Before you carry a rail up
- Roof age. Under 10 years of remaining life, the array comes off before the shingles do. Have that conversation before the deposit.
- Structural. Most residential arrays add 3–4 psf dead load. Old 2×4 rafters at 24" o.c. with a long span may still need an engineer's letter. Your AHJ decides.
- Fire setbacks. Typically 3 ft at the ridge and a clear pathway. Local amendments vary wildly — pull the actual adopted code.
Sequence
- Lay out and find raftersChalk lines both directions. Find rafters with a stud finder and confirm with a small pilot bit. Never trust a spacing assumption — old framing wanders.
- Set standoffsLag into the center of the rafter. Typical: 5/16" lag, minimum 2.5" of thread embedment into solid wood. Pre-drill a pilot per the manufacturer's table — too big and it strips, too small and you split the rafter.
- Flash itSlide the flashing up under the course above, sealant under the flashing and in the pilot hole, not smeared on top afterward. Water runs downhill; the flashing must shed over the course below.
- Rail and square upRails level, splices staggered off the standoffs. Respect the manufacturer's maximum rail span and cantilever — usually 24–48" span, cantilever no more than one third of the span.
- Bond as you goWEEBs or listed bonding clips at every rail-to-rail and rail-to-module connection. Bonding after everything is torqued means taking it back apart.
- Set modules and torqueMid clamps and end clamps to the clamp manufacturer's spec, not the rail's. Under-torque and the module walks; over-torque and you crack the frame or the glass.
- Manage wireNo conductor touching the roof deck. Ever. Listed clips to the rail, service loops at the junction box, MC4s of the same brand mated together.
Typical torque values — verify against your hardware
Lag into rafter: 5/16" — approximately 15–20 ft-lb, do not spin out the threads.
Module clamps: 8–12 ft-lb (about 96–144 in-lb) typical.
Rail splice bolts: 8–10 ft-lb typical.
Grounding lugs: check the lug — often 25–50 in-lb.
Use a torque wrench. Every published number is void the moment you use an impact.
Module clamps: 8–12 ft-lb (about 96–144 in-lb) typical.
Rail splice bolts: 8–10 ft-lb typical.
Grounding lugs: check the lug — often 25–50 in-lb.
Use a torque wrench. Every published number is void the moment you use an impact.
Do not do thisCross-mating MC4 connectors from different manufacturers. They mechanically latch and look fine, but the contact geometry is different — the joint runs hot, corrodes, and burns off the roof three years later. It also fails inspection and voids the module warranty.
Common callbacks
- Leaks at the standoff — sealant on top of the flashing instead of under it
- AFCI nuisance trips — one MC4 that was never fully seated (you hear a click; feel it too)
- Modules loose after one winter — clamps torqued by impact gun
- Bonding lug on painted rail — paint must be scraped or a listed piercing washer used