The roof is the foundation of the system for the next 25 years. Modules last that long; a cracked slab or a rusted sheet will not. During the survey you must decide whether the roof can carry the array, how to fix to it, and what could leak.
The four site types you will meet
A. RCC flat roofs
Most urban Indian homes have a flat reinforced cement concrete (RCC) slab, usually 100–150 mm thick, with a parapet of 0.6–1.2 m. Modules go on a steel structure anchored to the slab, either low or elevated so the terrace stays usable (module m05).
What to check
- Slab thickness. Look at the edge of a drain hole, the overhead tank opening or the staircase-room door frame, or ask for the structural drawing. Thin slabs (100–125 mm) need chemical anchors; ordinary wedge anchors can crack them.
- Cracks and spalling. Rust stains or exposed bars on the ceiling below mean corroded reinforcement. Do not anchor into that area until it is repaired.
- Leak history. Look at the ceiling of the top floor for damp patches and peeling paint. Ask the customer.
- Waterproofing layer. Brick bat coba (old buildings), cement-based coatings, membranes or elastomeric paint. Every anchor hole goes through it, so note its type and age.
- Drainage. Find the rainwater outlets and the slope. Structures must not block water flow.
- Tap test. Tap the screed with a small hammer. A hollow sound means the top layer has separated; anchors there will be weak.
How much load does solar add?
Indian slabs on accessible roofs are typically designed for an imposed load of 1.5 kN/m² (about 150 kg/m²) under IS 875 Part 2. A solar array is far lighter, but legs concentrate it in small spots, and wind can pull upwards.
Worked example (Chennai, 4.95 kWp, elevated structure):
- 9 modules × 28 kg = 252 kg
- Elevated galvanised steel structure: take the supplier's weight; assume 45 kg per kWp for this example (the top of Sunlectric's 30–45 kg/kWp benchmark for elevated structures), about 225 kg
- Total ≈ 475 kg spread over about 35 m² of roof = 13.6 kg/m² (0.13 kN/m²)
That is under a tenth of the design imposed load, so a sound slab carries it easily. The real questions are anchor strength and wind uplift, which can be several times the array's weight in coastal Chennai (module m05).
B. Tin sheds and metal roofs
Factories, warehouses, schools and many rural homes have trapezoidal galvanised or galvalume sheets on steel purlins.
- Sheet thickness. Common sheets are 0.40–0.50 mm. Thin sheets cannot hold screws alone, so mini-rails must line up with purlins, or you use rails clamped to the purlins.
- Rust. Check overlaps, screw heads and gutters. Rusted sheets should be replaced before solar goes on; replacing them later means removing the array.
- Purlin spacing and condition, usually 1.2–1.5 m. Look from inside the shed.
- Roof slope. Measure it with an inclinometer. Modules usually lie flush at the roof slope.
- Asbestos cement (AC) sheets. These are brittle and hazardous. Never walk directly on them, and never drill or cut them. Suggest replacing the sheets, a separate structure spanning over them, or a ground mount.
C. Tiled roofs
Mangalore-pattern clay tiles are common in Kerala, coastal Karnataka and Goa. Tiles crack under foot, and every hook passes between them.
- Fix roof hooks to rafters, not to battens or tiles.
- Grind or replace tiles where hooks pass, so tiles sit flat and do not leak.
- Carry spare tiles. Expect to break some.
- In heavy-rain areas, some owners prefer to replace part of the tiled area with metal sheet first.
D. Ground mounts
Used where the roof is small, shaded or unsafe, or for farms and institutions.
- Soil and flooding. Ask about water-logging in the monsoon; keep modules and inverters above flood level.
- Security. Fencing against theft, cattle and children.
- Shading from boundary walls, trees and buildings, often worse at ground level.
- Foundations. PCC blocks, piles or helical screws depending on soil. Large ground mounts need soil data.
- Cable run to the meter, often long. Plan for voltage drop and buried, protected cable.
Red flags that stop or change the job
| Red flag | What to do |
|---|---|
| Exposed or rusted rebar, wide cracks | Repair first; consult a structural engineer |
| Fresh waterproofing due in the next months | Do the waterproofing before the structure |
| Owner plans an extra floor | Wait, or design a structure high enough to build under (rare) |
| Asbestos sheets | No drilling or walking; propose alternatives |
| Rented property | Written owner consent |
| Heavy shade most of the day | Consider another roof, a ground mount, or a smaller system |
Key points
- Most Indian homes have RCC slabs of 100–150 mm; check thickness, cracks, rust, leaks and drainage.
- A solar array adds roughly 12–25 kg/m² on average, small compared with roof design loads, but legs concentrate the load and wind uplift matters more.
- On tin sheds, check sheet gauge, rust and purlin spacing; never drill asbestos sheets.
- Tiled roofs need hooks fixed to rafters and careful tile work.
- Photograph and record every defect before quoting.
Check yourself
- How can you find the thickness of an RCC slab without drilling?
- Why must mini-rails on a 0.40 mm sheet line up with the purlins?
- A customer plans a new floor in two years. What do you advise?
Answers
- Measure at a drain opening, overhead tank opening or staircase-room doorway, or check the structural drawings.
- Thin sheets cannot hold screws reliably against wind uplift; screwing into or near purlins gives proper holding strength.
- Either wait and install on the new roof, or accept that the array will need to be removed and refitted. Do not install a permanent structure that will block the construction.