By Vvon Engineering Team | Published | Updated
Compare ballasted and anchored roof arrangements, verify structural capacity and agree waterproofing responsibilities before installation.
Solar EPC & street lighting RFQ checklist (TXT)
A leak near an array fixing can be difficult to trace once modules cover the roof. Plan the roof interface, inspection access and responsibility for repairs before installation.
RCC mounting can use ballasted, anchored or combined arrangements. Compare roof load, uplift, drainage, waterproofing and service access for the actual building.
A ballasted system uses weight and the designed mounting arrangement to resist movement. It can avoid penetrations but still needs roof protection, drainage, load and warranty review. Determine the ballast distribution from the structural and wind design.
Two things make ballast work and are routinely skipped. The first is tying the array together so that it behaves as one block: uplift on any single module is then resisted by the weight of everything it is connected to, not just by the blocks under that module. The second is a protection layer between the ballast and the membrane, usually a slip sheet or a rubber pad, because a concrete block resting directly on a bituminous or liquid-applied membrane will abrade it as the frame moves with temperature. Check also that the friction assumption in the design matches the actual roof finish. A wet, algae-covered screed is not the surface the calculation assumed.
A penetrating system trades that load for holes. Fewer, smaller, more predictable fixings and much less dead load, at the price of a waterproofing detail that has to be right at every single point. On an older slab where the load capacity is the binding constraint, penetration is often the only workable answer, and the leak risk is entirely a question of workmanship rather than of concept.
The third option, still the most common on Bangladeshi roofs, is a cast concrete plinth. It has a habit of combining the disadvantages of both. It adds most of the weight of ballast, and if it is cast onto the existing waterproofing then that waterproofing can never be renewed. Plinths also dam water. Cast a continuous line of them across the fall of a roof and you have built a low wall, water ponds behind it, and the slab sits wet through the monsoon. If plinths are used, they need drainage gaps in every run and a properly formed detail where they meet the deck, not a smear of mortar.
Agree a method statement specific to the slab, anchor and waterproofing system. The following are hold points for the responsible professionals, not a universal installation recipe.
Sequence matters as much as detail. The waterproofing work should be completed, tested and signed off as its own hold point before a single rail is lifted. Where Vvon supplies the array, the fixing layout and the roof interface are engineered together under Design & Assembling PV Mounting Structure, which is civil engineer led for exactly this reason. If the array goes on first and the leak appears later, every subsequent conversation is about who pays for the demount.
Check the existing roof guarantee with its provider before selecting penetrations, ballast or protective layers. Obtain written agreement on the new work and who warrants each interface. A change may affect cover, but the outcome depends on the terms and approved detail.
A parapet is the shading object everybody forgets, because at noon in April it casts almost nothing and the roof looks clear. The design case is not April noon.
The geometry is simple. Shadow length on the horizontal equals the height of the obstruction divided by the tangent of the sun's altitude, projected along the sun's azimuth. Bangladesh sits at roughly 21 to 26 degrees north, so at the winter solstice the noon sun is somewhat above 40 degrees altitude, and in the hours either side of noon it is much lower. The worst case a designer normally works to is the shadow at a stated time either side of solar noon on the shortest day, and the choice of that window is a commercial decision about how much winter morning generation the owner is willing to give away. Make it explicitly, with a shading simulation on the surveyed geometry, rather than by applying a rule of thumb to a parapet nobody measured.
Two practical consequences follow. First, the setback on the southern parapet is the large one, and on a small roof it can cost a whole row. Second, and more important, a partly shaded module drags down every other module in its string. If a bottom row is going to be shaded for part of the winter, either leave it out or put those modules on their own MPPT input so the loss stays local. Bringing one shaded module into a healthy string is the most expensive way to gain a module.
While you are measuring the parapet, measure everything else: overhead water tanks, the stair headroom, lift machine rooms, exhaust cowls, the mobile tower on the neighbouring building, and any adjacent plot where a taller building is likely to go up. That last one is a judgement call, and it is better made openly at design stage than discovered in year three.
Assess the complete added dead load, local reactions and wind loads against the structure's verified capacity. Do not treat unused live-load allowance as automatically available for permanent PV equipment or ballast.
Three things need checking, and only a structural engineer can close them out:
Where drawings are missing, a structural engineer should determine the survey, reinforcement investigation, material assessment and calculations needed. Any load test must be professionally designed and justified; it is not a routine substitute for establishing capacity.
One last thing that gets designed out and then built back in by accident: keep the roof drains clear and keep a walkway. An array installed hard against a parapet with no access strip traps leaves, dust and bird litter behind it, blocks the outlet, and turns the roof into a shallow tank. That failure has nothing to do with the fixings, and it floods buildings all the same.
Consult the relevant edition and the documents for the exact equipment and site: Sika: project-specific solar roof and waterproofing arrangements; IEC 62548-1: PV array design requirements.
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