Heights, spans, anchoring, chemical anchors and ballast

Lesson 3 showed that wind can lift a module with several times its weight. This lesson follows that force down through the structure into the roof: through clamps, rails, rafters, legs and base plates to the anchors and slab. Most structural failures on Indian rooftops happen at the bottom of that chain, where steel meets concrete.

Height and tilt: the lever arms

Wind force acts on the module surface, well above the slab. The higher the module, the larger the overturning moment on the legs and the larger the pull on the rear anchors. Raising the front from 0.5 m to 2.1 m for walk-under use roughly triples that lever arm, which is why walk-under frames need heavier legs, cross-bracing in both directions and about 30% more steel.

Tilt matters in the same way: a 25° module catches more wind and sits higher at the back than a 15° one. That is a structural reason, not just an energy reason, for keeping rooftop tilts around 15–20°.

Spans, clamps and overhangs

Follow the structural drawing for:

• Rail (purlin) span between rafters, which depends on the rail section, the wind and the module load. Do not stretch spans on site to "save a leg".
• Overhang of rails beyond the last support: keep within the design limit; long cantilevers bounce in wind.
• Clamp positions: clamp modules only inside the clamping zones shown in the module's installation manual, usually on the long frame edges at set distances from the corners. Clamping elsewhere can overload the frame and void the module warranty.
• Clamp count: typically two mid-clamps per gap between modules and two end-clamps at each row end, all torqued to the value in the clamp maker's sheet.
• Module gap: about 20 mm between modules for thermal movement and clamp fit.

Anchoring into Indian slabs

Most Indian house slabs are 100–150 mm thick, often with a layer of screed, brick bat coba or tiles on top. Anchors must reach the structural concrete, not stop in the screed.

Mechanical (wedge or sleeve) anchors

Expand against the hole when tightened. Quick and cheap, but they push outwards on the concrete. An M12 wedge anchor needs roughly 80 mm embedment, a member about twice that thick and generous edge distance. In a 100–125 mm slab, or near a parapet, it can crack the concrete and lose holding power. Use them only on slabs of 150 mm or more, and only where the maker's edge and spacing rules are met.

Chemical (bonded) anchors

A threaded stud set in a resin (vinyl-ester or epoxy, capsule or injection type). The resin bonds along the hole without expansion stress, so they suit thin slabs and positions near edges. Sunlectric's default for slabs under 150 mm, or where the thickness is unknown, is an M12 stud with resin, at the embedment the resin maker specifies.

Installation steps that decide the result:

1. Drill to the specified diameter and depth with a depth stop. Never drill through the slab; stop short of the underside cover (look below for the ceiling of the room).
2. Clean the hole: blow, brush, blow, typically twice. Dust left in the hole can halve the bond.
3. Inject resin from the bottom up (or insert the capsule), then insert the stud with a twisting motion.
4. Respect the gel and cure times, which depend on temperature (shorter in a Chennai May, longer in a Delhi January). Do not load the anchor early.
5. Torque the nut to the maker's value only after curing.

Pedestals

Some installers cast PCC pedestals on the slab with J-bolts and fix base plates to them. Pedestals must still be tied into the structural slab (dowels or chemical anchors). A block simply sitting on the screed is just ballast, and the waterproofing around it needs care (lesson 5).

Ballast

Concrete blocks hold the structure down by weight alone, with no holes in the roof. It works for low-tilt, low-height arrays in low wind zones on strong roofs. For most Indian homes it does not.

Worked example: anchors versus ballast in Pune

From lesson 3, 9 modules see about 17.1 kN of uplift (illustrative Cf 1.2).

• Dead weight holding down: 9 modules × 28 kg + structure about 225 kg (45 kg/kWp) = 477 kg ≈ 4.7 kN
• Net uplift: 17.1 − 4.7 = 12.4 kN

Ballast option: to hold 12.4 kN with a safety factor of 1.5 needs 18.6 kN of extra weight, about 1,900 kg of concrete, plus more to resist sliding. That is far beyond what a typical house slab should carry in concentrated blocks.

Anchor option: 10 legs (about 2 per kWp), 4 anchors each. If the rear legs take about 60% of the net uplift: 0.6 × 12.4 = 7.4 kN over 5 rear legs = 1.5 kN per leg = 0.37 kN per anchor. A properly installed M12 chemical anchor in sound concrete is rated far above that (check the maker's data). The anchor itself is rarely the weak link; poor installation, embedment in screed, or anchors near a slab edge are.

Proof testing

On every job, pull-test a sample of anchors with a hydraulic anchor tester to the proof load stated in the design or specification. Record the readings with photos. On coastal and commercial jobs, many clients ask for this as a matter of routine; on homes it is good practice that costs little.

Key points

• Higher and steeper arrays create larger overturning moments; walk-under structures need heavier legs and bracing.
• Follow the design's spans, overhangs and the module manual's clamping zones; never stretch spans on site.
• In 100–150 mm Indian slabs, chemical anchors are the safer default; mechanical anchors need thicker slabs and large edge distances.
• Anchors must embed in structural concrete, not screed; clean holes properly and respect cure times.
• Ballast is rarely practical on Indian homes: resisting typical uplift needs tonnes of concrete.

Check yourself

1. Why are wedge anchors risky in a 110 mm slab near a parapet?
2. What does "blow, brush, blow" refer to?
3. A design gives 8 kN net uplift on 8 rear anchors with a safety factor of 3. What tested capacity per anchor is needed?