# Should rooftop solar on a flat factory roof be ballasted or anchored to the slab, going by the Ministry of New and Renewable Energy specification?

Canonical: https://senseible.earth/answers/flat-roof-solar-ballast-or-anchor-bolts

The Ministry of New and Renewable Energy (MNRE) specification gives a ballast structure for flat roofs, and separate foundation rules, with or without roof penetration, for raised reinforced cement concrete (RCC) structures. Ballast uses concrete blocks of M25 mix without penetration. A raised structure foundation uses a concrete block of minimum M20 grade. M25 and M20 are concrete grades.

## Ballast: no holes in the roof

These are clauses from the technical specification in Annexure 3 of MNRE's guidelines for its residential rooftop solar scheme, Pradhan Mantri Surya Ghar: Muft Bijli Yojana. A factory is not bound by them, but they are a published checklist to hold a quotation against.

M25 and M20 are concrete grades: the number is the strength in newton per square millimetre after 28 days.

- The mounting structure must be non-invasive ballast type, and any sort of penetration of roof is to be avoided.
- The minimum clearance of the structure from the roof level should be between 70 and 150 millimetres (mm), to allow ventilation for cooling, ease of cleaning and maintenance of panels, and cleaning of the terrace.
- The structure should be suitably loaded with reinforced concrete blocks of appropriate weight made out of M25 concrete mixture.
- The specification does not give the block weight, so ask for the designer's calculation.

## Raised RCC structure: foundation with roof penetration

- If penetration of the roof is allowed (based on the client requirement): anchor fasteners of minimum 12 mm diameter and minimum length 100 mm, with proper chipping.
- The minimum RCC foundation size is 400 x 400 x 300 (the document prints cubic mm; read it as millimetres), with material grade of minimum M20.
- Foundation design is to follow IS 6403, 456, 4091 and 875 (Indian Standards, IS), which state load bearing capacity and related parameters.

## Raised RCC structure: foundation without roof penetration

- If penetration is not allowed, the foundation can be done with a J bolt (refer IS 5624 for foundation hardware).
- Proper Neto bond solution should be used to adhere the foundation block to the RCC roof.
- J bolt: minimum 12 mm diameter and minimum 300 mm length.

## How to choose (my reading)

My reading, not the specification's: the specification gives a ballast structure for flat roofs and separate foundation rules for raised RCC structures, so the choice between them is a roof question first. A roof with a waterproofing guarantee, or a slab that is not designed for extra dead load, is one to settle with your structural engineer before a bolt or a block goes on it. The specification leaves the choice to the client requirement and asks only that the design follows the listed standards.

Note one difference inside the document: ballast blocks are named M25, while the anchored foundation is named minimum M20. They are different parts, so check each on your drawing.

## What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so the grid units you buy each month are on file before and after the plant goes live. It does not check roofs, loads or waterproofing.

- [Solar on a tin shed roof](https://senseible.earth/answers/solar-on-tin-shed-roof-mnre-spec)
- [Raised solar structure: sizes by height](https://senseible.earth/answers/solar-raised-structure-sizes-by-height)
- [Wind damage to rooftop solar: who pays](https://senseible.earth/answers/wind-damage-to-rooftop-solar-who-pays)
- [Solar on a rented factory roof](https://senseible.earth/answers/solar-on-rented-factory-roof-who-claims)

Sources: [Ministry of New and Renewable Energy (MNRE), Guidelines for PM-Surya Ghar: Muft Bijli Yojana, Central Financial Assistance to Residential Consumers, with Annexures 2 to 4](https://mnre.gov.in/static/uploads/2025/08/f20f6d61628866e66b612810b49abd78.pdf)

Last reviewed 2026-11-06. Re-check clause numbers and figures against the current version of the guidelines before you rely on them. Clauses are in Annexure 3, section 3.12 (i) and the foundation items. A structural engineer should approve the roof load.