Roof Trusses

Structural support, usually a beam or wall, that is designed by the building designer to carry the truss reaction loads to the foundation.

Every truss requires at least two bearings, or points, to transfer its loads to. Typically, it is a 2x4 or 2x6 wood frame wall or panel in residential construction but can also be a light gauge steel wall or block wall in commercial or industrial applications. Likewise, bearings can be beams, ledgers, headers, or posts made from any of the above-mentioned materials. Regardless, the bearing must be strong enough to support the loads transferred from the trusses above. If the bearing material is not strong enough, crushing can occur when the truss is under designed loads. This results in serviceability issues like cracked sheetrock and dips in ceilings above and floors below. In extreme cases failure can occur, albeit rare.

Other trusses can also be used to support adjacent trusses connected at some type of angle, usually 90 degrees with a metal connector joist or truss hanger. In some instances with very light loads, trusses can be nailed together to act as a bearing condition. The carrying truss that supports other trusses is known as a girder truss. The bearing supporting the girder truss is most likely to suffer from the crushing effects mentioned above. Enhancements can be made to the side of the truss where it connects to the wall known as bearing enhancers or squash blocks to better transfer the loads of the girder over a greater surface area of the bearing.

Additional bearings can be designed to the interior area to help support the truss mid span, lessening the load reactions at the outer bearing and potentially reducing the amount of lumber and the size of the metal connector plates required to build the truss. These bearings need to be supported by an adequate support structure downward though the wall to any floor systems below and into the foundation system of the structure.

Bearing width knowledge is critical to truss design to ensure the truss meets expectations in the construction process. An example of this is the seat cut of a scissors truss. If the bearing size varies from the truss design, the ceiling area can be impacted where it meets the wall and modifications will be necessary when installing drywall or other finish products.

Typically, trusses are connected to the bearing temporarily with nails and permanently with a metal connector designed for uplift and lateral load considerations.

Inclined, or horizontal member that establishes the bottom of a truss, usually carrying combined tension and bending stresses.

The bottom chord is one of three key components to any truss. In addition to the top chord and webs, the bottom chord is used to transfer forces to the truss bearings. The bottom chord typically carries combined tension and bending stresses. For traditionally spaced trusses, the bottom chord is cut out of 2x4 or 2x6 dimension lumber but can also be cut out of 2x8, 2x10, 2x12, and even LVL material for roof trusses. Factors that determine the size of the bottom chord include on-center spacing, dead loads applied to the truss, span, and if there is any pitch applied to the bottom chord. An example of this is a vaulted ceiling application framed by scissors trusses. In some cases a live load can be applied to a bottom chord, either for storage purposes or if there will be movement within the truss, such is the case in an attic truss.

In floor truss applications, 2x4 or 2x3 material is typically used with the wider portion of the board resting on the bearing or what is known as a 4x2 configuration. Orienting the chord material in a 4x2 manner creates a very stable truss that allows installers the ability to walk easily across members without tipping the truss while installing additional members or floor deck sheathing.

In standard heel applications of roof trusses, the bottom chord is cut on each end at an angle consistent with the roof pitch of the truss. This creates additional surface area for the two members to transfer forces and allows for them to be plated together, creating a strong, resilient joint. On the outermost part of a bottom chord a small vertical cut, known as a butt cut, is created to give the truss a starting and ending point that can be aligned with the bearings below. The butt cut is typically ¼” on standard heel trusses but can be increased for a taller heel. When combined with a slider or stacked chord, a raised heel, sometimes known as an energy heel, can be created to raise the profile of the roof and increase the amount of insulation and ventilation over the bearing of the structure.

The bottom chord is intersected by the top chord and web members at various points throughout the truss, creating a joint secured by metal connector plates. A bottom chord can be multiple pieces of lumber secured by a metal plate known as a splice, either at a joint or mid panel. While a vast majority of trusses will have bottom chords of the same species and grade, changes in both lumber species and grade are possible at splice points. This is most often seen in attic trusses where the bottom chord is increased in the living space and additional loads are applied and then reduced near the bearings to reduce expense.

Part of a truss that extends beyond its support, exclusive of overhang.

A cantilever is a portion of a truss that extends over its bearing, allowing both bottom and top chords to function beyond the limits of the bearings. Typically special webbing conditions are designed into cantilever trusses to ensure loads throughout the truss transfer properly to the bearings. Cantilevers can be up to one-third of the overall span on the truss.

A common application for a cantilever is a porch where a post and beam bearing may otherwise be used. Truss technicians that incorporate a cantilever into their truss designs can save the project considerable costs with minimal changes in the design of the truss. A cantilever can also be used in lieu of an overhang creating height within the truss over the bearing, similar to an energy heel. When this method is used for overhangs, considerations for sub-fascia should be taken into account and manufacturing tolerances are greatly reduced for variances in the truss and overhang.

Truss installers will sometimes ask what bracing is required when setting trusses, referring to the CLRs from the Truss Design Drawings (TDD). While the CLRs are specifically called out on the TDDs, they are not the only bracing required for proper truss installation. Temporary and permanent bracing are both required during and after installation. In fact, CLRs require additional bracing to effectively do their job. Please refer to BCSI as a reference for all required bracing during and after truss installation, including bracing of CLRs. If you have any questions on required CLRs for a specific truss application, please refer to the project truss technician or salesperson.

The point on the truss where the top and bottom chords intersect.

The truss heel is typically at the end of the truss, usually over the outer bearing. It consists of a minimum ¼” butt cut and the height of the top chord measured vertically or plumb. This is known as a “standard heel” application whose measurement varies based on the top chord material and the pitch of the truss. The greater the pitch and wider the material used, the higher the standard heel.

Raised heel applications increase the roof plane over the outer bearing walls to increase capacity for ventilation and insulation. This condition is often referred to as an “energy heel” and, while no standard measurement exists for an energy heel, anything over 9” is commonly considered applicable. Often, raised heel trusses will require blocking between each other for shear purposes. While solid wood blocking can be used, this is an opportunity for flat top truss blocking, furthering the use of components.

Multiple methods exist to increase the height of a heel. The butt cut can be increased from ¼” up to the full width of the bottom chord member. Additionally, a “slider” adjacent to the top chord or bottom chord can further increase the heel height and ultimately a “vertical web” of any length reinforced with a web member can meet any height necessary as required by design specifications. Alternatively to a slider, a reinforcing web can be used along the top or bottom chords that extend the entire length of the first panel.

Architecturally, the heel is quite significant to the overall building design, but is often not specifically defined in construction documents and can create confusion between various design professionals, including the truss technician. If the heel is not specified on the construction documents, the truss technician is required to scale the heel height from a section view, which can be less than desirable for accuracy. The heel dictates the plane on which the roof is established, and if it is designed incorrectly, can carry significant implications to the roof and how it interacts with the rest of the structure. An example of this is the heel of a truss that sits on the first floor that planes with a second floor roof.

Vertical distance between bearing and the uppermost point of the peak.

The overall height of a truss is a function of four variables: span, pitch, heel, and type of truss. The greater the span, pitch, and heel, the taller the truss will be. The truss type dictates where any pitch breaks occur, changing the direction of the top chord. Truss height is measured vertically between the uppermost point of the peak and the bearing. The truss height impacts overall building height, which is the sum of the truss height with sheathing, wall heights, and floor containers below. In a common truss, the peak will be located in the center of the truss. In a mono truss, the peak will be at one end of the truss. To better understand the wide variety of truss types, please refer to the truss configurations web page.

Manufacturing capabilities for individual component manufacturing facilities dictate the maximum overall height that can be manufactured at that location. In instances where design requirements require an overall truss height greater than manufacturing or delivery capabilities, a multi-piece or piggy back truss can be used where the truss is split into two different trusses stacked on top of each other during installation with special bracing considerations.

Overall height should not be confused with shipping height. Shipping heights take into account any overhang that may extend below the truss bearing. Shipping height is important when determining how trusses will fit on a trailer during the delivery process and if and when over width considerations should be taken. Typically, both overall height and shipping height should be called out on the individual Truss Design Drawings (TDDs). For more information on how to read TDDs visit the related SBCA Research Report.

Extension of the top chord of a truss beyond the outside of the bearing support.

Overhang is the portion of the truss that extends beyond the outside bearing of the structure, creating an eave for the building. Overhang length can vary from as small as 6” to as great as 36” in certain applications. Longer overhang lengths are certainly possible, but may require some sort of support, either as a structural fascia board or beam to support the overhang. For a pitched roof application, the overhang is subject to the slope of the top chord and in longer overhang scenarios can create conflicts with header heights for windows and doors.

Overhang material is typically the same as the top chord material, as it is usually the same member, just extended. Some truss technicians will create a splice joint near the bearing to differ the overhang material from the top chord in a cost saving method or to meet specific architectural requirements in the construction documents. Special considerations should be made in cases where the truss overhang will be exposed once the structure is complete. Overhang members should be culled for visual appearance and precautions are recommended to protect the overhangs of trusses shipped in inclement weather to protect from mud, road grime, and other hazards.

Most component manufacturers will manufacture and ship overhangs at the full width requirement anticipating the truss installers to trim them back for a fascia board to create an even, consistent overhang with a striking visual appearance. Trimming the overhang removes small, yet obvious, visual inconsistencies caused by a variety of factors: how the truss was set on the bearings, how it was manufactured, how the bearing is positioned, or any combination of these. An example of this is a truss with a 24” overhang that will be trimmed back to 22 ½” in the field with a 2X sub-fascia to finish at the full 24” per the construction documents. Alternatively, some component manufacturers will reduce overhangs by 1-½” anticipating a 2X sub-fascia member applied in the field. This is quite common when the overhang is actually a cantilever and any modification to the truss would jeopardize the integrity to the heel joint.

A majority of overhangs are cut and manufactured with a plumb Cut, where the end of the overhang is cut with a vertical orientation. In addition to a plumb cut, overhangs can have a square cut, horizontal cut, or double cut. A square cut overhang is cut square to the 2x member typical of how an unaltered rafter member would appear if it were used, resting on the bearing at a particular pitch. A horizontal cut overhang is cut level or parallel to the bearing. A double cut utilizes both a plumb cut and horizontal cut in order to minimize the fascia line. It is a great way to downsize fascia appearance on an oversized top chord member. For instance, a girder truss with a 2x8 top chord and overhang can be double cut to match adjacent 2x4 top chord trusses.

Overhangs can also include a soffit return that provides support for certain soffit types. It is a horizontal member that is plated to the end of the overhang that returns back to the bearing but is cut short so not to interfere with the wall sheathing or other parts of the bearing during installation.

Overhangs provide an important architectural element to a structure and truss design relative to overhangs have significant implications for exterior finishes.  

Panel Length: Horizontal distance between the centerlines of two consecutive panel points along the top or bottom chord.

Panel Point: Location on a truss where the web members and top or bottom chords intersect and are connected by metal connector plates.

Trusses are made up of a series of panels, or the horizontal distance between the centerlines of two consecutive panel points along the top or bottom chord. Panels are imperative to the triangulation of the truss that allows the transfer of forces through the truss members and ultimately to the bearings.

Panel length is determined by the truss technician and typically determines what size material is used for the top or bottom chord. The greater the panel length, the more likely the truss technician is to use wider width material. The truss technician’s primary task is to design the most efficient truss while meeting loading and design conditions as well as adhering to a variety of code, safety, and standards requirements.

Panel points are located where web members intersect with top or bottom chords, which are connected by metal connector plates. Panel points determine the length of the panel and webbing formation of the truss. Shifting panel points even small distances can impact the efficiency of the truss and transform a truss that doesn’t pass design requirements to a truss that does. This is particularly evident when point loads are applied to a truss.

Point on the truss where the sloped chords meet.

The truss peak is the uppermost point where sloping chords intersect. In a typical common roof truss that slopes up with the same pitch from each bearing, the peak is in the center of the truss span. If the heel heights or pitches differ on each side, the peak will be offset. Many trusses, such as hip or flat top chord trusses don’t have a peak.

The peak is important in determining the rafter length, or the measurement from the overhang to the top (peak) of the truss. This measurement is most commonly used for determining roof metal length in agricultural structures and other applications where cut to length metal roofing is used.

Another key measurement in roof truss production is the heel to peak measurement as many production crews will use it to measure the truss for ”square” or to make sure it is properly manufactured.

Incline of the roof described as inches of rise over inches of run. For example, 5/12 is 5 inches of rise over 12 inches of run.

Pitch is one of the most common characteristics of roof trusses and is required to determine the geometry of the roof. It can describe the roof pitch, or the angle of the top chord, as well as the pitch of a vault or the angle of the bottom chord in cathedral ceiling applications. Industry norm is to standardize all pitch values to a common denominator, 12 inches, or how much the roof rises over one foot. For example, a 5/12 pitch is 5 inches of rise over 12 inches of run and is simply described as saying the two numbers “five twelve.” Other regional variations for stating the pitch include “five over twelve” and “five and twelve.” Values can vary from 0/12 pitch for a flat truss to 24/12 for a very steeply pitched roof. Common values include 4/12 to 12/12 pitch roofs and can also include fractions (6.5/12) in the pitch to achieve a specific truss height for a given span.

The greater the pitch, the more area the truss technician has to work with to apply triangulation and the more efficient the webbing can be. However, increases in pitch directly affect top chord length. Pitch, combined with span and heel height, determine the overall height of the truss. Truss height has limitations in production and shipping and should be taken into account during the design phase of a project. Steps can be taken to work around these limitations but it’s best to know about them upfront to make proper accommodations.

In scissors or cathedral trusses where a vault is desired, the ceiling pitch is typically half the roof pitch. For example, a roof pitch of 8/12 can easily accommodate a 4/12 vault for most design considerations. Instances where a ceiling pitch greater than half the roof pitch is desired can be accommodated but are subject to other design specifications such as span, loading, building codes, and other engineering factors. Increasing the heel height can aid in designing these types of trusses, giving the truss additional space to incorporate webs and ultimately transfer loads. Parallel chord trusses have the same roof pitch as ceiling pitch and are able to accommodate this through a raised heel.

Horizontal distance between outside edges of exterior bearings.

Span is a necessary component of any truss. It is the horizontal distance between two exterior bearings and is typically the width of the structure. Special attention should be taken not to confuse any cantilever portion of the truss with its span. For example, if the walls of the structure are spaced 24’ apart and the truss has a 4’ cantilever on one side, the overall truss length would be 28’ with a span of 24’ and a 4’ cantilever. Truss span can vary widely from as small as 1’ (or smaller if necessary) to over 100’ in length. Truss length is typically limited by manufacturing and transportation capacities for a specific manufacturer. Trusses over 60’ in length are considered ”long span trusses” and require special installation and bracing considerations. 

Anyone wishing to order or request a quote for trusses will need to know the span. In simple structures it will typically be the width of the building. In more complicated buildings, it will be taken off the construction documents and likely determined by a truss design and/or layout software specific to the component manufacturing industry.

The location at which two chord members are joined together to form a single member. It may occur at a panel point or between panel points.

A splice is the point where two chord members are joined together by a metal connector plate (truss plate). It can occur at a panel point or between panel points depending on the specific design for that truss and the preferences of the truss technician and/or truss manufacturer. Splices allow multiple shorter pieces to be joined together to form a longer chord member and is a more efficient use of lumber.

Chord size can be changed (2x4 to 2x6 or greater) at a splice point if so desired either to meet engineering requirements or architectural effects. An example of this might be an attic truss where the bottom chord is increased in the living space where additional loads are applied and then reduced near the bearings to reduce expense. Another example is an oversized tail in an exposed overhang that then reduces to a more efficient chord nearer the peak.

Splice length is determined by the truss technician based on lumber inventory, span, and cutting preferences. Certain manufacturers determine splicing based on their equipment capabilities and will either splice from the center out or from the outside in, depending on what is most efficient. There’s no right or wrong way to splice, only manufacturer-specific best practices and preferences.

Inclined or horizontal member that establishes the top member of a truss.

The top chord is one of the three key components to any truss. In addition to the bottom chord and webs, the top chord is used to create the upper perimeter of the structure, or the roof. The top chord is configured to resist live loads such as those applied during construction, as well as wind loads, snow loads and others. It also resists more permanent dead loads like sheathing and roofing materials. For traditionally spaced trusses, the top chord is cut out of 2x4 or 2x6 dimensional lumber but can also be cut out of 2x8, 2x10, or 2x12 material for roof trusses. Factors that determine the size of the top chord include on-center spacing, dead and live loads applied to the truss, span, and pitch applied to the top chord.

The top chord is intersected by the bottom chord and web members at various points through the truss creating a joint that is secured by metal connector plates. A top chord can be multiple pieces of lumber secured by a metal plate known as a splice, either at a joint or mid panel. While a vast majority of trusses will have top chords of the same species and grade, changes in both lumber species and grade are possible at splice points.

In typical trusses with an overhang, the top chord is extended past the bearing over the bottom chord to create an eave condition for the structure. Most top chords are cut with a plumb cut at the peak and overhang and a square cut at splices.

For floor trusses, the top chord material is usually cut from 2x3 or 2x4 material, but instances utilizing 2x6 top chord material for floor trusses can also be found. Typically in floor trusses, the material is oriented in a flat or 4x2 position. Orienting the chord material in a 4x2 manner creates a very stable truss that allows installers the ability to easily walk across members without tipping the truss during installation of additional members or floor deck sheathing.

The act of forming rigid triangles with objects adequately fastened together.

Triangulation is the engineering practice of forming rigid triangles together with adequate fasteners at the joints. It typically involves the use of triangular shapes to give stability to structures. In roof and floor trusses, wooden triangles are used to pass compression and tension forces throughout the structure to its bearings. Triangles are the simplest geometric figure that will not change shape when the lengths of the sides are fixed and offer significant structural stability in design.  

Truss plates are manufactured with steel protected with zinc or zinc-aluminum alloy coatings or their stainless steel equivalent. They are designed to laterally transmit loads in wood and are manufactured to various sizes, thickness, and gauges.

Truss plates allow for dimensional lumber to be fastened together in plane in any number of configurations to create a truss using the properties of triangulation. Truss plates come in many sizes and are typically square or rectangular with multiple teeth “punched” in the manufacturing process to give them gripping strength when pressed into wood. They are manufactured with steel that is protected with zinc or zinc-aluminum alloy coatings or their stainless steel equivalent. They are designed to laterally transmit loads in wood.

Each component manufacturer has its own manufacturing methods but a majority of manufacturers utilize one of two methods to press truss plates into wood. The first method is a series of roller presses, the first a gantry type with 18-24” rollers that travel along a table to initially press the plate into place. It is then sent to a conveyor system and through a similarly-sized finish roller that completely embeds the plate. The second method is any variety of hydraulic presses that can be C-clamp pressed in place including a larger hydraulic press that travels on a gantry and presses plates into place using a large metal head. Regardless of manufacturing process, each plate is required to meet quality control tolerances for plate embedment, rotation, defective or rolled teeth, and other criteria.

Truss plates are required at each truss joint and splice. They are placed on both sides of the truss, essentially sandwiching the joint and splice. Plates can be rotated and shifted to increase surface area and maximize holding power. In some instances, like heel conditions, multiple smaller truss plates can be used to transfer forces opposed to a larger plate, improving the economic inputs of the truss.

A variety of manufacturers produce truss plates which on the surface look very similar. In reality they are a highly engineered product each with unique characteristics. Each plate manufacturer has a mechanical die that forms the teeth when punched through the metal creating a twist that adds to the strength and makes each truss plate brand unique. In addition to traditional truss plates, these manufacturers also produce unique products such as a hinge plate that allows for unique truss manufacturing and transportation capabilities.

Members that join the top and bottom chords to form the triangular patterns typical of trusses. These members typically carry axial forces.

The web member is one of three key components to any truss. In additional to the top and bottom chords, webs typically carry axial forces to the chords that eventually pass to the truss bearings. The vast majority of trusses utilize 2x4 dimensional lumber but webs can vary from 2x3 up to 2x12 lumber depending on the design. The grade and species of lumber used for webs is typically different from the chords of the truss to drive economic efficiency. Factors that impact the size of web members include on-center spacing, dead and live loads applied to the truss, span, and pitch, among others.

In floor truss applications, 2x3 or 2x4 material is used to cut webs. When oriented on the flat face of the board, webs will be cut with bevels on each end to create more surface area at the joint for a better fit. In some instances, square cut floor webs can be used but result in larger plates at the joint. Floor trusses oriented in a 4x2 or 3x2 manner are much more stable than floor trusses oriented on edge, allowing installers the ability to easily walk across members without tipping or rolling the trusses during installation while floor decking and bracing is applied.

Webs are typically cut with many angles on each end to allow for a tight fit at the joint. Sophisticated automated computer-driven saws are used to cut webs and other truss parts with very specific angles and lengths that allow for intricate joints and increased surface areas to aid in the transfer of forces. Linear saws are able to quickly cut differing webs out of one piece of lumber thereby minimizing waste. Component saws are utilized to cut high volumes of webs of the same configuration out of a fixed length board using 5 or 6 saw heads. Pull saws, either manual or semi-automated, are used to cut a variety of parts including webs in low quantities. Common trusses are typically designed with symmetrical webs to reduce the number of saw setups and make the overall truss fabrication process less complicated.

Often web members in compression require continuous lateral restraints (CLR) to prevent buckling under the applied design loads. Webs requiring CLRs will be denoted on the Truss Design Drawing with a rectangle with an “X” through it or a rectangle fully blackened on the web. CLRs attached to similar adjacent trusses are required to have a diagonal brace to transfer the forces from the CLR into a lateral force resisting system such as the roof or ceiling sheathing. Alternatively to CLRs, individual web restraints such as  scab, “T,” or “L” reinforcements can be installed in the field on the truss in instances where a CLR is not possible due to differences in web patterns of adjacent trusses. Another alternative to field applied restraints are factory installed stacked webs and proprietary metal reinforcements that have been specially designed for this purpose.

For more information on web member restraints and temporary and permanent truss bracing in general, refer to BCSI.

Triangular piece of lumber that has one side equal to the standard 2 inch dimension lumber widths, and is inserted between the top and bottom chords, usually to allow the truss to cantilever. Its use is determined through engineering analysis.

A wedge is often used near the heel of a truss to help plate the heel joint and transfer forces from the chords to the bearings. It is one of many options available to a truss technician to help find the best plating option at the heel joint. Wedges are typically cut from 2x dimensional lumber in varying widths and are widely used in cantilever truss applications. Alternatives to wedges include top and bottom chord sliders as well as top and bottom chord reinforcing webs.