Part VII: Solar ElectricityChapter 11. Grid-Tied and Off-Grid PV

Ground and Roof Mounts You Can Build

Rack geometry, uplift, and the point where a homemade mount should give way to a listed one.

Difficulty: Advanced
Cost: Varies

published

A photovoltaic module is a sail with a glass face. The mount's job is to hold it at a tilt, keep water out of the roof or the ground connection, and stay put in the wind and snow your site actually gets. Listed racking exists because those loads are calculated and the clamps land where the module maker allows. A lumber frame in a photo is not that calculation.

For a permitted array on a house, use the rack the module and the inverter paperwork expect. This page explains what that hardware is doing, and where a homemade mount is the wrong pride. Small portable panels are in the portable guide.

Difficulty

Level: Advanced
Time: A roof or a pier project with a permit, not a Saturday invention
Skills: Stopping when the wind load is a guess

Cost estimate

Varies

Listed roof attachments and rails are often a modest share of a whole system and a large share of the leaks if you invent them. Ground piers, concrete, and trench are their own project. A wood rack that rots or racks in the wind is the expensive cheap option.

Mount	Use it when
Listed roof rack, flashed, into rafters	The array is on a house roof
Listed ground rack on piers below frost	The roof is wrong and the yard is sunny
Lumber you cut yourself	Almost never on a permitted house array

Materials

Item	Qty	Notes
The module maker's clamp zones and the rack maker's spans	The pair	Clamps in the wrong place crack glass in a wind. Spans that are too long bend rails.
Flashed roof attachments or engineered piers	As the design specifies	Tar under a bracket is not flashing. A post set in a flowerpot of concrete is not a pier.
A wind and snow number for your site	1	From the code or the rack engineer. Not from a calm day when you were shopping.
Firefighter clearances	If the roof is a house	Many codes want open paths at the ridge and the edges. Filling every square is how you fail inspection and hinder a crew.

Roof

Attachments have to reach structure. Decking alone will not hold a sail. The pattern of lags, the rail span, and the distance from the eaves come from the rack design for your wind speed, not from a neat look. Every penetration is flashed in the order the roofing needs, the same way a vent is flashed. Sealant as the only water plan fails in the sun and then leaks into the ceiling you cannot see until the insulation is wet.

Leave the pathways the fire code wants. Modules also need an edge to clamp. Do not overhang a module past the clamp zone to gain one more inch of roof. Do not step on the glass. Do not use the array as a ladder.

Shared roofs, rented houses, and old decking are reasons to stop. If you cannot say what the rafters are and what the roofing warranty will do, the mount is not yours to invent. The working-safely page applies the whole time you are up there. So does the shade sketch. A perfect rack in a tree's shadow makes no power.

Ground

A ground mount wants sun, a setback from the lot line, and no overhead service drop you could hit with a rail. Piers go below frost so the array does not heave into a twist. The steel or aluminum is listed or engineered. Wood in the weather rots at the ground line, loosens the clamps, and is a poor match for a 25-year module. A detached shed with one small module is a different, smaller problem. A house-sized array is not.

Call before you dig. The trench for conductors is part of the electrical design, not a ditch you fill and then ask an electrician to use. Fencing, screening, and livestock that will rub a post are site problems. Snow that slides off the array needs a place to land.

Tilt is a compromise between winter sun, summer sun, and wind. Steeper catches more winter sun and more wind. The rack maker's tilt hardware is the adjustment. A propped board is not.

Listed hardware on the roof, into the frame, and piers in the ground. Wind is the design case. A lumber substitute is the one this drawing leaves out on purpose.

The homemade line

You can bolt listed rails to listed attachments and follow the sheet. That is building a mount. You cannot look at a module, guess a lag spacing, and call it the same thing once the array is large enough to hurt someone on the way to the neighbor's yard. Insurers and inspectors may draw the same line.

Galvanized steel, aluminum, and stainless fasteners are chosen as a set so the joints do not eat themselves. Random plated screws in aluminum rails are a five-year failure. Torque on clamps is the number on the sheet. Overtightening can crack the glass.

The electrical gear does not hang off the rack as an afterthought. Junction boxes need the shade and the drip the maker drew. The wiring page is the boundary. The mount does not include landing conductors in the house.

Pick roof or ground from the sun and the structure
A shaded roof loses to a sunny yard. A roof you cannot attach to loses to a ground mount. A yard under a power line loses to a different project.
Use listed racking sized for your wind and snow
Clamp zones, spans, and attachments from the sheets. Firefighter paths on a house roof. Flashing, not tar.
Put piers below frost, after you have called for locates
Engineered or listed ground hardware. No wood posts as a house array. No trench until the electrical route is designed.
Stop at the conductors
The mount can be ready and the system still unfinished. An electrician and a permit close that gap.

Difficulty

Cost estimate

Materials

Roof

Ground

The homemade line

Pick roof or ground from the sun and the structure

Use listed racking sized for your wind and snow

Put piers below frost, after you have called for locates

Stop at the conductors

Further reading