Two jobs, done by two people
A structural assessment is often described as if it were one task, but it is two. The first is a field job: someone goes onto the roof and into the building to record what the structure actually is. The second is an engineering job: a licensed professional engineer takes that record and checks whether the existing framing can carry the proposed solar system together with the loads the building code requires. Confusing the two is how projects end up with a stamped letter based on a drawing that no longer matches the building.
The survey does not decide whether a roof is adequate. Its job is to make the engineer's decision possible and defensible, by delivering measurements the engineer can trust and clearly labeled assumptions where measurement was impossible.
What the field crew measures
- Framing type. Open-web steel joists, joist girders, rolled steel beams, wood trusses, glulam beams or precast concrete planks. Each is analyzed differently.
- Member size and depth. Joist depth and chord sizes where visible, with any painted or stamped designation photographed.
- Spacing and span. Center-to-center spacing and clear span between supports, measured instead of assumed from the drawings.
- Deck. Steel deck profile and gauge, wood sheathing or concrete, and the way it is fastened to the framing. The deck governs what a mounting attachment can bite into.
- Roof buildup. Membrane, insulation type and thickness, and any previous overlays or additional layers, because each adds permanent weight.
- Column grid. The position of columns and heavier girders, which tells the engineer where load paths are strongest.
- Existing equipment. Rooftop units and other heavy items, and the framing modifications around them.
Where the underside of the roof is hidden by a ceiling, the crew should request access panels or check mechanical rooms, stairwells and other places where framing is exposed. If nothing can be seen, the record must say so, so the engineer can apply a conservative assumption or ask for a second visit.
What the engineer checks
The engineer starts from the building code adopted by the local authority and the national loading standard it references, commonly ASCE 7 for wind, snow and seismic loads. The question is whether the framing can carry the existing dead load plus the added solar dead load, and still resist the code-defined environmental loads with the array in place.
Dead load
Panels, racking and any ballast add permanent weight. Ballasted systems carry the most, because ballast is what resists wind uplift. The existing roof buildup is part of the baseline, which is why overlays and extra layers are worth recording.
Wind uplift
Wind loads on a low-slope roof are not uniform. Corners and edges see higher uplift than the field, and a parapet changes the pattern. A ballasted layout therefore concentrates weight where wind is strongest, which can clash with where the structure is weakest.
Snow and drift
Snow load depends on the ground snow load for the location, but the array itself and nearby parapets and rooftop units create drift zones where snow accumulates unevenly. A roof in a warm climate has little concern here, while a roof in a snowy region may have its layout set by drift.
Seismic
Added mass matters in seismic design. In higher seismic regions the engineer may require positive attachment even where a purely ballasted system would otherwise work.
Ballasted, attached or hybrid
The mounting method follows from the structural margin. A ballasted system avoids penetrating the membrane but adds weight. An attached system adds penetrations, and therefore a conversation with the roof warranty holder, but adds much less weight. Many commercial arrays end up hybrid. The survey cannot choose among these, but it can make sure the engineer has what they need to do so, and that the roof membrane and warranty constraints are known in parallel instead of discovered afterwards.
When the answer is no
Sometimes the assessment concludes that the roof cannot carry the full array. That is a valid outcome, and learning it early is cheap. Options include reducing the array to the stronger bays, reinforcing the framing, choosing a lighter attached design, or moving capacity to a carport or ground mount, which is why a carport and ground-mount survey is often scoped alongside the roof.
Common structural surprises on commercial roofs
A few patterns show up again and again. The first is the added load nobody wrote down: a second roof layer, a thicker insulation board, or a new rooftop unit installed years after the drawings were produced. The second is a bay that is weaker than its neighbors because a joist was cut or modified for a skylight, stair hatch or duct opening. The third is corrosion or water damage at the bearing ends of members under an old leak. None of these can be seen from satellite imagery, and each can change what the engineer is willing to approve.
The practical answer is to look for evidence of modification everywhere the crew can reach, and to photograph it. Cut members, added steel, patched deck and discolored insulation all tell the engineer where to be cautious. A short, honest note such as "deck underside not visible in the north bays" is far more useful than silence.
It also helps to ask the owner a direct question: has anything heavy been added to this roof since it was built? Facility teams often remember equipment changes that never reached the drawing set, and the answer costs nothing to collect.
Where Solar Survey AI fits
Solar Survey AI records the roof and the framing as part of a commercial on-site survey, in one visit, with the deliverable 48 hours after the survey. The record feeds C&I design and engineering, which is sealed by a licensed professional engineer, so the structural check is performed on the building as measured. For the wider scope of what a commercial survey captures, read what a commercial solar site survey needs.
One practical note for EPCs: ask your survey provider to label every framing value as measured, read from a drawing, or assumed. That single habit removes most of the ambiguity that turns a structural review into a back-and-forth.