Take a second look in the attic, then look under the eaves. If the eaves are open, you can see the rafters and note their alignment along one roof edge. Skylights, vents, chimneys, and ridges offer good clues to rafter location, especially if you were able to verify from inside the attic. Mark your estimated rafter locations on the roof near where you expect your rails to be running.
Make your best guess in locating an intersecting rail and rafter, then remove roofing material from there if necessary (for tiles or shakes) and look for clues of rafters in the underlayment. A hammer remains one of the best tools for finding rafters. Lightly tap on the roof, moving side to side along a rail line, and listen for where it sounds most solid, usually indicating the rafter location.
Probe the spot on the roof with a long pilot drill bit (usually a 1/8-inch-diameter bit). Since you know the rafter depth, you know when to stop drilling—you do not want your drill bit to punch through, especially if a finished ceiling is directly on the back of the rafters.
If your drill bit punches through after an inch or less, you didn’t hit a rafter. Seal that hole immediately with an appropriate roof sealant. If you take your eye off of it, you may forget to seal it later. Continue to work with the pilot drill until you are certain you have located a rafter. You can use your first verified rafter as a guide to locating the next rafter, and so on. However, you’ll still need to probe each assumed rafter location to make sure.
Install your rack hardware and torque your lag screw—if the lag continues to spin when tightening, it did not hit the rafter (this is often called a “spinner”). Pull the screw out, seal up the hole and try again. When installing lag screws directly to wood, it is important to hit the rafter as close to its center as possible. It is also important to drill the specified size of pilot hole for the lag you are using. If the hole is too small, you risk cracking or splitting the rafter. If it’s too large, the screw threads won’t bite as well as they should. The length of the lag should be determined by roof sheathing and decking layer thickness, wood type, the number of attachment points, and required pullout strength. There is a minimum embedding of the lag screw into the structural members, with 2 to 2.5 inches being a common requirement. Refer to the manufacturer’s specifications for minimum lag screw embedment requirements or consult a structural engineer if you are working with something atypical.
Other Tricks. In some cases, it can be helpful to drill from the underside of the roof with a long pilot drill. Be very careful doing this—coordinate with others on the roof! Electronic stud finders are a viable option, but tend to be expensive and aren’t always accurate.
Once you have your mounting points installed and the roof all back together, you’re ready to add the mounting rails. Attach them to the mounting points, but leave them loose and don’t worry about placing modules yet. Be sure to keep an extra rail or two and/or some string line around for the leveling process.
Take the time to level your array, or more accurately, to make sure the array lies in a single, uniform plane. Otherwise, irregularities will be quite visible from the ground. A rack system that has built-in leveling features is easiest to adjust. Railless systems, while often cheaper, can add substantial complexity (time) to the leveling due to not having long, straight rails to work with. Keep this in mind when considering these systems for your project. Rail systems are quicker and easier to level due to the rigidity of the rails and being able to level the rack structure before modules are installed.
Most systems require some rail splices. Consider the array’s expansion and contraction—improperly spaced splices can stress mounting points, which can lead to roof leaks (particularly if the system is not flashed) or even total failure. Follow manufacturer guidelines for gapping splices—a minimum gap of about 1/8 inch is typical, depending on local temperature ranges at your site and the lengths of rails used.
Roofs are rarely square. As you install modules, focus on getting them square to the lines of the structure that will be most visible from the ground. Minor discrepancies in rail alignment relative to module alignment are difficult to spot, but modules that are not aligned to the structure will be very visible.
Don’t assume that your modules are square; they often aren’t. Sometimes, the process of handling them from ground to roof can affect the alignment of the frames on the glass. To create the best-looking array, a common practice is to start with the bottom row of modules, since the alignment to the bottom roof edge is most visible. A string line can help with the initial line of this bottom row and will help you identify module “creep”—when the first module in a row is installed off square from the line of the structure. Creep is often hard to spot until you are three or four modules down the row, requiring you to make multiple adjustments to correct the alignment.