When specifying the Mounting BOS for a design using a **Unirac SolarMount** racking system, one important data point to consider is the maximum rail span for roof zones 1, 2, and 3.

### Why are these numbers important?

Correct values for these fields are critical. Everything from accurate maximum cantilever values (and thus accurate attachment layouts) to accurate calculations of racking hardware weights and quantities of racking hardware components on the bill of materials are affected by these numbers. If you use erroneous values to rush through the design process, some parts of your permit package (such as the attachment layout and racking hardware quantities and weights) may be inaccurate or missing.

### How do I find the maximum rail spans for my design?

Calculate the correct maximum rail span values for your project using the Unirac UBuilder Tool.

*Note: Maximum rail span, which is based on the mechanical properties of the rail, differs from maximum attachment spacing, which is based on the roof's structural limitations. Wider attachment spacing results in greater point loads on the roof at each attachment point. Optimal attachment spacing depends on many factors, such as the roof structure, roof condition, snow loads, wind loads, etc.*