How Solar Ballast Systems Avoid Penetrations on Membrane Roofs

What Is a Solar Ballast System for a Membrane Roof?

A membrane roof solar ballast system supports photovoltaic modules on low-profile racks above the waterproofing layer. Concrete pavers, also called ballast blocks, sit in ballast trays or in designated positions on the racking, and their weight does the work that anchors would otherwise do.

The term non-penetrating applies to the primary mounting method, meaning the connection between the racking and the building structure. Conduits, electrical equipment, fall-protection anchors, and hybrid attachment points may still require separately flashed roof penetrations.

The word ballast covers two different things on a roof-PV project. A ballasted roof assembly uses stone or concrete pavers to hold the roofing membrane in position, and that choice belongs to the roofing scope, settled before any solar equipment enters the project. A solar ballast system holds the PV mounting structure in position, and the membrane keeps whatever attachment method it already has. On an existing roof ballasted with concrete pavers, a PV array may still be feasible, subject to roof and structural review.

Roofs ballasted with loose aggregate need separate review. FM Data Sheet 1-15 advises against installing PV on roofs with aggregate, or where an adjacent higher roof has aggregate, because loose stone can become windborne debris and pedestals bearing on aggregate change the sliding resistance the calculation depends on.

How Ballasted Mounting Secures the Array Without Roof Penetrations

Four mechanisms keep a ballasted array in position, and they do not all work in the same way. System weight and interface friction supply resistance. Array geometry changes the wind pressure acting on each module, and the connections between racks change how an applied load spreads across neighboring units.

Ballast Weight Resists Uplift and Overturning

Wind creates pressure above and below a solar module. The net upward component is wind uplift. The same flow creates an overturning moment that tends to rotate the rack about one of its support edges.

The combined weight of modules, racking, and ballast counters both actions. How much resistance a support needs changes across the roof. Supports near roof corners, roof edges, array boundaries, gaps, and access aisles face different demands from supports located within the array interior.

An access aisle interrupts the continuity between connected racks, which reduces load distribution and removes the shielding that outer panels give to the ones behind them. For panels immediately around openings of that kind, FM recommends additional ballast or securement, typically on the order of 50 percent more.

Adding ballast raises resistance. It also raises structural load and local bearing pressure at every support that carries it. Engineers therefore assign ballast by roof and array zone, based on the local wind demand and the structural capacity available.

Friction Limits Horizontal Sliding

Friction between the mounting base and the roof surface resists horizontal movement. The coefficient depends on the exact material pair at that interface, typically a pedestal pad or slip sheet in contact with a TPO, PVC, or EPDM surface. A slip sheet is the thin sacrificial layer installed between the rack base and the membrane. Changing either contact surface changes the applicable coefficient.

ASTM D1894-24 provides the test method for static and kinetic coefficients of friction between plastic film or sheeting and another surface. Results shift with material age, surface condition, temperature, moisture, and the test procedure itself, so a coefficient taken from a product datasheet may not represent the installed material pair. FM Data Sheet 1-15 handles that variability by taking the lesser of the wet and dry tested values and limiting the assumed static friction coefficient to 0.4 unless testing justifies a higher figure. Those limits belong to the FM framework and do not govern every project or jurisdiction.

The sliding calculation should use friction data for the materials that will actually be installed, including the slip sheet. Changing the contact material later can invalidate that data, unless the substitute is covered by equivalent test results or written engineering approval.

Array Geometry Changes Wind Pressure

Module tilt, orientation, row spacing, edge gaps, parapet height, and wind deflectors all change the airflow around an array. A layout with lower wind demand can be held with less ballast, provided it stays inside the configuration range the pressure coefficients came from.

Deflectors show the effect clearly. In wind-test terminology, fitting deflectors on the high side of each row turns an open array into a closed configuration, and the two carry different pressure coefficients. Switching a module from portrait to landscape alters the rack geometry. A larger module, a higher tilt, a shorter connected row, or a different edge gap can also change the applicable pressure coefficients or the structural assumptions behind the original calculation.

Connected Rows Share Loads

Rails, braces, module connections, and rack-to-rack connectors transfer part of an applied load into neighboring units. Engineers call that behavior load sharing. Validated load sharing can reduce local ballast demand in some configurations by spreading wind action across connected racks.

A physical connection on its own does not establish how much of that sharing a design may credit. The amount depends on rack stiffness, connector strength, array dimensions, module arrangement, and ballast placement. The value must be supported by test data or structural analysis accepted under the governing design method.

Wind demand and the available resistance both vary across the roof, so the final drawing should give ballast quantities zone by zone.

Which Membrane Roofs Are Suitable for Ballasted Solar Mounting?

TPO, PVC, EPDM, and some modified-bitumen assemblies all carry ballasted solar arrays. What decides your case is the rest of the assembly: how the membrane is attached, what sits underneath it, and how much service life it has left.

Membrane Attachment Limits the Mounting Method

Attachment method is worth confirming early, because under some approval frameworks it settles whether a fully ballasted layout is permitted at all.

A fully adhered membrane is bonded continuously to the substrate below. A mechanically attached membrane is secured at spaced fastener rows and can move between them as pressure changes, a behavior called billowing. Rigid wind-tunnel models do not reproduce that movement, and vertical movement of the roof cover can raise the drag and lift coefficients acting on the panels. New ballasted PV systems within the FM framework are therefore limited to adhered roof membranes, and PV installed over a mechanically fastened roof cover must be mechanically fastened as well. Other jurisdictions and approval routes apply their own criteria, so confirm which framework governs your building before layout work starts.

Contact Materials Must Match the Roof Membrane

TPO and PVC are thermoplastic membranes. EPDM is a thermoset synthetic rubber. Each responds differently to prolonged contact with plasticizers, oils, rubber compounds, heat, and surface treatments, so compatibility should be checked for the specific membrane product and the proposed contact material.

A slip sheet or protection layer separates the rack base from the membrane, limits abrasion, and keeps incompatible materials apart over years of contact. The selected layer should be approved by the membrane manufacturer and match the interface used in the sliding calculation.

Roof Age Changes the Project Sequence

Check an existing roof for leaks, open seams, blisters, previous repairs, trapped moisture, and remaining service life before solar modules restrict access to it. FM Data Sheet 1-15 recommends replacing or re-covering a roof more than 10 years old before adding a PV system expected to remain in service for 25 to 30 years, and advises against installing PV over roofs showing leaks, cracks, delamination, or blisters. Outside an FM-governed framework, roof condition, remaining service life, warranty status, and the requirements of the roofing manufacturer and project engineer determine whether replacement or re-covering comes first.

A solar-ready roof should also carry a remaining warranty period that broadly matches the expected service life of the PV system. If the roof needs replacing inside that period, the array will have to be removed and reinstalled, which adds cost and lost production to a plant that is already running.

Roof slope and surface condition, cover-board type and thickness, insulation compressive resistance, roof-deck type and condition, drainage layout, parapets and rooftop equipment, and allowable structural loading all form part of the same review. Together these conditions decide whether a ballasted layout is suitable, and they supply the inputs for the ballast calculation.

What Determines Ballast Weight and Array Layout?

There is no standard ballast weight that applies across projects. The required quantity depends on site wind conditions, building geometry, roof characteristics, module configuration, racking, and the applicable wind-design data. Until those inputs are confirmed, any figure a supplier gives you is suitable for preliminary budgeting only.

The calculation follows the structural standards that apply to the project. ASCE/SEI 7-22 provides the U.S. framework for wind, snow, rain, seismic, dead-load, and load-combination calculations, and projects elsewhere work from the applicable national or local standard. Insurer criteria can add requirements beyond the code minimum and change the resulting ballast demand. FM wind design for these systems applies an importance factor of 1.15 and a safety factor of 2.0 on each panel, and assumes Ground Roughness C unless all conditions for Exposure B or D are met and documented.

The main design inputs connect as follows:

Roof Zones and Array Zones Are Different

Wind pressure rises near building edges and corners, so roof calculations divide the surface into interior, edge, and corner zones. A solar array has its own interior, edge, and corner positions. The roof zones and the array zones overlap, but their boundaries are not the same, and an exposed module at an array corner near a roof corner carries a different wind demand from an interior module near the middle of the building.

Where a design relies on wind-tunnel data, the applicable report defines both classifications for the tested configuration. A boundary layer wind tunnel study prepared under ASCE 49 should supply the pressure coefficients and the zoning for the tested array configuration. It also needs to state the limits on panel slope, building geometry, and roof slope, the edge and parapet factors, and the load sharing factor used. For an FM-based design, missing configuration limits or load-sharing information can prevent a report from being used for your layout.

Many racking systems are already supported by wind-tunnel data covering a defined range of buildings, modules, tilts, and array geometries. The project team then checks whether the building and layout fall within that tested range, rather than commissioning a new study. Prescriptive methods are also available within the limits set by the governing standard.

Structural Capacity Limits the Available Ballast

Structural capacity should be evaluated by a qualified structural engineer working from the project structural drawings and calculations. The review covers:

• Module and rack dead load
• Added ballast
• Local bearing pressure
• Installation and maintenance loads
• Applicable snow, rain, and other load combinations

For a first-pass screening on an existing roof, FM suggests allowing 2 to 3 psf (0.10 to 0.14 kPa) for the PV panels and their hardware, plus the calculated ballast, while confirming that the added dead weight does not reduce the capacity available for snow, rain, and live loads below the required level. That figure is a screening value. Actual equipment weights and project calculations govern the design.

If the calculated ballast exceeds the available roof capacity, the layout or the mounting method has to change. Tilt, module orientation, array boundaries, rack connections, and deflector arrangement all change wind demand and the resulting ballast requirement. A hybrid layout can use structural attachments in the higher-demand zones and keep ballast where structural capacity allows.

What Project Data Is Needed for an Engineered Ballast Plan?

A ballast schedule is assembled from building geometry, environmental loads, roof information, module data, and installation constraints. Roof area and target capacity are useful starting points, and they are not enough on their own to calculate a buildable ballast schedule.

Most of this information comes from you, your EPC team, or the building design team. The mounting engineer then applies the governing wind criteria and the validated interface-friction data to produce the ballast schedule.

Mibet Provides Ballast Plans for Membrane Roof Solar Projects

The Mibet engineering team develops project-specific ballasted solar mounting plans from the confirmed roof, building, and module data. The engineering package identifies the location of each rack and ballast block, the roof and array zones used in the calculation, and the module dimensions, interface materials, and layout conditions that must remain consistent through procurement and installation.

To begin a project-specific review, send the project inputs listed in the table above.