What array weight really means for your roof
Modern panels are lighter than most people expect
A 60-cell residential panel weighs about 40 to 50 pounds and covers roughly 18 square feet, which works out to around 2.2 to 2.8 pounds per square foot for the panel alone. Add rails, clamps, and flashing and a complete installed array typically lands at 2 to 4 pounds per square foot of distributed dead load. That is in the same range as one extra layer of asphalt shingles, which is why most roofs built to code carry it without structural work, though an older roof or a heavy snow region still deserves a professional check. The Department of Energy's homeowner's guide to solar covers what an installer should check on your roof before panels go up. The footprint figure matters as much as the weight, so confirm it with the usable roof area calculator before you trust a per-square-foot result.
Why panel count doesn't change the per-square-foot number
A bigger system means more total weight, but the load per square foot stays roughly constant, since a larger array also covers proportionally more roof area. A 10-panel system and a 30-panel system add essentially the same pounds per square foot to whatever roof plane they sit on, and the solar panel calculator is the place to settle how many panels you actually need. At the example values on this page, 16 panels at 45 pounds and 18 square feet each with 0.7 pounds per square foot of racking, the result is 3.2 psf, or 16% of a 20 psf limit. That 20 psf is the roof live load building codes set for flat and low-slope roofs, with steeper roofs designed for less, about 12 psf at a 12:12 pitch, so adjust the code limit field if your roof is steep. Comparing against live load makes sense because panels mounted close to the roof displace the workers and shingle bundles that load is meant to cover, which leaves reserve capacity for the array, as the California Solar Permitting Guidebook explains.
Why the average number isn't the whole engineering picture
The distributed pounds-per-square-foot figure this tool calculates is a useful sanity check, but real weight concentrates at the mounting points, not evenly across the roof surface. If lag bolts land on every other rafter instead of every rafter, those rafters can carry up to roughly double the average load before the roof sheathing shares some of it with their neighbors, which is why the rafter spacing and load calculator looks at attachment points rather than the average. Ballast changes the picture further on flat roofs, since ASCE 7 counts ballast as dead load, and this tool's mounting input tops out at 2 psf, so it is built for flush-mounted systems on pitched roofs.
What a favorable number does and doesn't mean
A low percentage of your roof's code limit is a good early sign, but it does not replace a structural check. Under the guidebook's checklist, a roof can be approved over the counter when it has a single layer of roofing, shows no sagging or damage, and carries an array that stays under 4 psf and covers no more than half the total roof area. Fail any one of those and the guidebook calls for stamped calculations from a licensed engineer. Roof age and rafter condition are also what a routine inspection looks at, and the annual inspection cost estimator works out what that yearly check costs. If a roof-based concern keeps coming up, the ground mount vs roof mount space calculator shows what a ground array would take instead.