Why flat roofs are a different shading problem
A pitched residential roof usually has a few obvious shading sources: a chimney, a tree, the next gable over. A flat commercial roof is a working surface covered in equipment. Rooftop air handlers, exhaust fans, vent stacks, skylights, screen walls, elevator penthouses and parapets all stand in the sun's path. On top of that, the array on a flat roof is usually tilted, which means every row casts a shadow on the row behind it. Shading is therefore partly something to measure and partly something the design creates.
The three sources of shade
On-roof obstructions
Anything standing above the roof surface casts a shadow whose length depends on its height and the sun's position. Rooftop units are the usual culprits, but also look for vent pipes, antennas, lightning protection, satellite dishes, signage frames, guardrails and screen walls. A parapet is shade on its own side of the roof, particularly in winter when the sun is low.
Off-roof obstructions
Taller neighboring buildings, trees, water towers, billboards and utility poles can shade a roof for part of the day. Trees grow, and new buildings arrive. The survey should photograph the horizon from the roof and note anything likely to change, such as a construction site next door or a large tree that will keep growing.
Self-shading
Tilted rows shade each other, especially when the sun is low in winter mornings and afternoons. Greater tilt and tighter spacing produce more self-shading. Many commercial flat-roof arrays use low tilts or east-west layouts to fit more capacity on the roof, trading some per-module output for density and reduced wind exposure. That tradeoff belongs to the designer, but it depends on the roof dimensions and obstruction map the survey supplies.
What the survey must record
- Height of every obstruction above the roof surface, measured with a laser or tape, not estimated from photos.
- Position of every obstruction relative to fixed references such as parapet corners, so the designer can place it accurately.
- Footprint dimensions, because a wide unit shades more than a narrow one of the same height.
- Parapet height on each side, and whether it varies.
- Roof edges and clear areas, including fire pathways and setbacks that reduce usable space.
- Horizon photographs from several points on the roof, with the direction of each noted.
- Future changes, such as planned rooftop equipment or nearby construction, from the owner.
Two roof points can differ. A tall unit at one end of a large roof may shade a modest area while leaving the rest untouched. This is why a single horizon photograph from the roof center is not enough on a big building.
How the analysis is done
Shading analysis combines the geometry from the survey with the sun's path across the year. The designer places obstructions in a model, computes where shadows fall at representative times, and evaluates how much each area of the roof is shaded. A common design convention is to avoid placing modules in areas shaded during the middle of the day around the winter solstice, because that is when shadows are longest. Other conventions weigh annual energy loss against usable area.
The metrics used to describe the result vary: solar access, total solar resource fraction (TSRF) and others. Our shading analysis page explains these measures and lets you move the sun yourself, and our survey methodology note documents the sources and limits of the measurements.
Design responses to shading
- Moving rows away from tall obstructions, using the distance to the shadow's end.
- Choosing row spacing and tilt to balance self-shading against density.
- Using east-west layouts where the roof is tight.
- Splitting strings or using module-level power electronics where partial shading is unavoidable.
- Dropping modules from the worst-shaded positions.
Each response affects equipment, cost and output, so the earlier the shading picture is complete, the better the engineer can choose. If the survey omits obstruction heights, the designer is forced to guess or to send a crew back to measure.
Common survey mistakes
- Photographing a unit without measuring its height.
- Forgetting small items such as vent stacks, which can shade a few modules at a time.
- Measuring from the wrong reference, such as a point on a sloped section of the roof.
- Ignoring parapets on the south side.
- Overlooking trees or buildings that are off the property but within shading range.
A note on seasons and time of day
Shading is not a single number. The sun is low in winter, so shadows are long and fall further, and high in summer, so shadows are short. A rooftop unit that barely affects a nearby row in June can shade it for much of a December afternoon. Designers therefore look at several representative days across the year, and the energy loss from a given obstruction depends on when the shade falls. Morning or late-afternoon shade costs less energy than shade at midday, because there is less sunlight available at the edges of the day.
This is why the survey should record geometry instead of conclusions. Heights and positions let the designer test any season, any layout and any tilt, while a note such as "minor shading" cannot be reused. The same measurements also support later questions, such as whether a taller piece of equipment can be added without hurting the array.
If the building is part of a larger campus, mention nearby planned construction as well, since a new building can reshape the shading picture for decades.
Where this fits in a commercial survey
Obstruction mapping is part of the roof zone of a commercial on-site survey, along with drains, penetrations and the structure beneath. Solar Survey AI captures the roof, structure and electrical room in one visit and delivers 48 hours after the survey, so the engineer can run shading, layout and structural checks from one record in C&I design and engineering. For the full list of roof items, see the commercial rooftop checklist.