A structural insulated panel (SIP) roof has no embedded lumber in the structure, and therefore nothing substantial for attaching PV array and solar hot water mounts. Single or double lumber splines could be put into the panels, or I-joists at 4-foot intervals could be added. However, the thermal bridging that they would create are at odds with the goal of an energy-efficient building. As buildings get tighter and more insulated, thermal bridging plays a relatively larger role in energy loss.
One solution for mounting solar equipment to a SIP roof is to drill completely through the roof, passing a threaded rod through to the bottom skin and placing a large washer under the nut. Since most SIP roofs are vaulted on the inside, though, few homeowners would be OK with seeing the nuts and washers on their ceilings.
On a recent SIP project in Hood River, Oregon, the PV contractor asked for test results for fastener pull-out so he could design an appropriate rack system. An independent test at Rigging Products in Portland, Oregon, provided the figures he needed. The first test, using hollow-wall anchors, gave a result of 405 pounds to failure. Dividing by three (which accounts for the industry standard safety factor) gives a working load of 135 pounds. Failure occurred as the hollow-wall anchor folded up and pulled through the anchor holes. In an area with 100 mph wind gusts, we did not think a hollow-wall anchor would hold up.
In the second test, two 3/8-inch, high-performance toggle bolts were used. Toggler brand bolts were used with cap screws instead of the original machine screws since the holes in the standoff were 3/8 inch. In hindsight, I would have used 5/16-inch bolts to give a little wiggle room for alignment. In this test, the failure of the SIP—when the OSB skin cracked—occurred at 1,105 pounds, or a working load of 368 pounds.