Roof Truss Dimensions, Spacing & Load
Sizing a roof truss correctly means getting five numbers right: span, pitch, spacing, overhang, and load. Get any one wrong and the resulting chord lengths, truss count, or support reactions will be off. This guide breaks down each measurement and links to a dedicated free calculator for it.
The Five Numbers Every Truss Design Needs
Span is the wall-to-wall width the truss must cover. Pitch is the roof slope, expressed as rise per 12" of run. Spacing is the on-center distance between trusses (commonly 12", 16", 19.2", or 24"). Overhang is the eave extension beyond the wall plate. Load combines dead load (material weight), live load (maintenance access), snow load, and wind load into the total force the truss must carry.
Change any one of these and the others shift too — a wider span needs deeper chords or closer spacing to carry the same load; a steeper pitch increases rafter length and material use per foot of span. The calculators below isolate each variable so you can see its effect on its own, then combine them in the main Roof Truss Calculator for a full design.
Span, Dimensions & Pitch
Load, Spacing & Force
Standard On-Center Spacing Reference
| Spacing | Relative Truss Count | Typical Use |
|---|---|---|
| 12" o.c. | Highest | Heavy snow load, smaller chord sizes |
| 16" o.c. | High | Heavier residential loads |
| 19.2" o.c. | Medium | Balanced material efficiency |
| 24" o.c. | Lowest | Standard residential default |
Always confirm final spacing against your local IRC/IBC requirements and your truss manufacturer's engineered design.
Frequently Asked Questions
What measurements does a roof truss need?
A complete truss design needs span (building width), pitch, spacing (on-center distance between trusses), overhang, and load (dead, live, snow, and wind). These five inputs determine every chord length, height, and quantity calculation.
What is the difference between span and truss dimensions?
Span is a single measurement — the wall-to-wall width. Truss dimensions is the full set of resulting measurements (top chord length, bottom chord length, overall height, overhang) calculated from span, pitch, and overhang together.
How does truss spacing affect load?
Wider truss spacing (like 24" on-center) means each truss supports a larger tributary roof area and carries more load; closer spacing (16" or 12") spreads the same total roof load across more trusses, so each carries less.