Solar PV Civil works: Tips on foundation block design

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Rooftop solar PV structures typically use a combination of purlins, rafters and posts mounted on the roof.

To provide for structural stability, the posts are fixed on concrete foundation blocks. These foundation blocks can be cylindrical or , typically, a concrete cuboid of size 400 x 400 x 300mm.

On several instances, the foundation blocks are casted on the site by pouring M25 or M30 grade concrete in a moulding frame of appropriate size. The J anchor bolts are inserted in the concrete at the initial stage. The simplest way to balance all 4 J-bolts is to mount them in a base plate while inserting.

Recently while visiting a local University, I observed from photographs that their solar contractor had fixed the post on the roof and poured the foundation block around it. While this gives a strong anchor to the structure, it is going to result in a high and hidden roofing cost as now it is very difficult to carry any replacement/ repair of the post if it corrodes. That is where a properly mounted anchor block system will ensure the flexibility and save you very costly repairs late on.


How do you prepare the foundation blocks? Typically, they are made by pouring M25 grade concrete (1:1:2 ratio of cement, sand, aggregate ratio) into metal or wooden frames for making the blocks  – and quite a few contractors will make it on site.

The basic purpose of the foundation block is to give a ballast dead-weight at the structure to help it maintain integrity when faced with high winds, storms or earth-quakes. Quite a few times, the contractors will bring pre-cast foundation blocks and deploy at a roof site. In such a case, it is quite normal to prepare the base area where the block is to be mounted by cleaning it and applying an appropriate coating (Lords-concrete-to-concrete coating) so that the blocks are anchored properly at the site. If one does not clean the area and does not use the right adhesive, the area will likely chip and will not give a good anchorage.

Do note that if you use the right adhesive, you can get a good anchorage on a RCC roof. So you need not go for the full 400x400x300mm foundation block size. You can reduce it to 250 x 250 x200mm. I have seen at least one roof where the post is pasted directly to the rooftop – not exactly the best deployment as again you will have high cost of replacement at a later date!

As you can see a little design care and workmanship can save major costs later on – and give an aesthetically appealing rooftop solar!

For a full collection of my tips on rooftop SPV,installation, please access: RoofTop Solar handbook of Tips Tricks and Traps v280320 AKA