Floor Trusses vs Floor Joists: Span, Rough-In and Jobsite Tradeoffs

Open-web floor trusses are worth pricing when long clear spans and substantial mechanical runs drive the layout. Solid-sawn joists are worth pricing when the spans are modest and the framing is straightforward. Engineered I-joists offer a third option: long-length framing with controlled openings through the web. None is automatically stronger, quieter or cheaper; compare complete floor designs under the same loads and performance requirements.

This comparison covers wood floor framing and uses U.S. design references. Before choosing, establish the support locations, clear spans, design loads, available floor depth and routes for ducts and plumbing—not just the price per member. Apply the locally adopted code and project-specific design requirements.

What you are comparing

A floor truss is a manufactured assembly of top and bottom chords connected by triangulated web members, commonly using metal connector plates. The open spaces between those members provide service routes. A solid-sawn joist is a lumber member, such as a 2×10 or 2×12, installed on edge. An I-joist is an engineered member with top and bottom flanges joined by a thin web; it is not an open-web truss. See SBCA’s floor-truss overview and Weyerhaeuser’s TJI joist information.

Decision	Open-web floor trusses	Solid-sawn joists	Engineered I-joists
Span selection	Project-specific truss design	Applicable span tables or engineered design	Product-specific tables or design software
Mechanical routes	Through open webs and coordinated chases	Within permitted hole and notch limits, or below framing	Through permitted web holes
Depth	Coordinate structure and service clearance together	Selected lumber depth limits the cavity	Available depths depend on product and series
Field changes	Follow the design; obtain approval for alterations	Cutting still has structural limits	Follow manufacturer cutting and hole rules

Span and depth: compare actual designs

Open-web trusses can provide long spans and girder-truss arrangements that reduce intermediate walls, beams, columns and footings. But removing an intermediate support does not remove its load: the remaining bearings and supporting structure must carry the resulting reactions. SBCA specifically emphasizes adequate bearing capacity and a load path down to the foundation. Floor-truss design and bearing guidance

For solid-sawn joists, allowable span depends on lumber species, grade, size, spacing, loading and deflection criteria. The AWC’s 2024 span tables also state installation assumptions, including properly supported and sheathed members. A table value is not transferable to a different grade, heavier floor assembly or concentrated-load condition without checking the design.

I-joists also require product-specific selection. A long stock length is not an allowable clear span, and two joists of the same depth can have different capacities because their flanges, webs and design properties differ. TJI series and design properties

Depth deserves a separate check. SBCA describes most floor trusses as 16–24 inches deep, while Weyerhaeuser lists TJI products from 9½–24 inches, depending on series. Those are product ranges, not equivalent-load comparisons. SBCA · Weyerhaeuser

For reference, dry surfaced 2×10 and 2×12 lumber have actual depths of 9¼ and 11¼ inches, respectively—not their nominal depths. AWC lumber dimensions

For the project, compare the finished assembly height: framing, any service space below it, ceiling system, subfloor and flooring. A deeper open-web member may accommodate services that would otherwise require a soffit. Where headroom or existing floor elevations are fixed, that tradeoff can decide the system.

Rough-in: open webs help, but the route must fit

The main coordination advantage of trusses is access through the webs without drilling structural members. Large ducts running perpendicular to the trusses may need a deliberately designed, aligned chase. Ordinary triangular openings are not a continuous rectangular duct corridor. SBCA identifies chase design and overall depth as important mechanical-system decisions. Mechanical access through floor trusses

Before fabrication, send the truss supplier the duct’s outside dimensions, insulation allowance, drain routes and slopes, major penetrations, and stair openings. Check those routes across the whole floor, including girders and headers—not just through one typical truss.

Solid-sawn joists permit limited drilling and notching, not unrestricted cutting. For example, ICC’s explanation of the 2021 IRC prohibits notches in the middle third of a solid-lumber joist’s span and limits other notch dimensions. Confirm the locally adopted code and the complete detail before cutting. ICC cutting, drilling and notching guidance

I-joists can accommodate substantial web openings, but allowable size and distance from supports depend on the exact product. Weyerhaeuser’s TJI guidance permits specified web holes while prohibiting flange cutting or notching. Do not apply solid-lumber cutting rules to engineered members. TJI hole restrictions

Strength is not the same as a firm-feeling floor

A floor can meet structural code requirements and still produce objectionable walking vibration. Neither “truss” nor “engineered joist” guarantees a stiff-feeling floor.

WoodWorks identifies member stiffness, span and spacing; sheathing stiffness; blocking, bridging or strongbacks; ceiling attachment; and supporting-beam stiffness as relevant factors. It also explains why a uniform-load deflection limit alone may not control long-span floor vibration. Floor vibration analysis in light-frame projects

Ask each bidder to use the same floor-performance target, not merely the same minimum load capacity. Identify heavy finishes and concentrated loads before design. If wider framing spacing is proposed, verify the subfloor and finish-floor assembly too; our subfloor selection guide covers that separate check.

Field changes and installation controls

Follow the truss placement layout and member-specific designs. Do not shift a truss to clear plumbing without checking with the manufacturer: changing spacing changes the load carried by adjacent members. Alpine’s floor-truss guide

Likewise, do not cut a web or chord and improvise a repair. The 2021 IRC, Section R502.11.3, requires registered-design-professional approval for truss alterations and verification where added loading exceeds the design load. Use the approved detail for any designated trimmable end; it is not permission to shorten an ordinary truss. IRC truss alteration requirements

Include unloading, staging and bracing in the installation plan. Trusses need the specified temporary and permanent restraint and bracing; SBCA’s BCSI-2025 guide addresses those controls. I-joists also require construction bracing and are unstable until laterally braced, according to Weyerhaeuser’s installation guidance.

Compare installed cost, not member price

Request competing quotes against one coordinated scope:

• Structure: loads, support locations, reactions, openings, cantilevers and floor-performance target.
• Assembly: framing depth and spacing, subfloor, ceiling, and required fire or acoustic details.
• Rough-in: duct chases, permitted holes, soffits and service supports.
• Jobsite work: delivery date, unloading equipment, staging space, framing labor, connectors and bracing.
• Change risk: fabrication cutoff, replacement lead time and responsibility for engineered repair details.

The useful winner is the system that meets the same structural and comfort requirements with the least total work—not the lowest-looking lumber line.