Open-Web Floor Trusses
Open-web floor trusses use a triangulated web of wood or steel between parallel lumber chords to span long distances with minimal depth. The open web design allows ductwork, plumbing, and electrical to pass through without drilling or notching.
Fast Facts
- What Is It?
- Engineered floor/roof framing members with lumber top and bottom chords connected by diagonal wood or tubular steel webs in an open lattice pattern.
- Common Uses
- Floor framing (residential and light commercial), flat roof framing, long-span ceiling support. Standard spans of 20–32 feet.
- Cost Range
- $4–$12 per linear foot depending on span, depth, and load rating. Custom-manufactured to order.
- Durability
- Indoor use only — same service life as the building when properly installed.
Specifications
| Property | Value |
|---|---|
| Depths Available | 12", 14", 16", 18", 20", 24" (custom depths available) |
| Maximum Span | Up to 32' (residential floor loads); varies by depth, spacing, and load |
| Spacing | 12", 16", 19.2", or 24" OC |
| Chord Material | 2×3, 2×4, or 2×6 MSR lumber (machine-stress rated) |
| Web Material | Wood (2×3/2×4 diagonal) or tubular steel (18 ga) |
| Design Standard | ANSI/TPI 1 — National Design Standard for Metal Plate Connected Wood Truss Construction |
When to Use Open-Web Trusses
Open-web trusses are ideal for first-floor framing over basements where HVAC ductwork runs between floors, or in any application where long clear spans (20–32 feet) eliminate the need for interior bearing walls. They cost more than dimensional lumber but save labor in mechanical rough-in.
Ordering and Lead Time
Unlike off-the-shelf lumber, open-web trusses are custom-engineered and manufactured for each project. Provide floor plans to the truss manufacturer 3–6 weeks before delivery. Shop drawings require engineer review and approval before fabrication begins. Trusses arrive on flatbed trucks and require crane placement.
Installation Considerations
Trusses are installed with the top chord bearing on walls or beams. Bearing length is typically 3.5" minimum (full 2×4 wall width). Hangers (Simpson HU/LUS series) are required when trusses frame into beams or headers. Strongback bracing — a 2×6 or 2×8 standing vertically at mid-span and fastened to each truss — reduces bounce and improves floor feel.
Frequently Asked Questions
What is the advantage of open-web trusses over I-joists?
Open-web trusses allow mechanical systems (HVAC ducts, plumbing, electrical) to pass through the open webs without drilling. I-joists require either careful hole placement per manufacturer specs or dropping below the framing. Trusses also span longer distances at equivalent depths.
Can open-web trusses be cut or modified on site?
Never cut, drill, or modify a truss without the truss engineer's written approval and a repair detail. Any modification voids the engineering and can cause structural failure. If field conditions require changes, contact the truss manufacturer for a redesign or repair plate.
Do open-web trusses require special bracing?
Yes. Trusses require continuous lateral bracing (typically 1×4 or 2×4) nailed to the top and bottom chords at intervals specified on the truss shop drawings. Without bracing, individual trusses can roll or buckle during and after construction.
Also Consider
I-Joists (Engineered Wood Joists)
I-joists are engineered wood floor and roof framing members with an I-shaped cross section — LVL or solid lumber flanges bonded to an OSB or plywood web. They are the dominant floor framing material in new residential construction, offering long spans, dimensional stability, and lightweight handling.
LVL (Laminated Veneer Lumber)
Laminated veneer lumber (LVL) is a high-strength engineered wood product made from thin wood veneers bonded with structural adhesive under heat and pressure. It is the most commonly specified engineered beam and header material in residential and light commercial construction.
Joist Hangers
Joist hangers are metal connectors that support wood joists, headers, and beams at the end of a member where bearing cannot occur directly over a wall or beam.