The glass area is arithmetic. The roof area is not. Row pitch, walkways, parapet shadows and neighbouring buildings are why a flat roof needs roughly twice what a sheet roof does for the same 100 KWp.
The number quoted in most conversations is the glass area, and the glass area is the easy part. It is a division and a multiplication, and it is the same anywhere in the world.
Take the module you intend to use and read two figures off its datasheet: rated power and physical dimensions. Divide 100,000 watts by the rated power to get the module count, then multiply by the module's area. On the large-format modules commonly used on industrial projects here, that arithmetic lands at somewhere around 380 to 420 square metres of glass for 100 KWp. A higher-wattage module needs fewer panels of slightly greater area, and the total barely moves.
Then the useful question begins, because that is the area of the modules, not the area of roof you need to put them on.
There is a factor of roughly two between the two common industrial cases, and it dwarfs every other consideration.
On a pitched sheet roof, modules are mounted coplanar with the sheet, lying parallel to it. No row shades the row behind, so the panels sit close to edge to edge and the only losses are walkways, ridge and eaves clearances and the edge setbacks. This is the efficient case, and it is why factory sheds carry so much more capacity than their footprint suggests.
On a flat RCC roof, modules are tilted on frames so they face south, and each row casts a shadow onto the one behind it. Row pitch has to be set so the array is unshaded through the design window, usually somewhere around the middle of the day at the winter solstice, when the sun sits lowest. At Dhaka's latitude the required pitch is more forgiving than it would be in Europe, but the ground coverage ratio still commonly settles between about forty and fifty per cent. In practical terms, budget roughly two square metres of flat roof for every square metre of module.
| Roof type | Module glass area for 100 KWp | Roof area to allow | What drives the difference |
|---|---|---|---|
| Pitched sheet roof, coplanar mounting | About 380 to 420 m² | About 500 to 600 m² | Walkways, ridge and eaves clearance, edge setbacks |
| Flat RCC roof, tilted rows | About 380 to 420 m² | About 800 to 1,000 m² | Inter-row pitch to avoid self-shading, plus walkways and parapet setback |
Those roof-area figures assume a clear rectangle with nothing on it, which no real roof is.
The most frequent mismatch between two quotations for the same roof is not the module make. It is that one contractor has quoted 100 kilowatts of inverter capacity and the other has quoted 100 kilowatts peak of modules, and on a plant designed with a DC oversize those are meaningfully different quantities of glass, of area and of money.
Ask the question directly, in writing: is the 100 KWp figure the DC array capacity or the AC output rating, and what is the DC to AC ratio. Then compare the two proposals on the same basis. Everything else in a rooftop comparison is easier once that is settled.
And when a contractor quotes a capacity for your roof, ask for the layout drawing that goes with it, showing the walkways, the setbacks, the roof obstructions and a December shading study. A capacity number without a layout is an estimate made from a satellite image, and that estimate is always optimistic.