How to find rafters on a roof for a solar installation

Safety Note:  Whenever working on a roof, use adequate fall protection. 

An essential part of installing PV racking on most roofs is accurately locating the underlying rafters. How one goes about doing this varies because each roof is a little different, and therefore  no one method will always work. Rafters are supposed to be spaced 16 or 24 inches apart on center but structures can shift and settle, designs and installations can be unorthodox, and remodels or additions can create unexpected changes. So, each roof can present a unique challenge to a PV installer.  There are some accepted attachment methods that allow a connection to the roofing material where is it unsupported by a rafter;  this article will not cover those methods.

There are a number of techniques that can solve the rafter location problem and one or another is likely to work on most roofs. Probably the easiest technique is simply to look at the overhang of the roof.  In many cases, the ends of the rafters are visible there and one can then easily project the course of the rafters from those ends.  

In the absence of such visual cues, it is possible find the rafters using a density meter or high-density stud finder. These work like ordinary stud finders but are more sensitive (and more expensive.)  In order to use them to find rafters, it will probably be necessary to remove several shingles or tiles from the roof to reduce the distance between the stud finder and the rafter.  It can then sense through the plywood and find the rafters inside the attic.  Some installers are skilled at finding rafters with nothing more than a rubber mallet. By tapping the roof with the mallet, one can hear a noticeably different sound when a rafter is struck.  Compared with hitting the space between rafters, the sound when hitting a rafter is duller and less reverberant. For those with enough experience, this technique can suffice.

If these techniques are not possible, then it is necessary to climb into the attic space, which allows direct observation of the structural layout. Attics often present dangerous temperature conditions and can also be precarious places to walk, but the attic space does provide the best access to the rafter structure.   In the attic, one measures the distance from the outside wall to the center of the first rafter. Then one directly measures the center-to-center spacing of the rafters. Usually this is a constant, but direct observation will establish this with certainty. Once the rafter structure is completely measured in the attic, these measurements can be transferred to the roof using the original outside wall as the starting point.  With care, this method will provide an accurate map of the underlying rafters.

Important: When each rafter is found, it's important to mark it in a durable manner. For an asphalt or tile roof, a crayon or marking paint will work. For a gravel or foam roof, the area around the attachment point will be disturbed, so a large finish nail driven into the roof deck directly above the rafter is a better choice. If the waterproofing of the attachments will happen before the next chance of rain, it's best to open up the roofing material all the way to the attachment surface before marking.

Here is a quick time lapse video of marking rafters prior to installing solar stand-offs on a flat roof tar and gravel roof.