Manufactured Trusses Explained! Why they’re replacing stick-framed roofs

Introduction

The last few decades have seen roofing trends shifting significantly from stick-framed roofs to manufactured trusses. This has been driven by efficiency, cost, and engineering computational advancements. According to the Home Builder Innovation Labs, 66 percent of surveyed homebuilders used factory-built roof trusses for new single family detached homes in 2017. These statistics were published in the 2020 winter issue of Evidence Matters, which is a publication by the Office of Policy Development and Research in the United States Department of Housing and Urban Development.

Stick framed roofing involves framing a roof using rafters and ceiling joists. The rafters are framed to a ridge board at the ridge or supported by a ridge beam. The framing elements, in this case, are cut and assembled on site. This requires highly skilled carpenters and is very time-consuming. This was the dominant method of roof construction all the way to the later parts of the last century when manufactured trusses began to take over.

Manufactured trusses have great versatility and may be used to frame all types of roofs including gable roofs, hip roofs, shed roofs, flat roofs and other types of roofs. This versatility is derived from the fact that you can literally configure individual framing elements to come up with a truss of any shape as long as the members and connections have sufficient capacity to support the load. The most common truss shapes and types used in residential construction include the fink truss, scissor truss, Hip truss, the mono pitch truss, flat truss and others.

A gable roof typically consists of common trusses only. Scissor trusses may also be used where a vaulted ceiling is desired. On the other hand, a hip roof is framed using a combination of different trusses to create sloping 'ridges' that extend from the corners of the building up to the ridge at the top of the roof. A hip roof consists of common trusses, hip trusses, and smaller mono-pitch trusses that are commonly referred to as jack trusses. Jack trusses span between the last hip truss, which is also known as a hip girder truss, and the exterior walls that are parallel to the hip trusses.

This way of framing where some trusses are designed to support other trusses without needing interior columns is one of the reasons why trusses are able to provide diverse possibilities in roofing styles without undermining their ability to span the expected distances.

Parts of a Truss

Trusses are made up of two groups of structural elements. These are the framing members and the connections. The framing members consists of three major parts. First, there is the top chord, which is the horizontal or diagonal member or members at the top of the truss. Then there is the bottom chord which is the horizontal or diagonal member that forms the bottom of the truss. Finally, there are web members which are the interior diagonal members extending between the top and bottom chord.

Mechanics of Trusses

Let us now look at the internal mechanics of how trusses support load.

In general, structural element supporting loading will typically be subjected to one or a combination of the following loading effects.

First, when a load is acting axially on a member, then the member is subjected to axial load. Axial load may either be compressive where the forces pushed axially into the member or tensile where the force pulls away from the member. A column supporting vertical gravity loads is subjected to axial compressive forces. On the other hand, steel rods that are supporting mechanical equipment that is hanging from the ceiling are subjected to axial tensile forces. As we shall see shortly, truss members are also subjected to axial compression and tensile forces.

Secondly, when a member is loaded laterally along its length, then the member is subjected to shear forces and bending moments. This is typically the case with floor joists and floor beams which are horizontal elements that are primarily loaded from above and thereby subjected to bending moments and shear. These members will also experience deflections resulting from the flexural or bending stresses and shear stresses. Deflection is a critical design consideration especially for long span members where deflection limits will govern over bending strength. One of the primary reasons why trusses are preferred over bending members such as rafters is their ability to span long distances with relatively minimal overall deflections.

Thirdly, there are torsional forces which are moments acting radially about the central longitudinal axis of a member. A good example is a guard rail post that is attached to a beam on one side. When a lateral force is applied at the top of the guard post, the post is subjected to a bending moment which becomes a torsional moment on the beam supporting the guard post.

Trusses are designed so that the truss elements will be subjected to axial load without seeing any significant bending, shear or torsional loading. This allows the designer to use framing members with small cross-sectional area to efficiently configure long span trusses. In most cases, the top chord members will see bending moments and shear when there is uniform loading along the length of the member. The loads will however become axial forces once they get into the nodes or connections between the top chord and the web members. These forces will be transmitted throughout the truss framing elements as axial forces. The presence of multiple top-chord to web-member connections along the length of the top chord minimizes the deflections due to bending when the top chord is uniformly loaded. In most cases, truss designers typically convert the uniform load over the truss top chord into nodal reactions and use the nodal point loads to design the truss framing elements as axially loaded elements.

The top chord will be subjected to axial compressive forces when the truss is supporting vertical gravity loads that are acting downwards. If uplift loads are applied, then the top chord will see tensile forces. On the other hand, the bottom chord will have axial tensile forces when the truss is supporting net vertical loads acting downwards. When the truss is supporting uplift loads, the bottom chord will be in compression. Just like the top and bottom chord, some of the web members will be under tension while others will be under compression depending on the direction of the net loading acting on the truss.

The Design of Trusses

Manufactured trusses are designed by licensed civil or structural engineers and are built off site after which they are transported to the building site for installation. The design of manufactured trusses in the united states is governed by the international building code and international residential code. These codes are adopted by nearly all states and jurisdictions across the united states. For most states, the adoption process involves amendments where the final documents takes the name of the state adopting it. For example, the international building code becomes the California building code after amendments and adoption in the state of California. That being said, there are other states that use the international residential code and building codes without amendments or without changing the name.

The structural design of manufactured trusses is almost entirely done by specialized software. Some of the most common tools used by designers include the Mitek Truss Designer, Simpson Truss designer, Alpine Truss Designer and many others.

For homes or buildings designed to use manufactured trusses, the design of the trusses is often delegated to a specialty truss design engineer while the primary designer of record, designs the rest of the structure. Additionally, in most cases, the design of the trusses is often deferred which means that the design of the trusses does not need to be completed and reviewed before permit issuance. In this case, the rest of the design is reviewed and a permit issued so that construction can begin while the design of the trusses is ongoing. This helps to save time.

When the truss design is deferred, the designer of record must still define the loading and layout of the trusses. In most cases, the designer of record or engineer of record works very closely with the truss engineer to define the truss layout. Additionally, the designer of record should also have a good estimate of the reactions at the girder trusses which may require posts and pad foundations which are part of the primary structure design that needs to be completed before permit issuance.

Advantages of Manufactured Trusses over Stick Framed Roofs

Manufactured trusses have several advantages over stick framed roofs.

First, Manufactured trusses are pre-engineered and often delivered to the site pre-assembled or ready for quick installation. This can significantly speed up the building process compared to stick framing, where each piece must be cut and assembled on-site.

Secondly, Since trusses arrive pre-manufactured, less on-site labor is required for assembly. Stick framing requires skilled carpenters to cut and fit each piece, which can be time-consuming and costly. Additionally, trusses are designed and cut to exact specifications, minimizing material waste.

Thirdly, trusses are often more structurally efficient than stick framing especially if you consider the larger spans that you can get with trusses compared to bending elements of the same weight.

Fourth, trusses come in a range of designs and configurations to suit different types of roofs or load requirements. Trusses can be engineered to fit specific design needs without sacrificing strength or functionality. Additionally, trusses allow for larger open spaces without the need for interior load-bearing walls or posts which is beneficial where large clear spans or vaulted ceilings are desired.

Other advantages include the fact that trusses are often pre-assembled and delivered to the site in a compact, organized manner, making them easier to transport and manage compared to the various lengths of lumber needed for stick framing. There is also minimal or no need for on-site cutting which reduces the sources of dust and debris.

We have provided a comprehensive course on how to incorporate engineered manufactured trusses into a residential structural design, please check out our conventional roof framing design course at https://www.conventionalframing.com/conventional-roof-framing

If you are a plan reviewer or inspector, we also have a plan review course on conventional roof framing which includes plan review of manufactured/ engineered trusses. The plan review course is provided at: https://codexpca.com/courses/2021-irc-roofs-and-manufactured-trusses/ (International Residential Code) and https://codexpca.com/courses/conventional-roofs-and-trusses/ (California Residential Code). All the courses are approved to provide continuing education units (CEUs) by the International Code Council.