Quick Facts
What ballasted mounting is
Ballasted mounting is a solar panel racking system that holds the array in place using weight (typically concrete blocks) rather than physical attachment to the roof structure. The mounting frame sits on the roof surface, and concrete or other heavy material is placed on the frame to resist wind uplift and lateral forces. No holes are drilled through the roof membrane.
The technique originated in commercial flat-roof solar installations where roof waterproofing is critical and any penetration creates a potential leak point. By avoiding penetration, ballasted systems preserve the roof warranty and reduce future maintenance risk.
Ballasted mounting is common on Indian commercial and industrial buildings with flat or near-flat roofs, particularly where the roof is leased or where the building owner is sensitive to membrane integrity.
How ballasted mounting works
The system has three main parts. First, a low-profile aluminium or steel frame that supports the solar panels and spreads the load across a footprint of plastic or rubber pads (to protect the roof membrane). Second, ballast material, typically pre-cast concrete blocks of 10 to 40 kg each, placed in trays or pockets within the frame. Third, panel clamps that fix the modules to the frame.
The frame is engineered so the ballast weight, distributed across the array, exceeds the calculated wind uplift force by a safety margin. Most designs use a southern tilt of 5 to 15 degrees for ballasted rooftop systems, lower than the latitude-optimal tilt because steeper angles require more ballast.
The weight of ballast for a typical Indian commercial rooftop ranges from 30 to 100 kg per square metre of array, depending on wind zone, tilt, building height, and exposure.
Design considerations
Wind load: IS 875 Part 3 defines six wind speed zones in India, from 33 m/s (basic wind speed in inland low-wind regions) to 55 m/s (coastal cyclone zones). The required ballast weight scales sharply with wind speed. Coastal Tamil Nadu and Odisha sites need 2 to 3 times more ballast than inland Karnataka.
Building geometry: Tall buildings, buildings near sea cliffs, and buildings with sharp parapets all experience higher wind uplift at the corners and edges of the roof. Ballasted designs use heavier ballast at the perimeter (called the “zone of high pressure”) and lighter ballast in the centre.
Roof structural capacity: The added dead load (panels plus ballast) must be supported by the existing roof structure. A structural review is mandatory. Older Indian commercial buildings sometimes need reinforcement before a ballasted solar system can be installed.
Seismic considerations: IS 1893 governs seismic design. In high-seismic zones (parts of north and northeast India), the frame’s lateral resistance must include seismic forces alongside wind.
Roof slope: Ballasted systems are limited to nearly flat roofs (slope under 5 to 10 degrees). Steeper slopes are unstable for unanchored systems.
Membrane protection: Plastic or rubber pads under the frame prevent damage to the roof membrane. The pads spread the point load over a larger area.
Ballasted versus penetrating mounting
| Factor | Ballasted | Penetrating |
|---|---|---|
| Roof membrane | Preserved | Punctured (with flashing) |
| Waterproofing risk | Low | Moderate (with proper flashing, low) |
| Added load on roof | High (30 to 60 kg per sq m) | Low (panel weight only) |
| Installation speed | Faster for some designs | Slower (requires flashing) |
| Use on sloped roofs | Not recommended | Standard practice |
| Wind resistance | Limited by ballast weight | Mechanical attachment to structure |
| Removability | Easy | Moderate (requires patching holes) |
| Cost | Higher (ballast material) | Lower |
For Indian commercial flat-roof projects, the trade-off usually favours ballasted when the roof is in good condition and structural capacity allows, and penetrating when structural capacity is limited or when the roof is older and patching after removal is acceptable.
Common Indian applications
Commercial warehouses and logistics centres with PEB (Pre-Engineered Building) flat roofs. Ballasted is often required by the building owner to preserve the roof.
Industrial factories with concrete flat roofs. Structural capacity is usually adequate, and the no-penetration approach is preferred.
Schools, hospitals, and institutional buildings on flat roofs where future expansion or removal is anticipated.
Leased commercial buildings where landlord disallows penetration.
Ground-mount installations on rocky or contaminated sites where pile driving is impossible or costly.
Common mistakes
Skipping the structural review. Adding 50 kg per sq m of dead load to a roof not designed for it can cause structural damage.
Underestimating wind zone. Coastal Indian sites are in the highest wind zones and need significantly more ballast than inland sites.
Ignoring edge and corner uplift. Wind pressure is much higher near roof perimeters and corners. Designs that use uniform ballast across the array fail at the edges.
Using inadequate membrane protection. Frame feet without proper pads can wear through the roof membrane over years.
Combining ballast with bifacial without modelling. Bifacial gain depends on rear-side exposure. Heavy ballast trays that block the rear face of panels reduce bifacial gain.
Best practices
Always conduct a structural review before specifying ballasted mounting.
Use a wind load analysis tool that accounts for site-specific wind zone, building height, exposure category, and array geometry.
Place heavier ballast at perimeter and corner zones; lighter ballast in centre zones.
Use protective pads under all frame feet to spread point loads.
Document the ballast layout and quantities. Future maintenance teams need this for any repositioning or repair.
For coastal and high-wind sites, consider hybrid mounting with limited penetration anchors at high-uplift zones, ballast in lower-uplift zones.
Standards and references
Ballasted mounting design in India follows IS 875 Part 3 (wind loads), IS 1893 (seismic), and IEC 62893 (PV mounting structures). Structural calculations must be signed by a chartered structural engineer. Site-specific wind zone and exposure data are taken from IS 875 and local meteorological records.
Related glossary terms
Key takeaways
Ballasted mounting holds solar panels in place using concrete weight instead of penetrating the roof. It preserves roof waterproofing and is common on flat commercial and industrial rooftops in India. The trade-offs are significant added structural load and higher capex for ballast material. A structural engineer’s review per IS 875 (wind) and IS 1893 (seismic) is mandatory. The choice between ballasted and penetrating mounting depends on roof condition, structural capacity, building owner preferences, and project economics.