When specifying the Mounting BOS for a design using a **SnapNRack Ultra Rail** 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?

To find the correct maximum rail span values for designs using SnapNRack Ultra Rail racking, first go to SnapNRack's online configuration tool for Ultra Rail systems at the link below and input the necessary parameters for your PV design:

Alternatively, look up the rail span values in the span tables. Find the UR-40 rail span tables here and the UR-60 rail span tables here.

After putting in your system parameters and specifying the layout of your array, the tool will give you the maximum rail spans for each roof zone.

*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.*