Types of Roof Trusses and Their Applications in Residential Design

Roof trusses can be used in a variety of roof shapes, including gable, hip, flat, and gambrel roofs. Roof trusses are becoming increasingly popular in residential home design due to their versatility, strength, and cost-effectiveness.

In this blog post, we look at the different types of residential roof trusses and their advantages for various applications in residential design.

Common Types of Roof Trusses

King Post Roof Truss

King Post Roof Trusses are simple designs consisting of a central vertical post, two sloping rafters, two sloping struts and a horizontal tie beam.

In a king post roof truss, the load path starts with the weight of the roof being transferred to the top of the two sloping rafters. These rafters are then supported by the central vertical post, which is typically located directly under the peak of the roof. The weight is then transferred down the king post to the two sloping struts on either side. These struts are angled and extend outward from the king post to meet the horizontal tie beam, which spans between the ends of the struts. The horizontal tie beam then distributes the weight of the roof evenly across the two struts, which in turn transfer the load to the foundation or supporting walls.

They are commonly used in small or medium-sized buildings with a span of up to 8 meters. King post trusses are symmetrical and relatively easily replicated and assembled in a straightforward manner. This simplicity reduces the potential for errors during construction and makes the truss easier to analyze using structural engineering software, which makes them one of the most cost-effective options.

Figure 1.1: Components of a King Roof Truss (Reference)

Figure 1.2: An example of a King Post Roof Truss in a residential building (Reference)

Queen Post Roof Truss

Queen Post Roof Trusses are similar to King Post Trusses, but have two vertical posts that provide additional support for longer spans and can be more visually appealing.

They are commonly used in larger buildings with a span of up to 12 meters. However, they are typically more expensive to manufacture and install than King Post Trusses due to the additional members and longer spans.

Figure 2.1: Components of a Queen Post Truss (Reference)

Figure 2.2: An example of a Queen Post Roof Truss in a residential building (Reference)

Scissor Truss

Scissor Trusses are designed with sloping bottom chords that intersect at a central point, creating a scissor-like shape. Scissor trusses are characterized by their angled bottom chords, which cross each other in the shape of an "X" at the center of the truss. In contrast, other truss types such as common trusses or king post trusses, typically have horizontal bottom chords that do not cross each other. While it's true that some other types of trusses, such as raised heel trusses, may have angled bottom chords, they are not considered scissor trusses because the chords do not cross over each other in the center of the truss.

This design provides additional height and openness to the space below, allowing for open planned living or commercial spaces with a vaulted or cathedral ceiling.

They are typically used for spans up to 12 meters, and their design and construction are typically more expensive than simpler trusses such as the King and Queen Post.

Figure 3,1: Components of a Scissor Truss (Reference)

Figure 3.2: An example of a Scissor Truss used in residential roof construction (Reference)

Attic Roof Truss

Attic Roof Trusses are a modified version of the king post truss design that includes a room-like space within the roof.

This type of truss is ideal for creating additional living space or storage within a roof without needing an additional level or more land space. Attic roof trusses are designed to create additional living or storage space in the roof area.

These trusses typically cost more than king and queen post trusses due to the additional complexity of their design, including the need for a floor and ceiling structure which results in additional materials.

Figure 4.1: Components of an Attic Roof Truss (Reference)

Figure 4.2: An example of an Attic Truss in residential roof construction (Reference)

Fink Roof Truss

Fink Roof Trusses consist of a network of triangles that distribute the load evenly across the structure, providing maximum strength and stability allowing for long spans that are often utilised in commercial buildings.

Fink trusses are cost-effective, easy to install, and can span up to 30 meters. Notably, the fink truss design features the distinctive W-shape. They are relatively cost-effective due to their simple design and material efficiency.

Figure 5.1: Components of a Fink Truss (Reference)

Figure 5.1: An example of a Fink Truss used in residential construction (Reference)

Truss Designs to Match Roof Type

Builders often have to match truss types used to the architect's requirements. The three main types of roofs in residential construction are hip, gable, and flat roofs.

Figure 6: Different roof types for residential buildings (Reference)

Hip Truss

Hip Trusses are used in hip roofs, where all four sides of the roof slope down to the walls.

Hip trusses are designed to support the weight of the roof and provide additional support for the corners of the building. Hip trusses are commonly used in residential and commercial buildings.

They are complex design to design, which can make them more expensive than simple trusses, especially for larger spans.

Figure 7.1: Hip Truss Setout (Reference)

Gable Truss

Gable Trusses are used in gable roofs, which consist of two sloping sides that meet at a center ridge.

Gable trusses are designed to provide support for the weight of the roof and to transfer loads to the walls. They are one of the simplest truss designs, which makes them very cheap for smaller spans.

Figure 8.1: Components of a Gable Truss (Reference)

Figure 8.2: An example of a Gable Truss used in residential roof construction (Reference)

Flat Truss

Flat Trusses are used in flat or low-pitched roofs often where there are building height limitations due to council regulations, overhanging trees or architectural requirements.

They are designed to provide support for the weight of the roof and to distribute loads evenly across the structure. Flat trusses can be used for roofs in residential buildings or substituted for steel beams in composite concrete slab systems in commercial buildings, as shown in Figure 9.1.

They are more complex to design and install than simple trusses, which can make them more expensive. When using flat trusses, a designer should consider how to adequately provide drainage, so water does not pool on the roof.

Figure 9.1: Composite slab using Flat Trusses (Reference)

Figure 9.2: An example of Flat Trusses system used in construction (Reference)

The internal chords of a flat truss can be arranged differently to be a Warren, Pratt, or Howe truss.

Warren Truss

A warren truss is a type of truss with a series of equilateral triangles within the structure. This truss has diagonal members that alternate in direction and cross over each other in a zigzag pattern. It is commonly used in bridges and other structures that require a lightweight and efficient design. If large steel members are used the warren truss can be used in spans up to 250 feet (75 metres).

Pratt Truss

A Pratt truss is similar to the Warren truss in that it has diagonal members that alternate in direction, but it features vertical members as well. This truss is characterized by its vertical members that form the posts, and its diagonal members that slope towards the center of the span. The Pratt truss is commonly used in spans up to 250 feet (75 metres) where large steel members are used, and it is often used in bridges, industrial buildings, and other structures that require a strong and rigid design.

Howe Truss

A Howe truss is a type of truss with vertical members that form the posts, and diagonal members that slope away from the center of the span. This truss is characterized by its diagonal members that are under compression, and its vertical members that are under tension. The Howe truss is commonly used in spans up to 200 feet (60 metres), and it is often used in bridges, roofs, and other structures that require a strong and efficient design.

Figure 9.3: Chord arrangements in Flat Trusses (Reference)

Mono Roof Truss

Mono Roof Trusses (also known as monopitch and skillion trusses) are similar to Fink trusses but are designed to span a greater distance with a shallower pitch which are often used in structures such as sheds, garages, and small workshops.

This design is commonly used in commercial and industrial buildings where a low-pitched single-sloping plane roof is desired due to building height limitations or architectural requirements.

Their simple design makes them a cost-effective option for smaller spans, but they typically need additional reinforcement to make them structurally adequate for longer spans in comparison to king and queen post trusses.

A mono vaulted truss is a variation of the mono truss that has a raised or vaulted center section, creating a more open and spacious interior. The raised center section of the mono vaulted truss is achieved by extending the center portion of the top chord upwards, while maintaining the same angle of the bottom chords. This creates a peaked or arched shape in the center of the roof, which provides additional headroom and allows for larger windows or skylights.

While the basic design of the two truss types is similar, the addition of the raised center section in the mono vaulted truss requires additional engineering and support to ensure that the roof structure can handle the additional weight and stress.

Figure 10.1: Different types of Mono Trusses (Reference)

Figure 10.2: An example of Mono Truss being used in residential roof design (Reference)

Design Standards for Roof Trusses

In Australia and the United States, there are several design standards that must be followed when designing roof trusses.

In the United States, the most common standard is the National Design Specification for Wood Construction (NDS), which provides guidelines for the design and construction of wood structures, including roof trusses.

In Australia, the most common standard is the Australian Standard for Timber Structures (AS 1720), which provides guidelines for the design and construction of timber structures, and the Australian Standard for Installation of nailplated timber rooftrusses (AS 4440) which provides specifications and performance requirements and for the bracing, connection and installation of nailplated timber trusses.

These standards are conveniently built into ClearCalcs Truss Analysis Wizard.

Deciding Which Truss Type to Use

When deciding on what type of trusses to use for roof design, engineers must consider a variety of factors, including the shape and span of the roof, height restrictions, architectural requirements, ceiling type, material availability, and load applied.

King Post and Queen Post trusses are commonly used in small to medium-sized buildings, while Scissor, Attic, Fink, and Mono trusses are used in larger and more complex buildings and are typically more expensive.

Hip, Gable, and Flat trusses are used to meet architectural requirements for the shape of the roof.

Additionally, engineers must consider procurement times and ease of installation. Prefabricated trusses are typically better quality (constructed by professionals), cheaper (reduction in onsite labor costs), and easier to install (fewer onsite connections) but procurement times can often be 1-6 months.

If the lead times in your area for prefabricated trusses are too long, structural engineers can design trusses to be stick-built on-site. This involves cutting timber and assembling the necessary components on-site, using nail-plated connections. Stick-building trusses can be a time-consuming process, but they can be a cost-effective solution for smaller projects or projects with unique design requirements.

ClearCalcs Truss Analysis Wizard can be incredibly useful for trialing different truss types to determine what the most efficient design will be.

Cost Ranking for Trusses

It's difficult to provide an exact ranking of trusses in terms of cost, as prices can vary depending on a variety of factors such as size, design complexity, location, and local market conditions. However, in general, the trusses listed above can be ranked from least expensive to most expensive as follows: including their maximum span in meters:

• Flat truss: Simplest and least expensive, with spans up to 6 meters.
• King post truss: Relatively simple and inexpensive, with spans up to 10 meters.
• Queen post truss: Slightly more complex than a king post truss, with spans up to 12 meters.
• Gable truss: Common and relatively simple, with spans up to 15 meters.
• Fink truss: Commonly used in residential and light commercial construction, with spans up to 25 meters.
• Attic truss: More complex truss designed to create additional living space in the attic, with spans up to 30 meters.
• Mono roof truss: Simple truss with a single sloping plane, with spans up to 35 meters.
• Hip truss: More complex truss with sloping rafters on all sides, typically used in larger buildings, with spans up to 45 meters.
• Scissor truss: More complex truss with angled bottom chords that cross over each other to create a vaulted or arched shape, with spans up to 50 meters.

Again, it's important to note that these maximum spans are general and can vary depending on a variety of factors such as truss design, building materials, and local building codes