficiency in spanning moderate distances, the Queen post truss offers several structural and aesthetic advantages. This blog will cover everything you need to know about Queen post trusses, including their design, components, benefits, disadvantages and applications.
What is a Queen Post Truss?
The Queen post truss is a structural framework commonly used in roof construction. It adapts to the more basic King post truss, but the Queen post truss spans longer distances. It is used in roofing systems that need medium spans; the maximum span of a queen post roof is around 10 meters (33 feet).
Unlike a King post truss with a single vertical post in the centre, a Queen post truss has two vertical posts called queen posts that divide the span into three sections. This results in a lightweight and structurally sound truss that supports the roof’s weight and any external loads like snow or wind. The Queen post truss is used in timber-framed buildings, bridges, and other constructions where aesthetic appeal and structural strength are needed.
Components of a Queen Post Truss
A Queen Post Truss has a number of components, each playing an important role in its strength and stability. These components are:
1. Top Chord: The top horizontal member of the truss, running from one end of the truss to the other. It forms the upper edge of the truss and undergoes compression.
2. Bottom Chord (Tie Beam): This horizontal member runs along the base of the truss, connecting the ends of the truss and helping to keep the queen posts from spreading apart. It is under tension.
3. Queen Posts: The two vertical posts in the truss, located at the third of the span from each side. These posts transfer the load from the top chord down to the tie beam.
4. Struts: These diagonal members connect the queen posts to the top chord. They help distribute the load and provide additional support to prevent bending or sagging.
5. Rafters: The rafters extend from the top of the queen posts to the ends of the top chord. These inclined members form the pitch or slope of the roof.
6. Purlins: Horizontal members that run parallel to the ridge of the roof and rest on the rafters. They provide intermediate support to the roofing material, allowing it to span the distance between the rafters.
7. Soffit: A covering under the eaves of the roof, providing a finished interior appearance and helping with weatherproofing.
These components work together to create a triangular structure that distributes forces evenly, ensuring the truss remains rigid and stable under load.
Queen Post Truss Design
The Queen post truss is designed using principles of geometry and engineering to efficiently support the loads acting on the roof. The design uses the basic triangular form, which is naturally stable and strong.
Here’s a step-by-step explanation of the factors affecting Queen post truss design:
1. Triangular Framework
Like most trusses, the Queen post truss uses triangles to ensure structural integrity. The triangular shape is naturally rigid, meaning it cannot deform unless one of its sides changes length.
2. Load Distribution
The vertical queen posts divide the top chord into three sections, allowing even load distribution across the truss. The tie beam at the bottom resists the outward thrust produced by the roof’s weight.
3. Forces in the Members
- The top chord experiences compression due to the weight of the roof and any applied loads.
- The bottom chord (tie beam) is in tension as it resists the outward forces exerted by the roof.
- The queen posts are primarily under compression, carrying the loads from the top chord down to the bottom chord.
- The struts are under compression and tension, depending on the forces acting on different parts of the roof.
4. Span Capabilities
While a King post truss is suitable for spans up to 8 meters (26 feet), a Queen post truss can span distances up to 10 meters (33 feet). This makes it a preferred option for medium-sized roofs.
5. Pitch
The pitch (angle) of the roof can vary depending on the application, but it is usually between 30 and 45 degrees for most Queen post truss systems. The steeper the pitch, the better it is for draining rain and snow, but it also increases the forces on the top chord.
Uses of Queen Post Truss
Queen trusses have been used for centuries in various types of structures due to their simplicity, efficiency and ability to cover moderate spans. Some applications are:
1. Residential Buildings
Queen post trusses are used in residential roofing systems, especially in houses with pitched roofs. They balance structural and aesthetic appeal, making them ideal for modern and traditional homes.
2. Bridges
In addition to roofing, Queen trusses have historically been used in bridge construction. The truss can support the weight of pedestrian or vehicular traffic, making it suitable for small to medium-sized bridges.
3. Timber-Framed Buildings
The Queen post roof truss is a popular choice due to its relatively lightweight design for timber-framed structures. Timber trusses also enhance the aesthetic of exposed ceilings, which are often featured in rustic or heritage-style buildings.
4. Agricultural Buildings
Barns and agricultural structures frequently use Queen post roof trusses. These trusses allow for wide-open spaces without the need for interior supports, which is beneficial in buildings that need large, unobstructed areas.
Benefits and Drawbacks of Queen Post Truss
| Advantages | Disadvantages |
| Efficient for medium span distances and medium-sized roofs and other structures. | The queen post truss spans are limited to around 10 meters (33 feet). |
| Cost-effective as they are simple to design and construct for residential and commercial buildings. | For larger buildings, more complex trusses or additional supports may be needed. |
| The exposed truss serves as a decorative element to the interior of a building, making it an attractive option for homes, churches and other buildings. | Constructing a queen post truss involves more joinery and components than simpler trusses like a king post. |
| It has good flexibility as it can be constructed from various materials like wood, steel, and engineered timber. Queen truss is adaptable to different types of construction projects. | The construction time and labour costs are high for queen post trusses. |
| Truss design ensures that loads are evenly distributed across structures, reducing the risk of sagging or failure over time. | Even though queen post trusses are strong, they are not suitable for structures that need to bear very heavy loads, like industrial buildings or bridges with high traffic. |
Conclusion
The Queen post truss is a versatile, cost-effective and aesthetically pleasing option for medium-span roof structures. It offers a good balance between strength and simplicity that makes it ideal for residential, agricultural and small commercial buildings. While the queen post truss has limitations in terms of span and load capacity, its advantages in terms of efficiency and design flexibility make it a popular choice in various construction applications.