A rooftop array is designed to sit where it is for a long time. The roof covering underneath it has a life of its own, and the two are not synchronized. If the covering is near the end of its service life when the panels go on, you will eventually pay to take the array off, replace the roof, and put the array back, which is labor you could have avoided by reversing the order of two jobs.
This is the most expensive avoidable mistake in residential solar, and nobody has to behave badly for it to happen. It happens by omission. Nobody asked how old the roof was, so nobody raised it. Ask first, and ask somebody whose income does not depend on the answer being that there is plenty of life left up there.
Start with the age of the covering, from your closing documents, permit records, or a previous owner. Then have a roofer look at it, ideally one who is not part of the solar transaction and has nothing riding on the outcome. Have them state the remaining life in years, in writing, and ask what they saw that supports that number.
If the answer is that the roof has many years left, proceed. If the answer is that it is getting close, do the roof first. If the answer is uncertain, treat uncertain as close. The cost of replacing a roof somewhat early is bounded and knowable. The cost of finding out you were wrong after the array is up includes the roof plus removing and reinstalling everything sitting on top of it.
Which directions your roof planes face, how steep they are, and what casts shadows on them through the day drive production more than anything on the equipment list. A roof broken into several small planes facing different directions is a harder design than one large uninterrupted plane, and it may mean the array gets split, which affects layout, hardware, and output.
Shade is the factor homeowners underestimate, because they picture the roof at noon. Look at it in the morning and again in the late afternoon and note the trees, chimneys, vents, dormers, and neighboring structures. Then ask the installer how shading was accounted for in the production model, which is a sharper question than asking whether shading was considered.
Mounting an array means putting holes in your roof and flashing them so they do not leak. Done properly, this is routine work that has been performed on a great many roofs without incident. Done carelessly, it becomes a water problem that shows up months later in a spot where the source is not obvious and the array is sitting directly on top of the evidence.
So ask concrete questions. How are the attachments flashed. What does the workmanship warranty cover specifically with respect to roof penetrations, and for how long. Who do you call if a leak appears. Then ask how all of that interacts with any remaining warranty on the roof itself, because roofing warranties can carry terms about work performed by others.
Winter output here is lower. Days are short, the sun sits low, snow can rest on an array until it slides or melts, and inversion in the valleys reduces the light reaching your roof for stretches at a time. This is normal rather than a defect, but it should be visible in the numbers you were shown instead of buried inside an annual average.
Roof pitch matters here too, since steeper planes shed snow more readily than shallow ones. Ask how the production model handled snow and soiling, ask to see the monthly production shape, and set your expectations against that shape rather than against a yearly total. A homeowner who already knows December will be low does not panic in December, which is worth something by itself.
Get a roofer outside the solar transaction to put remaining life in writing, then decide with that number in hand. Replacing a roof somewhat early costs a known amount. Removing and reinstalling an array later adds a cost you can eliminate completely by sequencing correctly, so a genuine tie generally goes to doing the roof first.
Mounting approaches differ by material, and some are more involved than others. Tile and metal are not the same job as asphalt shingle. Tell the installer what your roof is made of early in the conversation, and ask how the array attaches to that specific material and how those attachments get flashed.
Panels shade the covering beneath them, which is not nothing, but the array also introduces penetrations and makes any future roof work harder and more expensive. Do not treat solar as roof protection. Treat the roof as its own system with its own remaining life, and settle that before adding anything on top.
Twelve months of usage and the age of your roof covering are all we need to begin.