Roof Truss Design Guide for Builders and Engineers

A well-designed roof truss is essential for any building, providing structural support and ensuring the safety of occupants. The truss's shape and size are critical factors in its design.

Roof trusses can be designed to accommodate various roof pitches, with the most common being between 4:12 and 9:12. This range allows for optimal snow load distribution and water runoff.

A typical roof truss consists of a series of interconnected rafters and ceiling joists, with the rafters spanning between the exterior walls and the ceiling joists spanning between the rafters. The truss's overall depth and rafter spacing are determined by the load-bearing capacity of the truss.

The International Residential Code (IRC) sets minimum requirements for roof truss design, including a maximum rafter span of 16 feet for trusses with a slope of 4:12 or greater.

A roof truss is essentially a triangular structure that supports the weight of a roof. You can analyze a roof truss using specialized software like the SkyCiv S3D.

To start designing a roof truss, you'll need to decide on the type of section you want to use, such as an AISC L shape. The example in the article uses a 2.5”x2.5”x3/16” L shape.

When fixing the supports, you'll want to release the local Y- and Z-axis to model the members as trusses. This is what the article does by adding supports on each node with code RRFRRR to only fix the Z-axis displacement.

Suggestion: Roof Form

When designing a roof truss, it's essential to consider the "Open" type, which is characterized by a space between the top chord and the bottom chord.

In an open truss, the top and bottom chords are typically spaced 2-3 inches apart to allow for insulation and other components to be installed.

The open design also allows for easier access to the space between the trusses for maintenance and repairs.

A common application of open trusses is in cathedral ceilings, where the space between the trusses can be used for insulation and to create a sense of openness in the room.

The static system of a truss roof is built up by two inclined timber beams connected at the top with a hinge. This connection allows for flexibility and movement.

The beams are supported at their lowest point, either by a pinned or a roller support. In some cases, the support may be located close to the lowest point if the roof has a cantilevered overhang.

Four diagonals connect the top and bottom chords of the truss, taking up normal forces only. These diagonals or webs can be modelled as bar elements due to the nature of the forces they experience.

Roof trusses are the backbone of a sturdy roof, and there are several types to choose from.

The most common type of roof truss is the Scissor Truss, which is made up of two triangular trusses that intersect in the middle.

Scissor Trusses are great for larger homes and can span long distances without the need for additional support.

Another popular type is the Fink Truss, which has a distinctive "fink" shape and is often used in residential construction.

Fink Trusses are typically used for smaller homes and can be more cost-effective than Scissor Trusses.

The Howe Truss is a type of truss that is commonly used in commercial construction and is known for its strength and durability.

Howe Truss is also more resistant to wind loads and can withstand heavy snowfall.

A king post truss is a simple yet effective design that's commonly used in conjunction with two angled struts. The king post truss has two principal rafters, a tie beam, and a central vertical king post.

The king post is typically under tension, which requires quite sophisticated joints with the tie beam and principal rafters. This can be a challenge for builders, but the end result is a sturdy and reliable roof structure.

In a variation known as a king bolt (rod) truss, the king post is replaced by a metal bolt (rod), usually of wrought iron. This design can be a cost-effective alternative to traditional king post trusses.

For more insights, see: How to Cut Roof Rafters for Shed Roof

The queen post truss is a type of roof truss that's particularly useful for extending the span of a roof. It typically has two principal rafters and two vertical queen posts.

One of the key features of a queen post truss is that it can be combined with a king post truss to create a compound truss. This is done by using the straining beam of the queen post truss as the tie beam for the king post truss above.

In Italy, the queen post truss is often known as a Palladian truss, named after the Renaissance architect Andrea Palladio who frequently used it in his designs. Palladian trusses can also be defined as compound trusses with a queen post and king post truss in the same assembly.

The queen post truss is often used in place of the more traditional "Stehender Stuhl" type of roof truss, which features simple vertical posts instead of the more elaborate support structure of the queen post truss.

The arch-braced truss offers a more open look to the interior of the roof compared to other truss types.

This is because it lacks a tie beam, allowing for a more airy feel inside the roof space.

The principal rafters in an arch-braced truss are linked by a collar beam supported by a pair of arch braces.

These arch braces stiffen the structure and help transmit the weight of the roof down through the principal rafters to the supporting wall.

A double arch-braced truss has an additional pair of arched braces lower down, from the rafter to a block or inner sill, giving it a cylindrical appearance.

The hammerbeam roof is a type of truss that allows for greater spaces to be spanned.

It was the culmination of the development of the arch-braced truss, and its design enabled the creation of larger timber-roofed spaces.

The hammerbeam roof of Westminster Hall in London, designed by Hugh Herland and installed between 1395 and 1399, was the largest timber-roofed space in medieval Europe, spanning a distance of just over 20 metres (66 ft).

Hammer beam trusses can have a single hammerbeam or multiple hammerbeams.

A false hammerbeam roof (truss) has two definitions: 1) There is no hammer post on the hammer beam as sometimes found in a type of arch brace truss or; 2) The hammer beam joins into the hammer post instead of the hammer post landing on the hammer beam.

A scissor truss is essentially shaped like a pair of shears, or scissors.

Two key features define a scissor truss: the joint where the bottom chords pass must be firmly connected, and the rafter feet must land on the bottom chords.

The joint where the bottom chords pass is crucial, as it's the hinge of the scissor truss.

If the bottom chords join to the underside of the top chords, the assembly is considered "scissor braced" rather than a traditional scissor truss.

Pre-fabricated wood trusses offer advantages in building construction through machine-made accuracy. They tend to use less timber, which can be a cost-effective solution.

However, this doesn't account for site-specific design alterations that require customized truss design.

When designing a roof truss, it's essential to consider the type of support it will have. A simply supported roof truss is a good starting point, as it allows for easier analysis in 2D.

The SkyCiv S3D software can be used to analyze the roof truss, and by adding supports on each node with code RRFRRR, you can fix the Z-axis displacement. This is a crucial step in the design process.

Using the AISC L shape – 2.5”x2.5”x3/16” section, you can model the members as trusses and release the local Y- and Z-axis node fixity. This will help you accurately calculate the loads and stresses on the truss.

Applying the roof loads and converting them to nodal loads by multiplying each load by the member length can be done using the SkyCiv S3D software. This will give you a more accurate picture of the loads on the truss.

Curious to learn more? Check out: Home Renovation Design Software Free

In the past, truss design was heavily influenced by traditional carpentry methods. The top chords of a truss were often referred to as rafters, and the bottom chord was called a tie beam.

Roof trusses have been around for a while, and there are two main types: closed and open. Closed trusses have a horizontal bottom chord at the foot of the truss, while open trusses have raised bottom chords to create more space.

Historic carpentry used a lot of creative problem-solving to design trusses that would work with the materials available. The closed/open distinction is used in two ways to describe truss roofs, as shown below:

These distinctions are still relevant today, and understanding them can help with truss design and construction.

Closed trusses offer a unique aesthetic appeal by exposing the roof framing to view. This type of truss is often used in open-plan living spaces where the ceiling area is vaulted.

A closed truss can be identified by an interrupted tie beam or a scissor truss. These features allow for a vaulted ceiling area, which can be a striking design element.

Roof framing that is open to view is a key characteristic of closed trusses. This means that the trusses are not hidden by a ceiling, giving the space a sense of openness and airiness.

Here are some key features of closed trusses:

Bracing is a crucial aspect of roof truss design that can greatly impact the stability and safety of a building. Permanent bracing designs can improve the lateral stability of roof trusses, especially those with j chords.

Continuous lateral braces can be set diagonally to prevent negative effects of lateral forces by bracing the chord. The length of the chord determines the number of brace locations.

A well-designed bracing system can make all the difference in withstanding strong winds and other external forces. By understanding how bracing works, builders can create stronger and more reliable roof trusses.

Connections in wooden trusses have a fascinating history. The earliest connections consisted of mortise-and-tenon joints, which were often crafted at the construction site with the posts.

These joints were prone to cracking due to the subsequent shrinkage of unseasoned posts. This is because mortise-and-tenon joints were typically located at the weakest point in the post, accelerating failure.

Drilling holes slightly off-centre allowed the peg to naturally pull the posts together with gravity, anticipating structural behavior under loads.

Some of the earliest truss connection designs were quite clever. They anticipated how the truss would behave under different loads, which is impressive considering the technology available at the time.

Engineering Criteria for Roof Truss Design is crucial for a safe and durable structure.

To design a roof truss, you'll need to consider the span and load requirements. For instance, Brent Wall, a retired construction professional, plans to build a new home with a 32-foot span and 26 trusses for a flat roof.

The span of the roof truss is a critical factor in determining the design. A span of 32 feet, as mentioned by Brent, requires careful consideration of the truss configuration and material selection.

For your interest: Roof Rafter Span

In terms of load requirements, the trusses will need to support the weight of the roof, including any snow or water accumulation. This is especially important for flat roofs, which can be more prone to water accumulation.

A key consideration is the delivery of the truss product to the Denver area, as mentioned by Brent. This will require coordination with a supplier who can meet the specific needs of the project.