Comparing Floor Systems: Dimensional Lumber vs. Floor Trusses vs. I-Joists – When to Use Each
When designing a floor system, builders and contractors must choose between three primary options: dimensional lumber, I-joists, and floor trusses. Each system has its own advantages, cost considerations, and best-use cases, making it critical to understand when and why to use each one.
In Colorado, the shift toward engineered wood products (I-joists and floor trusses) has accelerated due to the demand for longer spans, better material stability, and efficient mechanical coordination. Below, we’ll compare these three floor systems, outline their strengths and weaknesses, and help you determine the best choice for your next project.
Dimensional Lumber: The Traditional Approach
Dimensional lumber (e.g., 2x10 and 2x12 floor joists) has been a standard in construction for decades. These solid wood joists are spaced 12", 16", 19.2", or 24" on center and offer a simple, cost-effective solution for floor framing.
Pros of Dimensional Lumber:
✅ Cost-Effective: Typically the least expensive upfront option.
✅ Availability: Readily stocked at most lumber yards.
✅ No Engineering Required: Easy to work with and does not require custom design.
Cons of Dimensional Lumber:
❌ Limited Span Lengths: Requires interior bearing walls or beams for longer spans, which can reduce design flexibility.
❌ Potential for Warping & Shrinking: Solid wood contains natural moisture, making it prone to movement over time.
❌ Difficult to Run Mechanical Systems: HVAC, plumbing, and electrical must be routed around or notched, leading to more labor and potential issues.
When to Use Dimensional Lumber:
Small-scale residential projects with short spans.
Budget-conscious builds where longer spans and mechanical coordination are not major concerns.
Renovations or additions where existing framing matches traditional joists.
I-Joists: Engineered for Strength and Versatility
I-joist floor system installed in a production single family home in the Painted Prairie Neighborhood of Aurora, Colorado. This home was built using Advanced Framing techniques (24” o.c. framing) with I-joists and Huber Advantech subfloor.
I-joists are a modern engineered wood product that combines wood flanges (top and bottom) with an OSB or plywood web, creating a lightweight yet strong structural member. These joists are available in depths from 9 ½″ to 24″ and are commonly used in residential and commercial construction.
Pros of I-Joists:
✅ Longer Spans: Can span longer distances than dimensional lumber, reducing the need for interior load-bearing walls.
✅ Consistent & Stable: Unlike traditional joists, I-joists are manufactured for uniformity, meaning no warping, twisting, or shrinking.
✅ Lighter Weight: Easier to handle and install without requiring a crane.
✅ Flexibility for Mechanical Runs: Holes can be cut on-site (following manufacturer guidelines) or pre-cut before delivery.
Cons of I-Joists:
❌ Limited Mechanical Openings: While holes can be cut, large ductwork placement must be carefully planned.
❌ Less Fire Resistance: Requires additional treatment or fire protection under many building codes.
❌ Not as Strong for Deep Floor Systems: Beyond 14" depths, floor trusses often become more cost-competitive.
When to Use I-Joists:
Single-family homes and light commercial projects where long spans are needed.
Builders looking for a stable, consistent floor system that is easy to install.
Projects where site modifications may be necessary, since I-joists can be cut or trimmed in the field.
Floor Trusses: The Ultimate in Strength and Design Flexibility
Floor trusses pre-assembled into a floor cassette being installed on a multifamily project in Denver, Colorado.
Floor trusses are engineered, open-web structures made from wood chords and metal plates, allowing for exceptional span lengths and integrated mechanical runs.
Pros of Floor Trusses:
✅ Longest Spans: Floor trusses can span longer distances than both dimensional lumber and I-joists, reducing the need for interior supports.
✅ Best for Running HVAC & Plumbing: The open-web design allows easy mechanical routing without the need for drilling or notching.
✅ Highly Engineered for Strength: Designed specifically for each project, optimizing structural performance and efficiency.
✅ Reduces Structural Steel Needs: Fewer load-bearing points can lower overall construction costs in some cases.
Cons of Floor Trusses:
❌ Higher Upfront Cost: Typically more expensive than dimensional lumber and I-joists—but cost-competitive at greater depths (16″+).
❌ Longer Lead Time: Since floor trusses are custom-fabricated, they require detailed design coordination before manufacturing.
❌ Not Field-Modifiable: Unlike I-joists, trusses cannot be cut or trimmed on-site.
When to Use Floor Trusses:
Multifamily, custom homes, and commercial projects where long spans are required.
Projects with complex mechanical systems where plumbing, HVAC, and electrical routing efficiency is critical.
Jobs where reducing soffits and bulkheads is a priority, ensuring a clean ceiling design.
📌 Key Consideration:
👉 The quality of the design team is critical for floor trusses. If HVAC and plumbing aren’t coordinated upfront, soffits may still be required, adding unexpected costs. This is especially true in truss depths below 18", where space is more limited.
💰 The big question - how do the costs compare?
Floor trusses stacked at our manufacturing facility in Longmont, Colorado.
Dimensional lumber joists are almost always going to be the cheapest option per lienar foot of span.
I-joists tend to be cheaper than floor trusses in 9 ½″, 11 ⅞″, and 14″ depths.
Beyond 14" depths, floor trusses usually become the most cost competitive option
When comparing costs, it is important to understand total costs not just the cost per linear foot. How much extra will you be spending on soffits for mechanical and plumbing? Will you incur extra labor cost fixing damaged joists
Final Thoughts: Choosing the Right Floor System for Your Project
When comparing dimensional lumber, I-joists, and floor trusses, it’s essential to consider:
✅ Span requirements – Longer spans favor I-joists and trusses.
✅ Mechanical system coordination – Floor trusses offer the most flexibility.
✅ Budget – I-joists are cost-competitive at shallow depths, while trusses become more economical beyond 14″+.
✅ Construction complexity – Dimensional lumber is the simplest but most limited option.
For Colorado builders and contractors, floor trusses and I-joists have largely replaced dimensional lumber due to evolving building designs and mechanical coordination needs. Choosing the right system depends on your project goals, budget, and construction timeline.
