Ballasted Rooftop Solar: Structural Design for Commercial Roofs

Commercial and industrial rooftops are where solar structural engineering gets expensive to get wrong. Warehouses, manufacturing plants and big-box retail roofs are often designed close to their limits, and a ballasted array can add more dead load than the original designer ever anticipated. The structural engineer has two jobs: find the capacity that exists, and design ballast and attachments that are code-compliant without overloading the roof.

This guide focuses on ballasted rooftop arrays. For our full scope on C&I projects, see commercial solar structural engineering.

This article covers reserve capacity, wind and seismic ballast design, drift and the IEBC thresholds that decide whether a roof needs reinforcement. For the overall code framework, see our solar structural engineering guide.

The Commercial Roof Problem: Reserve Capacity

Steel Joists

Open-web steel joists are selected from load tables for a uniform load. Ballast trays and racking feet are concentrated loads, and a concentrated load between panel points puts bending into a top chord designed mainly for axial force. A reserve capacity analysis checks the joist with its actual loads, not just the table rating. When a joist falls short, targeted reinforcement at a few panel points is usually far cheaper than blanket strengthening.

Pre-Engineered Metal Buildings

PEMB roofs use light cold-formed purlins, often lapped for continuity and braced by the roof deck. They typically have the least reserve of any commercial roof, and added load can change both strength and stability checks. Ballasted systems on PEMBs need careful layout to keep loads near frames, or an attached system instead.

Ponding and Deflection

Low-slope roofs must be checked for ponding stability (IBC Section 1611). Added dead load increases deflection, which increases ponding. Arrays that obstruct drainage paths make it worse.

What We Need from the Field

Original structural drawings are ideal. Without them, a field survey of joist depth, designation, spacing and span, girder sizes and roof assembly lets us rebuild the model. Existing rooftop equipment and its loads must be included.

A Typical Reserve Capacity Outcome

On a typical warehouse, a screening check using the joist table rating and a uniform ballast load can show the roof as overstressed. A member-level analysis that uses the actual joist designation, the real ballast tray locations and the existing roof assembly often shows most joists pass, with a few at drift zones or under equipment needing reinforcement. That difference decides whether a project is feasible.

Wind Design for Ballasted Arrays

Three Accepted Approaches

• ASCE 7 Section 29.4.3 prescriptive coefficients for low-slope roofs, with parapet, chord and array edge factors.
• SEAOC PV2-2017, Wind Design for Solar Arrays, which provides a method for low-profile arrays on flat roofs.
• Wind tunnel testing per ASCE 49 and ASCE 7 Chapter 31, typically performed by the racking manufacturer.

Wind tunnel data often produces the lightest ballast, and AHJs know it. Some require peer review. Los Angeles County, for example, sets a floor of 65% of the SEAOC PV2 prescriptive value for designs based on wind tunnel data without peer review. Confirming the local rule before sizing ballast avoids a full redesign at plan check.

Friction, Sliding and Uplift

Ballast resists uplift by weight and resists sliding through friction between the system and the roof membrane. The friction coefficient must be justified, usually by laboratory testing on the actual membrane, and shown on the plans. FM Global Data Sheet 1-15, which governs FM-insured buildings, uses 0.4 unless a higher value is justified. The design must check uplift, sliding and overturning at every array zone: interior, edge and corner.

Seismic Design for Unattached Arrays

ASCE 7 Section 13.6.12 treats rooftop arrays as nonstructural components. Ballasted, unattached arrays on low-slope roofs are permitted when a set of conditions is met, including:

• a calculated seismic displacement, from the standard's formula, shake-table testing or nonlinear analysis,
• clearance to roof edges of at least twice the displacement and not less than 4 ft, or a parapet designed to resist impact,
• an internal load path that holds the array together, and
• conductors and conduit that can accommodate the movement.

The 2021 IBC limits ballasted, non-penetrating systems to roofs with a slope of 1:12 or less, and requires friction to be determined by acceptable engineering principles. SEAOC PV1-2012 provides the background and separation criteria used by many engineers and AHJs. ASCE 7-22 added that panels cannot be counted as part of the interconnection load path unless they have been tested or evaluated for it, which affects some racking designs. California also modifies these provisions in the CBC.

The final ballast must satisfy whichever governs, wind or seismic, at every location. In high-seismic regions, seismic often controls ballast at the interior even when wind controls at edges.

Snow Drift and Sliding at Arrays

Tilted arrays on flat roofs act like a series of small roof projections. Snow can drift between rows and against the array, and snow sliding off panels can load the roof below. In snow regions, these loads may govern joist design. With ASCE 7-22, ground snow loads are strength-level values, and the calculation basis must be stated clearly to avoid mixing with allowable stress values.

IEBC Thresholds on Commercial Roofs

• Gravity (503.3): elements with more than a 5% increase in gravity load must meet current code. This is where reserve capacity analysis pays off.
• Lateral (503.4): an element may remain unaltered if its demand-capacity ratio increases by no more than 10%.
• 2024 IEBC solar exemption: seismic demand-capacity evaluation is not required for rooftop PV adding 5 psf or less of dead load, including ballast, and no more than 10% of the existing roof dead load.

The lateral exemption is a strong argument for light ballast layouts and hybrid systems where the building is near its seismic limit.

Ballasted, Attached or Hybrid

Fully ballasted systems avoid roof penetrations but carry the most weight. Fully attached systems carry the least weight but need structural attachments and flashing. Hybrid systems use attachments at edges and corners, where wind demand peaks, and ballast elsewhere. The right answer comes from the roof's reserve capacity, the seismic region and the owner's roofing warranty. We run the options before the layout is locked.

Material Staging During Construction

Pallets of modules and ballast blocks are some of the heaviest loads a commercial roof will see. A staging plan shows where material may be placed, in what quantities, and on which structural members, so the roof is not overloaded during installation. It is a short document that prevents expensive damage.

Documentation Plan Checkers Expect

• code editions and design criteria on the cover sheet,
• wind method and source: ASCE 7 Section 29.4.3, SEAOC PV2 or wind tunnel report,
• friction coefficient with test reference,
• seismic displacement and required clearances,
• ballast layout by zone, with total added dead load,
• existing roof analysis and IEBC threshold evaluation,
• reinforcement details where required.

Commercial projects also bring equipment loads: inverters and combiners on dunnage, cable tray runs, and BESS and switchgear pads at grade. We design those with the array so the whole package goes to plan check together. For ground-level equipment and foundations, see utility-scale solar foundation design.

Frequently Asked Questions

How much ballast does a solar array need?

It depends on wind speed, exposure, roof height, parapets, array zone, seismic demand and the tested friction coefficient. Interior zones may need little ballast, while edges and corners need much more. Every layout is designed for its own building.

Can a ballasted system go on a metal building?

Sometimes. PEMB purlins usually have very little reserve, so ballast must be kept light and close to frames. An attached or hybrid system is often the better fit.

Will my AHJ accept wind tunnel data for ballast design?

Many do, often with conditions such as peer review, a research request or a minimum percentage of the SEAOC PV2 prescriptive value. We confirm the local requirement before sizing ballast.

What if the roof does not have enough capacity?

Reserve capacity analysis often finds capacity that a screening check misses. Where it does not, we design targeted reinforcement or recommend a lighter system or layout.

This article is general technical guidance, not engineering for a specific project. Code editions and local amendments vary by jurisdiction.