To Scan? Or Not to Scan? That is the Question.
I recently attended a large event for satellite dish installers who asked me for advice and tips on how to use Zircon® StudSensorTM or MultiScanner® stud finders on the roof to locate rafters. They were really surprised to hear that Zircon does not recommend these tools for outdoor use. I hear the same reaction from solar panel installers.
The reason is that stud finders are designed to scan through sheet materials, such as drywall or plywood that have a fairly smooth surface and consistent thickness and density. A typical asphalt shingle roof has a layer of wood based sheet material, usually plywood or chipboard, and normally with a layer of tar paper.
Then, the asphalt shingles are fastened to the roof with roofing nails. The difficulty with scanning for rafters is that the asphalt shingles now create an irregular scanning surface that can result in false positives (stud indication when a stud may not actually be present).
On many other irregular surfaces, I recommend switching to metal scan mode and locating screws, but that won’t work on the roof because you are unable to differentiate the screws that fasten the plywood to the rafters from the roofing nails.
Although our newest MultiScanner® x85 OneStep® wall scanner (capable of finding studs, detecting thermal changes, and locating live AC wiring) is also not recommended for outdoor use, it is more technologically advanced in scanning over inconsistent and irregular surfaces.