This is my first post, so I am new to the forum...but not new to the field.... in business since 1980 installing PV systems.
I have done many projects with Standing Seam Roofs. The key element is how the roof itself is fastened to the substrate or roof deck. The loads must be transferred from the array to the building foundation, and how this is done is the key element to how its fastened. We have seen many systems with an upload force reaching values close to the take off of some big airplanes, so these are real issues to contend with. The upload on these connections is great, and if the roof was not installed to allow for this up load force, it may not be able to handle a non rail system. Typical rule of thumb is double the roof pan fasteners to add solar. So if the roof manufacture will allow solar installed (some do not allow attachment of additional equipment) in the first place, what is the recommended fastener spacing? If they say 12" on center, then you would use 6" on center to attach the roof pans to the roof deck. Also if they say a #8 screw, we would use a #10 or 12, based upon our engineers report. I also have been required to use extra long screws to reach roof joists and also to add blocking below the roof deck between joist. On existing roofs we have many times had to add a S-5 to every seam and use rails, so that we had enough connections to as many seams as possible to spread out the load. Even to the extent of extending the rails beyond the array to hit several more roof seams. Our design wind load can be as high as 140MPH, especially if near canyons or along the coast, so again much is dependent on your location, height of structure and surrounding structures or terrain and the design loads you are having to deal with. For sure this is an area for an engineer to do some calculations on the loads and determine a best method forward.
I hope this sheds some light on why we like rails on systems of this type!
