Deciding Between I Joists and Floor Trusses

When designing and building a home's floor framing system, one of the most important decisions is choosing between I-joists and floor trusses. Both offer unique structural capabilities, installation methods, and budgetary impacts that must be weighed against the specific needs of each project.

In this comparison guide, we'll analyze the key differences between I-joists and floor trusses to help inform your selection process. We'll look at the structural design, performance factors, pros and cons, and costs of each system. With the right information, you can make an informed decision on the optimal floor framing solution for your build.

What Are I-Joists and Floor Trusses?

Before comparing these two popular options, let's briefly define what they are:

• I-joists are lightweight engineered wood products made by bonding together boards of lumber, usually oriented strand board (OSB). The shape resembles an I-beam, with a vertical web in the center and horizontal flanges on top and bottom.
• Floor trusses are made by connecting multiple lengths of dimensional lumber, often 2x4s, together in a web-like triangular pattern. The truss transfers loads to reaction points at the ends.

Both I-joists and trusses serve to support floor loads and provide structural integrity. However, they achieve this through different designs.

Structural Performance

When it comes to structural design, I-joists and trusses have some key differences:

• I-joists come in standard depths, such as 9.5", 11.7", and 14". Deeper I-joists can span greater distances between supports.
• Floor trusses can be designed in various depths to optimize spans based on load requirements. Long truss spans may require additional intermediate supports.
• The standard materials in I-joists result in consistent, predictable performance. Truss performance depends on the lumber grade, connection details, and workmanship.
• I-joists are very uniform, enabling modular layouts. Trusses may have slight variations that require field adjustments during installation.

Understanding these structural differences will inform decisions on applicability for a given floor plan. For example, I-joists may be better suited for simple layouts with uniform spans, while trusses offer more flexibility for complex designs.

Load Capacity

Load-bearing capacity is another key consideration when selecting between I-joists and trusses:

• I-joists can reliably handle up to 50 pounds per square foot (PSF) for floors. Special heavy-duty I-joists are available for heavier loads.
• Floor trusses are commonly designed for 50 PSF residential loads but can be engineered for commercial and industrial applications requiring 75 PSF or greater.
• Trusses can accommodate point loads through nodal reinforcing, whereas I-joists require sistering additional joists or substituting beams.
• Both systems may require closer spacing, smaller spans, or other modifications to handle higher loads.

When expected floor loads are calculated during the planning and design process, the structural engineer can then specify the optimal floor framing system and components to withstand those loads.

Evaluating Performance Factors

When deciding between I-joists and trusses, here are some key performance factors to consider for your building project:

Span Ranges

The maximum clear span between supports is an important factor influencing floor framing choice. Here's how the two systems compare:

• I-joists typically span 10' to 16' safely, though longer spans are possible with heavier products.
• Floor trusses commonly span up to 24', with some designs achieving 48' or more by using intermediate supports.

If your floor plan requires long, open spans, trusses may be the better option. For shorter spans, I-joists are often adequate.

Deflection Control

Minimizing bounce and flexing in the floor surface is crucial for function and comfort. Here are key differences:

• I-joists are prone to bowing and twisting, requiring bridging between joists.
• Trusses exhibit less deflection across their rigid triangular structure.
• Special web stiffeners can enhance I-joist rigidity and control of deflection.

For floor plans with large open areas, trusses may result in a stiffer, less bouncy floor compared to I-joists.

Accommodating Complex Designs

Irregular floor plans with bump-outs, notches, openings, or variance in span lengths present unique challenges, such as:

• I-joists require substituting beams and additional framing at openings.
• Trusses can be designed to accommodate notches, holes, and custom spans cost-effectively.

Trusses offer high design flexibility, an advantage for complex floor plans. I-joists work better for straightforward layouts.

Integration of MEP Systems

Both I-joists and trusses impact the coordination of MEP systems like ductwork. Key factors include:

• I-joists allow running ducts between joists but limit space below for large ducts.
• Open web trusses enable MEP components to pass through cost-effectively.

Trusses can make routing mechanical systems easier. However, value engineering the floor-to-floor height is critical regardless of framing type chosen.

Noise Transmission

Floor assemblies must provide sound dampening between levels. Here's how the two systems compare:

• I-joists require resilient channels and insulation for sound control.
• Trusses inherently have more dead air space, boosting sound insulation.

Trusses tend to provide better noise reduction between floors due to their increased cavity space.

Cost Comparisons

When evaluating I-joists versus trusses, cost is often a primary factor. Here are key considerations:

• Material costs can be higher for I-joists compared to dimensional lumber in trusses.
• Truss engineering, manufacturing, and transportation add costs not present with I-joists.
• I-joists require less labor for installation compared to assembling trusses onsite.
• Truss framing may allow using fewer foundation footings or interior bearing walls.

There is no universally cheaper option--a detailed cost analysis should be performed based on lumber pricing, building design, and other project-specific factors.

Budget Impacts

The floor framing system impacts multiple parts of the overall budget:

• Foundations: Trusses allow fewer interior supports but may require enhanced footings for longer spans.
• Steel: Additional beams/posts are often needed with trusses for long spans.
• MEP: Trusses can reduce mechanical system costs by easing coordination.
• Labor: I-joist installation is simpler than truss assembly and craning into place.

Developing detailed pricing for each aspect provides a clearer budget comparison and helps optimize value.