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A commercial rooftop array is not a ground-mount array that happens to sit higher. The roof is the constraint, and it is the one part of the project you cannot change. Everything else — module format, string layout, inverter siting, even the maintenance plan — has to be designed around a surface that was built for weather, not for equipment.
This page is written for the person who has to get a rooftop array onto an existing building without creating a leak, a warranty dispute, or a structural review that comes back twice. It covers the decisions in the order they actually need to be made, and what we need from you to quote rather than guess.
Start with the roof survey, not the module
The cheapest stage to discover a problem is before anyone prices a module. A roof survey that answers five questions will save more money than any component choice: how much life is left in the covering, what the structural reserve is, where the water goes, what is already up there, and how the fire service expects to move around it.
If the roof covering has limited remaining life, compare replacement before installation with the cost and disruption of removing the array for later roof work. That is not a reason to abandon the project, but it does change the sequence and the budget, and it is much better found in week one than in week ten.
The attachment method follows the covering
Three methods cover almost every commercial roof, and the covering picks one for you:
- Standing-seam metal — non-penetrating seam clamps. The clamp must match the seam profile and the approved load requirements. Confirm the roof manufacturer’s warranty conditions and the proposed attachment method before installation.
- Asphalt shingle and tile — penetrating hooks or tile replacements, sealed and flashed. Penetrations are normal and reliable when they are flashed properly; the risk is workmanship, not the method.
- Single-ply membrane and concrete — ballasted trays, sometimes with a limited number of mechanical anchors. No penetration, but ballast adds dead load, and that load has to be checked against the structure rather than assumed.
Whichever method is used, the fixing torque is supplied with the order. Excessive clamp torque can damage the module frame. Follow the selected module and mounting instructions; claim coverage depends on the cause of damage and the written warranty terms. This is one of the few places where a five-minute instruction removes a real financial exposure.
Ballast is calculated by roof zone, not per building
On a flat roof the wind does not load the surface evenly. The corner zones and the perimeter see substantially higher uplift than the middle of the roof, so a single ballast figure for the whole array is either over-built in the middle or under-built at the edges. The ballast schedule that goes with the layout is issued per zone, and that is the document your structural reviewer will want to see.
Two things change the number: parapet height, which shields the perimeter, and building height, which increases the wind pressure. Both come from the building, not from the equipment, which is why we ask for them up front.
Layout has to respect the building, not just the roof
A layout that maximises module count will usually fail review. Fire service access routes, walkways to rooftop plant, drainage paths and clearance around existing equipment all remove area, and they are not negotiable. On flat roofs, leaving the drainage paths clear matters as much as the setbacks: ponding water under an array is a maintenance problem that shows up in year three, not on handover day.
Where the roof has plant equipment, the shading it casts is worth modelling before the layout is frozen. A row that is shaded for two hours each morning will underperform for thirty years, and moving it costs nothing at design stage.
The electrical side is decided by the roof shape
Most commercial roofs are not a single plane. Where a roof has two or more orientations, each orientation needs its own MPPT input, and the string lengths have to match within each input. That usually decides inverter count before the total array size does.
Inverter siting is the other decision worth making early. String inverters are commonly mounted on an exterior wall at ground level or on a rooftop plant screen, and the DC and AC runs are very different in cost. Deciding where the inverter goes before the conduit route is drawn avoids the most common mid-project change.
What the structural review actually needs
A structural reviewer is not looking at the modules. They need the dead load per unit area including ballast, the attachment method and spacing, the uplift figure per zone, and confirmation that the load path into the structure is understood. Confirm the required load figures, calculations and layout documents in the project scope, including who prepares and signs the structural review.
Specifications at a glance
Configuration reference
Swipe horizontally to view all columns →
| Item | Typical specification | What decides it |
|---|---|---|
| Array size | From 30 kW on a single building to several megawatts across a portfolio | Usable roof area after setbacks and obstructions |
| Roof types handled | Standing-seam metal, asphalt shingle, tile, single-ply membrane, concrete | What is on the building now and its remaining life |
| Attachment | Non-penetrating seam clamps, penetrating hooks and rails, or ballasted tray | Roof covering, roof warranty position and uplift requirement |
| Tilt | Roof-following on pitched roofs; 5 to 10 degrees on flat roofs | Roof pitch, or a ballasted low-tilt design |
| Wind uplift | Calculated per roof zone to ASCE 7 or EN 1991-1-4 | Building height, terrain category and roof zone |
| Ballast | Per-tray ballast schedule issued with the layout | Roof zone, parapet height and structural reserve |
| Module | Typically 540 to 600 W, TOPCon, 182 or 210 mm wafer | Roof area, wind class and rail layout |
| Inverter | String inverters, 100 to 125 kW, or 30 to 60 kW on smaller roofs | String count, roof orientations and where it can be sited |
| Setbacks | Confirmed against the applicable local fire and building requirements | Roof geometry, fire-service access and local authority review |
| Walkways | Designated maintenance routes and edge protection where required | Roof traffic and the safety plan |
| Documentation | Roof layout, ballast schedule, structural calculation and torque specification as required | Document scope, availability and responsible parties confirmed in the proposal |
| Warranty | Modules 12 years product, up to 30 years performance; racking 10 to 12 years | Model and order terms |
Figures are selection references. Confirm the final ratings, dimensions, available certifications and warranty against the current datasheet and written proposal for the selected model or system.
Questions buyers ask about rooftop arrays
Will the array void the roof warranty?
On standing-seam metal with non-penetrating clamps, usually not. On any roof where penetrations are made, the roof manufacturer’s position depends on who does the work and how it is flashed, and that should be confirmed in writing before the method is chosen rather than after.
Do we need to replace the roof first?
If the covering has less remaining life than the array is expected to run, yes. Recovering a roof under an array means stripping and rebuilding the array, and that costs several times the roof work alone. It is the single most common reason a rooftop project should be sequenced differently.
How much roof area do we actually lose to setbacks?
It depends on the code and on the building, but on a flat roof with perimeter setbacks and access routes the usable area is often noticeably less than the measured area. The reliable way is to model it against the actual roof plan rather than apply a percentage.
Can we add storage later?
Yes, and it is worth saying so now, because the AC coupling point and the space for the cabinet are much cheaper to reserve at design stage. The array does not have to change.
Who is responsible if a clamp is over-torqued?
Follow the selected module and mounting instructions, including the specified clamp position and torque. If damage occurs, responsibility and warranty coverage are assessed from the installation records, the cause of damage and the written order terms.
What to send us
Send the roof plan if you have one, the roof covering and its age, the building height and location, and the target array size or the electricity spend you are trying to offset. We come back with a layout, the attachment method we recommend and why, the ballast schedule by zone, and a written delivery date.
EXPLORE FURTHER
Equipment & planning for this application.
Start a B2B enquiry
Send the roof age and the interval load data
With the roof age and covering, and twelve months of interval load data, we can tell you whether the roof should be dealt with first and what the array can realistically offset.
Send your requirementsOr reach us directly: +86 136 7367 3267 · sales@gtpsolar.com