By AGILE Consulting Engineers, Solar PV and Battery Energy Storage Systems (BESS) specialists.
Ground-mount solar design starts with a mostly blank site. Commercial rooftop design starts with someone else’s building, built to someone else’s loading assumptions, years or decades before solar or a Battery Energy Storage System (BESS) was ever part of the plan. That difference shapes almost every decision in the design, and it is why a rooftop system cannot simply be a scaled-down version of a ground-mount one.
Table of Contents
- Assessing the Existing Structure First
- Roof Loading and Structural Capacity
- Wind Loading and Uplift
- Adding BESS to a Rooftop Commercial Project
- Access and Maintenance Planning
- Integration with Existing Building Electrical Infrastructure
- Fire Safety and Building Services Coordination
- What to Do Next
- FAQ
Assessing the Existing Structure First
The first real design question on a commercial rooftop solar or BESS project is not how many panels fit, it is whether the existing structure can carry what is being proposed to put on it. Every building was designed to a set of structural loading assumptions current at the time of construction, and those assumptions rarely anticipated a rooftop solar array, let alone the additional weight and access requirements of a battery installation. A structural assessment of the existing roof, and in some cases the supporting building frame, is the starting point that everything else in the design depends on.
In New South Wales, mandatory structural assessments are most likely to apply to larger commercial systems requiring development approval and to heritage-listed buildings, while in Victoria the Victorian Building Authority recommends structural assessment for all rooftop installations and treats it as effectively mandatory for commercial buildings and larger systems requiring a building permit. Requirements vary between states and between local authorities, so confirming the specific obligation for a given building and jurisdiction is a necessary early step, not an assumption to carry over from a previous project.
Roof Loading and Structural Capacity
Solar panel systems add dead load to a roof structure, generally in the order of 12 to 15 kilograms per square metre for the panels and mounting system, though the actual figure depends on the specific equipment and mounting method chosen. That load needs to be checked against the roof structure’s actual capacity, not its nominal design capacity from decades ago, because factors like corrosion, prior modifications, or accumulated point loads from other rooftop plant can all reduce the margin available for new equipment.
This is also where rooftop design diverges most clearly from ground-mount. A ground-mount array’s foundation can generally be sized to whatever the array requires. A rooftop array is constrained by what the existing structure can actually take, which sometimes means the optimal electrical design, the array configuration that would maximise generation, is not the array the roof can support, and the structural assessment ends up shaping the electrical layout rather than the other way around.
Wind Loading and Uplift
Wind action is a major structural consideration for rooftop solar, particularly for systems with any tilt, which generally attract higher uplift forces than flush-mounted arrays. Structural design actions for wind in Australia are governed by AS/NZS 1170.2, and a rooftop system’s mounting and fixing design needs to be assessed against the specific wind loading conditions for that building’s location, height and exposure category, none of which can be reliably assumed from a similar-looking project elsewhere.
Uplift is a case where getting the design wrong has consequences that are not merely theoretical. A mounting system under-designed for the site’s actual wind loading is a structural and safety liability, not just an underperforming asset, which is part of why this element of rooftop design is one that should not be simplified past what a qualified structural engineer’s assessment actually supports.
Adding BESS to a Rooftop Commercial Project
Placing a Battery Energy Storage System (BESS) on or within a commercial building introduces a different order of structural and safety consideration than solar panels alone. Battery enclosures are considerably heavier and more concentrated in their loading than a distributed PV array, which means rooftop placement is not always structurally viable, and ground-level or basement plant room placement is frequently the more practical outcome once the structural assessment is complete. Where rooftop BESS placement is pursued, it needs its own dedicated structural assessment against the building’s actual load-bearing capacity at that specific location, not an assumption based on the roof’s solar loading margin.
Fire safety is a parallel consideration that shapes placement as much as structure does. Lithium-ion battery installations carry fire risks that differ from solar panels, and Australian requirements for electrical installations and safety of battery energy storage systems are addressed in AS/NZS 5139. At a conceptual level, this generally means giving early thought to separation distances from occupied spaces, escape routes and other building services, and working with the project’s fire engineer and the applicable building code requirements rather than defaulting to a placement chosen for electrical convenience alone.
Access and Maintenance Planning
A rooftop system needs to be maintained for the life of the asset, and access planning is a design consideration in its own right, not something resolved on the day a technician first needs to get onto the roof. That includes safe access pathways to and around the array and any rooftop equipment, compliance with height safety and fall protection requirements, and clearances that allow for panel cleaning, fault-finding and eventual component replacement without creating a safety hazard or requiring disruptive scaffolding every time routine maintenance is due.
Ground-mount projects generally have more flexibility to plan access around the array from the outset. Rooftop projects are constrained by the building’s existing access provisions, lift capacity for equipment, and whatever fall protection infrastructure is already, or is not yet, in place, which makes early coordination between the solar design and the building’s existing access arrangements more important than it might first appear.
Integration with Existing Building Electrical Infrastructure
A commercial rooftop system also has to integrate with an existing electrical installation that was designed around the building’s own load profile, switchboard capacity and metering arrangement, none of which were built with embedded generation in mind. Detailed design needs to assess whether the existing switchboard has capacity for the new connection, whether existing protection and metering need to be upgraded, and how the new generation or storage capacity interacts with the building’s existing maximum demand and any tariff structure it operates under.
This integration work is where a rooftop project’s Single Line Diagram (SLD) often ends up more complex than a comparably sized ground-mount system’s, because it has to represent not just the new solar or BESS equipment, but how that equipment ties into an existing electrical installation with its own history and, sometimes, its own undocumented modifications.
Fire Safety and Building Services Coordination
Beyond BESS-specific fire considerations, a rooftop solar installation needs to be coordinated with the building’s existing fire safety systems and services, including maintaining required access and setback provisions for fire fighting operations, and avoiding conflicts with existing rooftop plant such as air conditioning units, lift overruns and other services already competing for the same limited roof space. This coordination is as much a layout exercise as an electrical one, and it is generally easier to resolve properly during detailed design than to retrofit once equipment is on order.
What to Do Next
Commercial rooftop projects carry a different risk profile to ground-mount, structural capacity, wind loading, access and integration with an existing electrical installation all need to be resolved against the specific building, not a generic assumption. AGILE’s solar and BESS system design service covers the structural, electrical and access planning work that a commercial rooftop project needs before it goes to construction.
FAQ
Does every commercial rooftop solar system need a structural assessment?
Requirements vary by state and local authority, but larger commercial systems requiring development or building approval, and heritage-listed buildings, are the most likely to require a mandatory structural assessment, so this should be confirmed for the specific project.
How much extra load does a rooftop solar system add?
Typical panel and mounting system dead load is in the order of 12 to 15 kilograms per square metre, though the actual figure depends on the specific equipment and mounting method and needs to be checked against the roof’s actual structural capacity.
Can a BESS always be installed on a commercial rooftop alongside solar?
Not always. Battery enclosures are heavier and more concentrated in their loading than a distributed PV array, so rooftop placement depends on a dedicated structural assessment, and ground-level or plant room placement is often the more practical outcome.
What standard governs wind loading for rooftop solar in Australia?
AS/NZS 1170.2 governs structural design actions for wind, and rooftop mounting systems need to be assessed against the specific wind loading conditions for the building’s location, height and exposure category.
Why is integration with the existing switchboard a design consideration?
The existing electrical installation was designed around the building’s original load profile, so detailed design needs to confirm the switchboard has capacity for the new connection and that protection and metering are adequate for the added generation or storage.
Does rooftop design need to plan for future maintenance access?
Yes, safe access, fall protection and clearances for cleaning, fault finding and component replacement should be planned as part of the design, since retrofitting access provisions after installation is generally more disruptive and costly.
