- The roof has to carry rooftop solar
- Shade and sun, before the layout is frozen
- The utility and the paperwork
- What “ready for rooftop solar” means
- Frequently asked questions
- Do I need a new roof before rooftop solar?
- Can rooftop solar be installed on any slope?
- Is a sunny photo enough to approve the roof?
- Sources and references
Rooftop solar is a construction project that happens to make electricity. The questions that belong before the modules are ordered are the roof, the shade, the utility service, and who can maintain what gets built. A catalog of panel watts does not answer them. Pacific Northwest National Laboratory’s operations notes are useful here even though they were written for plants that already exist: access drives cost, penetrations have to stay dry, and modules are live once they see the sun. Those facts are cheaper to face in the design than on the roof.
The roof has to carry rooftop solar
PNNL describes rooftop mounts as typically fixed, either anchored into the structure or held with ballast. Anchors make holes. Ballast moves the load into weight, including wind. A visual “the roof looks fine” is not a structural check. Remaining life of the membrane or the shingles matters because lifting an array to replace the roof is the expensive version of a job you could have done first. If the covering is near the end of its life, replace it, then build. That sequence is dull and correct.
Snow, wind, and hail are loads, not moods. A buyer’s specification can say framing should resist them and still not state your local ground-snow load. The engineer uses the local number. No hail diameter is given here. Winter installation is possible and is a crew question, covered in installing solar panels in winter. The mount choice, if the roof cannot take the array, is a ground row or a canopy, not a tighter clamp spacing. That comparison is in roof-mounted versus ground-mounted solar.
Shade and sun, before the layout is frozen
A vent, a chimney, and a neighbor’s tree cast beam shade that moves with the month. A layout that is clear at noon in June can be covered on a December afternoon. Shade losses are nonlinear. A small shadow can pull more of a series string than its area suggests. NREL’s 2016 update found that module-level electronics recovered about 25 to 35 percent of shading losses for the products evaluated, and the larger experimental difference was how many strings were in parallel, not the brand. That is recovery of a loss. It is not a reason to string modules through a shadow you can already see. Walk the roof, or the model, at more than one hour. The mechanism is in how shade affects PV output.
Tilt on a house is often the tilt of the roof you already have. A steep south roof and a flat membrane are different annual numbers, and a flat roof holds snow and dust. Production estimates belong in a tool such as PVWatts with that tilt stated. PNNL names PVWatts when the installer did not leave monthly estimates. An annual total with no winter month is how people are surprised in January.
The utility and the paperwork
Ask the utility what service you have before you buy an inverter. On one utility’s 2024 handbook, houses are single-phase 120/240 volts, or 120/208 where a network exists, and commercial service may be single-phase or three-phase. The handbook’s instruction is to ask before purchasing equipment. A rooftop array does not pick its own voltage. Export rules are a second ask. Net metering, net billing, and a ban on new export credits are different bills. They are explained in what net metering means. Permits and inspection are local. This article will not list a city’s form. It will say the job is unfinished if the disconnect is not where the design can be operated, and if nobody has said who may shut the array down.
Warranties should be read before the deposit, not after a module fails. Product years and performance years are different. Labor on a roof is often outside both. A 25-year power warranty that ships a panel and leaves the crane to you is a roof problem. How to read the PDF is in what a solar panel warranty covers.
What “ready for rooftop solar” means
| Before you order | Done looks like |
|---|---|
| Structure | A letter for this roof, this mount, and the local wind and snow loads. Anchors or ballast named. |
| Shade | More than one hour and one month. Strings not drawn through a known shadow. |
| Service and export | The utility’s voltage and the actual credit for exports, in writing. |
| Access for the next 10 years | Someone can inspect and, if needed, wash without improvising on wet glass. PNNL ties cost to that access. |
If a row is a shrug, you are not early. You are about to buy a module count. Module counts come after this list, by the method in how many solar panels a home needs.
One more item belongs on the walk, and it is easy to leave until the electrician arrives. Where will the disconnects sit, and who can reach them without moving a car or a shelf? Pacific Northwest National Laboratory notes that disconnects are expected within sight of the inverter in the practices it describes, and that larger systems may need a transformer while small ones often land on the existing service equipment. A rooftop array that is electrically fine and operationally trapped is not finished. Walk the path from the meter to the proposed inverter wall in daylight, with the homeowner, and write down what has to move. That conversation is cheaper than a redesign after the rails are on.
Frequently asked questions
Do I need a new roof before rooftop solar?
If the covering will need replacement while the array is still up, yes, do that first. Lifting modules to reroof is the cost you are trying not to pay twice. If the roof has decades left and a structural letter says it can carry the mount, the array can go on what you have.
Can rooftop solar be installed on any slope?
The slope you have is the tilt you get, unless you add a tilted rack, which is a different structure and a different wind load. A steep roof can be a good tilt and a bad place to work. Both facts belong in the decision.
Is a sunny photo enough to approve the roof?
No. Shade moves. Structure is a calculation. The utility is a letter. A clear photo at noon in June misses December, the snow load, and the export tariff.
Sources and references
Roof mounts, penetrations, access, soiling, and PVWatts: Pacific Northwest National Laboratory. Shading-loss recovery for the module-level products tested: NREL, 2016. One utility’s instruction to confirm service before buying equipment: SWEPCO handbook, 2024.
