Ballasted Racking

Key Takeaways

• Ballasted racking = non-penetrating flat-roof solar.
• Concrete or steel weights resist wind uplift.
• Pros: no penetrations, fast install. Cons: structural capacity needed.
• Not suitable for HVHZ (high-wind zones).
• ASCE 7-22 wind analysis required for ballast sizing.

How Ballasted Racking Works

Instead of bolting or lag-screwing a mounting foot through the membrane and into the deck, a ballasted system sets modules on tilt-legged trays or rails that sit directly on top of the roofing. Concrete blocks (sometimes steel or plastic trays filled on site) are placed along the array to counteract the uplift wind creates as it passes over and around the panels. Because nothing pierces the TPO, EPDM, or PVC membrane, the roof’s original waterproofing warranty stays intact — a major reason facility owners and roofing contractors prefer it on commercial low-slope roofs.

The trade-off is weight. Every pound of ballast becomes an added point load the roof deck and structural framing must carry, so the racking layout can’t be decided in isolation — it has to be checked against the building’s live and dead load capacity. That’s also why ballasted systems are typically limited to low tilt angles (around 5–10°): a steeper tilt increases the wind uplift coefficient, which in turn drives up the ballast weight needed, often past what the roof can reasonably support.

Worked Example

Picture a 500 kW array on a warehouse roof in a moderate-wind region. A designer running the ASCE 7-22 components-and-cladding analysis will vary ballast per zone — corner and edge zones see higher uplift pressures than the field of the array, so blocks cluster more densely there, while interior rows need comparatively less. The output isn’t a single number for the whole roof; it’s a zone-by-zone ballast schedule that the structural engineer then confirms the roof can carry without exceeding its rated load. If the wind zone is severe enough — as in coastal HVHZ regions — the required ballast weight can climb to the point where penetrating racking, or a hybrid ballast-plus-mechanical-attachment approach, becomes the more practical choice.

Related Reading

Because ballast weight is derived directly from wind uplift pressure, anyone specifying a ballasted layout should be comfortable with the underlying load calculation — our ASCE 7-22 wind load walkthrough for solar rooftops breaks down how designers actually read the standard’s zones and coefficients into a ballast schedule. Once the loads are known, the structural verification of the roof deck and framing typically runs through the same tools compared in SAP2000 vs STAAD Pro vs manual calculations for solar structures. If a site’s wind exposure or roof capacity rules out ballast entirely, the alternative is usually a penetrating, ground-level foundation instead — a very different design problem covered in our guide to pile foundation design for solar ground-mount arrays.