Q: After purchasing a PV permit package it appears that the plans provided to me show the relative positions of the roof penetration points, the approximate positions of the rafters, and rail directions. However, few measurements are given. Do the measurements appear in some other document? Some of them appear to have the penetration points some distance from the mounting holes on provided on each PV array (which I understand we're supposed to use as a guide).
A: This information will generally be provided in the PV permit package by the assumed distance between rafters. For example, if you have 2’ on center rafters, and the flashing is mounted on every other rafter, then you should be installing a system with 4’ on center design.
Note that it is typically assumed that rafters will be 2' on center from each other, unless otherwise indicated on the form submitted before the initial creation of the PV permit package. If you plan to have a 4’ on center design, You can simply skip mounting a flashing near the end of the rail, then skip a rafter before mounting the next flashing. This sufficient to support most systems. Occasionally, you can mount flashings 6’ on center or even wider apart. You will always want to have the end of the rail supported by the nearest rafter as well.
For more details on this, please check the installation manual and/or design guide/tool from the racking manufacturer. Mounting distances are typically local weather condition and roof style dependent, so it will be important to check and confirm suggested maximum distances between mounting points for the particular system you are using.
Be sure to check these links for Unirac and Ironridge design.