What Are Floor Trusses?

Designing and building a house or any structure requires knowledge of building codes and building materials available. Using floor trusses generates larger spaces between supports and makes the installation of electrical, plumbing, and HVAC systems easier. What are floor trusses you ask?

Floor trusses are engineered open web trade-friendly ‘joists’ that can span further than dimensional lumber or I-joists. Manufactured using construction grade lumber, a top and bottom chord are connected using diagonal and vertical pieces. Wood components are oriented to maximize strength and spread the load and fastened together with steel connector plates.

In this article, we’ll explain what floor trusses are, their advantages, and identify different types of floor joists. We’ll discuss how to build and install floor trusses, their span, spacing, and how much they cost. To further assist you, we’ll also compare floor trusses with dimensional joists and with I-joists. By the end of the article, you’ll know whether floor trusses will work for your project.

What Are Floor Trusses

What Are Floor Trusses?

Floor trusses are designed, engineered, and manufactured to exact requirements in climate-controlled factories using quality dimensional lumber to fit each build’s specifications. Floor trusses have a top and bottom rail or chord separated by diagonal and vertical components that form a series of triangles. All joints are fastened with barbed heavy gauge steel connector plates.

The open webbing allows for easy placement of plumbing, HVAC, and electrical, while also minimizing bounce. Load forces are transferred through the triangles to the bearing walls or other supports. Floor trusses can span further than dimensional lumber or I-joists, thus accelerating build speed by reducing interior supports, which decreases overall construction costs.

What Are the Advantages of a Floor Truss System?

Flooring trusses

There are numerous advantages to using floor truss systems. Floor trusses don’t require interior supports which creates larger clearspan flexibility, so interior wall design and placement are easy to build and alter. There are fewer building materials required, no pads to pour or steel beams to place, plus HVAC, plumbing, and electrical are easier to run. The result is accelerating build speed and closing times, and a reduction in material and labor costs.

Floor trusses are manufactured using construction grade 2×3 or 2×4 lumber that is more readily available than long lengths of 10” or 12” dimensional lumber. The trusses can span greater distances and improve structural efficiency, resulting in fewer pieces to go the distance, another cost saver. The trusses are also less likely to twist, shrink, or warp too. Since the trusses are made to exact measurements there is less waste, and job site theft of large lumber joists is also reduced.

The top and bottom chords of the trusses are 2-1/2” or 3-1/2” wide, making gluing, nailing, or screwing easier for subfloors or ceilings. Floors laid on floor truss systems are less likely to bounce or squeak than those on other joist systems. They also support more concentrated loads and bearing walls than other methods. Additionally, there’s no need to drill or cut holes in the trusses since the open web design makes it easier to run ducts, wiring, and plumbing, and also to make changes or alterations without compromising the truss design – another cost-saving feature.

Floor truss systems can be designed and built for step-down floor level changes or cantilevered balconies, a design innovation not offered by other floor joist systems. Plus, floor truss systems don’t require blocking panels, web stiffeners, squash blocks, or even rim or band boards to support edges or exterior walls.

Another bonus of using floor trusses is that the separated top and bottom chords make it easier to insulate and also help reduce sound and impact noise transmission, making for a quieter barrier.

Floor Truss Types

 

Floor Truss Types

Floor truss discussions can include any type of floor joist, including wood web trusses, metal trusses, I-joists, parallel strands, plywood, solid wood, and laminated veneer joists. Aside from the metal floor truss though, the wood web truss is the only one typically referred to as a truss, not a joist. It uses diagonal and vertical components to form open triangles between the top and bottom chords, similar to roof truss designs. So, our focus is on that type of floor truss.

Floor trusses are customized to each build resulting in a vast array of floor truss designs. However, there are really only two basic designs that are modified exponentially to adapt as needed. The ‘Pratt’ configuration has an ‘N’ web separating the top and bottom chords, while the ‘Warren” configuration utilizes a ‘W’ web. A modification to the ‘Warren’ configuration adds a vertical member between the arms of the ‘W’ to form an upside-down ‘M’ web between the upper and lower chords.

To provide greater use options, floor trusses are designed for different bearing conditions too – top chord bearing, bottom chord bearing with or without trimmable ends, chase way, and mid-block bearing. Trusses can also be manufactured with a built-in ribbon notch to tie trusses together and facilitate edge nailing and act as the band or rim board.

Another alternative to rim or band boards is the ladder or gable end floor truss. Manufactured with the other floor trusses, they are the same dimensions and run parallel to other trusses to close the outer floor platform edge and to support exterior walls.

Floor trusses may have double top or bottom chords, or both to increase span and load-bearing. For more floor design versatility, the trusses can be stepped for raised or sunken floor depths, or for step-down cantilevered exterior balconies.

The trusses can be customized with beam pockets and headers for load-bearing at openings and to support perpendicular run floor trusses. Additionally, they can be keyed to join together, or for shared-bearing to ensure alignment of two structural components. The different design options make the floor truss a versatile structural choice for floor construction.

How to Build a Floor Truss

How to Build a Floor Truss

Floor trusses are typically designed and engineered for specific spans and projects and fabricated in climate-controlled factories. Once the plan and cut schedule leave the Engineer and go to the floor for building, the carpenters take over. The top and bottom 2×3 or 2×4 chords of the required length are laid down on special tables at the specified spacing. Diagonal and vertical web components are angle cut using a powerful miter saw, and set between the two chords.

Once all components are in place, steel gusset or connector plates are tacked over all joints. A heavy, automated roller then moves over the truss, pressing the connector plate barbs into the wood and fastening all the components together.

The trusses are then flipped over, connector plates again tacked into place, and the roller used to press that set. With both sides securely fastened, the floor truss is lifted and placed for shipment to the job site. Depending on the factory, the truss may be rolled through another press prior to stacking for shipment.

How to Install Floor Trusses

How to Install Floor Trusses

Floor trusses commonly arrive at job sites on flatbed trailers. They may be slid off the trailer and onto blocks on the ground, or they can be hoisted directly onto one end of the bearing walls. Included with the trusses is a spec sheet that details the installation and corresponds to the number or letter on each truss.

The spec sheet not only identifies the layout and placement of each floor truss but the type and number of nails for securing the trusses in place, including any reinforcing gussets or plates. Trusses are delivered right-side up, so don’t accidentally turn them over or the triangular configuration that provides the strength will be inverted, and mess with the weight-bearing integrity.

Mark the layout onto the sill or top plate to identify where trusses need to be placed. The spacing is set by the Engineer based on design calculations. Lifting and placing the trusses is typically a two- or three-person job as the trusses should remain in an upright or vertical position.

Each truss has a 3-1/2” x 1-1/2” ribbon notch at the upper end to accommodate a 2×4. The 2×4 stabilizes the top of the trusses, maintains spacing, and acts as a rim board and nailing edge for both the subfloor and wall plates.

Once the floor trusses are in place and properly spaced, they are nailed to the sill or top plate and the 2×4 ‘rim board’ attached with the appropriate size and number of nails. Often there are two ladder or gable end floor trusses that run along the top of the outer non-floor truss bearing walls parallel to the other trusses. The ladder trusses act as the rim board and carry the outer subfloor edges and exterior walls.

Floor Truss Span

Floor truss span is determined by wood species used in the construction, depth and width of the truss, web geometry, gauge of connector plates, load and bearing requirements, and spacing between trusses. Depending on the factors, the maximum span for a 24” deep floor truss is 47-feet at 12” centers, while a 12” deep truss is 26-feet.

Floor Truss Spacing

Floor truss spacing is determined by the Engineer or software used to design the trusses. Due to the dimensional aspects of subflooring and other building materials, floor trusses are spaced at the same distance as standard dimensional floor joists, I-joists, roof trusses, or rafters. They are typically spaced at 12”, 16”, 19.2”, and 24” on center, with 16” being the most common and 12” achieving the greatest span.

How Much Do Floor Trusses Cost?

The cost of floor trusses depends on several factors, including suppliers and manufacturers. The depth, width, wood species, and length all impact the price too, as does transportation to the building site. Most floor trusses are custom made for each build, although some home building supply companies stock several different depths and lengths for standard dimension builds.

At one building supply store that carries 8- to 28-foot-long premade floor truss with trimmable ends, an 11-1/4” deep truss is $8.10 a linear foot, a 14” deep one is $8.77, a 16” is $9.05, and an 18” is $9.50.

In some cases, longer floor trusses, especially those over 16-feet, may cost more per linear foot, so shop around for the best price. Prices are from February 2022 and will fluctuate with lumber prices, supply, and demand as the building season warms up.

Floor Trusses vs Floor Joists

Floor trusses are designed and engineered open web ‘joists’ that often span the width of a building and don’t commonly require intermediate support, resulting in large open-span spaces. Trusses provide for a more solid and less bouncy floor deck, don’t require rim or band boards, or blocking. The open webbing allows ductwork, plumbing, and wiring to be easily run without cutting or interference. Floor trusses must be laid out in accordance with the provided engineer-approved plan.

Floor joists are solid dimensional lumber that is span limited by wood species, grade, depth, and other factors. They do not span great distances and typically require support beams carried by posts on concrete pads, resulting in smaller open spaces.

The joists are more likely to bounce, twist, or crack, and require blocking. While small diameter holes can be drilled through the joists to allow threading of electrical and distribution plumbing pipes, ductwork and plumbing waste pipes typically are suspended below the joist.

Floor trusses are more expensive than the dimensional lumber used for joists. However, since they are fabricated using more readily available 2×3 or 2×4 material, supply is seldom the issue it can be with 2×10 and 2×12 lumber. Additionally, trusses span further, require fewer building materials, less time to install, and labor, so there is often an overall saving in construction costs. The top and bottom cord of floor trusses are either 2-1/2” or 3-1/2” wide, making for a wider surface for fastening subflooring or wall plates.

Floor Trusses vs I-Joists

Floor Trusses vs I Joists

I-joists have a 2-1/2” or 3-1/2” top and bottom cord with the space between filled with 1/2″ or 5/8” OSB or plywood. Some use larger materials but are commonly custom orders. The joists are engineered and manufactured off-site in factories to ensure quality control and provide a longer spanning alternative to joists made from standard dimensional lumber. The solid panel between the chords needs to be cut or drilled for plumbing, HVAV, or electrical, which can affect its bearing strength and span.

Floor trusses are also engineered and factory built with top and bottom chords of similar dimensions, but they have an open web of 2×3 or 2×4 material. They cost slightly more than I-joists of similar-sized depths and lengths but can span further. Since they are custom-built for the job, minimal trimming, if any, is required. Additionally, the open web makes running utilities much easier and quicker.

I-joists can be cut and trimmed on-site to fit, so do not require the lead time of floor trusses that are manufactured to build specifications. However, floor trusses are faster to place, require no drilling or cutting for utilities which means sub-trades prefer them and often charge less for their services.

Floor trusses also result in a stronger, less bouncy floor, have a better fire rating, and reduced sound and impact noise transmission. In the long run, floor joists generate greater savings making them less costly than I-joists and often result in faster build and closing times.

Conclusion

Floor trusses are engineered ‘joists’ made of readily available 2×3 or 2×4 lumber. The top and bottom chords are connected with diagonal and vertical components that form load-bearing triangles. All joints are fastened on both sides with steel connector plates.

The trusses create larger open span areas and more solid, less bouncy floors. They are lighter, easier and faster to install, span further, and the open webbing makes them more trade-friendly. Although more expensive than some other options, the saving in materials, labor, and time typically generate substantial savings when compared to other floor support methods.

Hopefully, you have a better awareness of what floor trusses are, and how they may improve your building project.

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