A price per module does not define a ballasted flat-roof PV mounting system. The same module count can require very different chassis quantities, ballast distribution, roof attachments, pads and labor when the building height, parapets, wind exposure, roof zones, array edges, seismic conditions, snow, roof membrane, drainage paths or structural limits change. Even within one project, two identical-looking bays can carry different ballast because one sits at an array corner and the other lies in an interior field.
A useful RFQ therefore asks for a project-specific roof plan and ballast map, not a generic kilograms-per-module allowance. It fixes the building and array geometry, identifies the adopted loading standard and site parameters, records the exact mounting-system model and module, and requires the supplier to return every ballast and anchor location with a calculation report. The roof engineer must then check total and local dead load, load combinations, membrane compatibility, drainage and the building’s capacity.
This guide is a procurement method, not a structural design, wind-tunnel study, roof warranty decision or installation instruction. The engineer of record, mounting-system manufacturer, roofing manufacturer, building owner, insurer and authority having jurisdiction remain controlling. No statement here claims that SINAWATTS provides structural engineering, roof approval, system certification, ballast blocks, stock or installation. For general corrosion and load evidence, also use the solar mounting bracket corrosion and load guide.
Begin with one controlled roof model
Issue one roof model to every bidder. Include a dimensioned roof plan, roof elevations, building height, parapet dimensions, roof slopes, steps between roof levels, corners, setbacks, expansion joints, drains, scuppers, overflow routes, curbs, hatches, vents, ducts, screens and other obstructions. Mark the proposed array boundaries, module orientation, tilt, row spacing, maintenance aisles and fire access paths.
The inputs must also identify the project location, risk category, governing code and loading-standard edition, wind speed basis, wind exposure, topographic effects, elevation, seismic parameters, snow and rain criteria. Do not copy a nearby project’s wind data. In the United States, the controlling edition may depend on the adopted building code rather than the latest publication. The current ASCE page checked on 2026-09-21 identifies ASCE/SEI 7-22 as the current edition and describes coverage of dead, live, snow, rain, seismic and wind loads, among other hazards. The project authority must state which edition applies. ASCE/SEI 7-22 official overview.
Provide roof construction and condition: deck type and gauge or thickness, support layout, insulation type and compressive properties, cover board, membrane manufacturer and system, roof age, warranty status and areas of ponding or repair. Existing-building drawings should be field checked. A roof called “TPO over steel deck” still lacks the insulation stack, fastening pattern, support spacing, condition and approved interface needed to evaluate concentrated reactions and membrane protection.
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| Input group | Required RFQ record | Why it changes the offer | Stop condition |
|---|---|---|---|
| Site and code | Address, adopted code, load-standard edition, risk category and hazard inputs | Changes wind, snow, seismic and rain design | Bidder substitutes default location data |
| Building geometry | Roof dimensions, elevations, height, parapets, steps and adjoining structures | Changes wind zoning and flow | Aerial image replaces dimensions |
| Array geometry | Exact module, orientation, tilt, row spacing, setbacks, aisles and subarray boundaries | Changes aerodynamic response and load sharing | Module count only |
| Roof assembly | Deck, supports, insulation, cover board, membrane, warranty and condition | Changes load capacity and interface requirements | Roof called merely “flat” |
| Drainage | Primary and overflow drains, scuppers, slopes, crickets and flow paths | Ballast and bays can obstruct water | Drains omitted from design model |
| Structural limits | Allowable distributed and local loads, combinations and weak zones | Screens ballast and point reactions | Average kg/m² is used as sole check |
Version the model. A moved array edge, deleted module, new roof obstruction or changed parapet can alter the ballast solution. Every bidder should return the model revision used, and the purchase order should prohibit field redistribution unless the approved design process is repeated.
Separate building roof zones from array zones
“Edge,” “corner” and “interior” can refer to several different things. Building-code roof zones describe pressure regions on the building. A mounting-system calculation or wind-tunnel method can also distinguish array perimeter, exposed module edges, internal bays, gaps or subarray corners. They are related but are not interchangeable labels.
Require the calculation report to show how the adopted roof geometry and wind inputs become forces or reactions at each mounting location. The roof plan should display the boundaries used by the software or engineer, with dimensions. If a module crosses a zone boundary, the controlling method should state how it is assigned. Do not let purchasing convert colored software output into a generic rule such as “corners get two blocks.”
The current Unirac U-Builder design-assumption page, checked on 2026-09-21, illustrates why assumptions must remain with the output. It lists product-specific roof-slope and roofing-material boundaries, stated wind and snow assumptions, and says the user is responsible for project inputs while Unirac is not the engineer of record. It also notes that the most restrictive roof-zone span applies when a module is attributed to that span in the cited assumptions. These are Unirac tool statements, not universal design rules. Unirac U-Builder design assumptions.
The supplier should return the complete calculation package, not only a bill of materials. Retain input pages, roof plan, array layout, zone map, ballast map, anchor map if applicable, component schedule, reactions, warnings, system limitations and software/report version. Any manual override should identify its author and engineering basis.
A ballast map is a controlled installation drawing
A ballast map assigns a specific quantity, block size or mass and position to each bay, tray or chassis. It should use stable grid references and match the roof plan. “Typical ballast” is not enough because wind demand and load sharing vary across an array, and roof capacity can vary beneath it.
The current IronRidge BX System Installation Manual, version 4.2, checked on 2026-09-21, tells installers to place ballast blocks in BX chassis as indicated on the site layout. Its installation sequence and optional anchoring detail belong to the BX system. They do not authorize a buyer to replace its site layout with an average block count. IronRidge BX System Installation Manual.
The current K2 D-Dome 6 Xpress assembly instructions likewise refer to the ballast plan from K2 Base and show model-specific ballast positions and components. The current K2 system page describes defined roof-slope and system conditions for the named family. Those documents demonstrate the link between configuration software and assembly; their positions and limits cannot be transferred to another rack. K2 D-Dome 6 Xpress assembly instructions; K2 Dome 6 system official page.
Require the map to state the ballast unit definition. A drawing that says “2” is ambiguous unless the approved block dimensions, minimum verified mass, material requirement and placement are known. Actual concrete units can differ in weight. The installer or receiving inspector should verify the project-required minimum mass using the approved sampling and documentation plan.
Unirac’s official ballast-block technical bulletin, available from its technical library and checked on 2026-09-21, distinguishes capacities among its named systems and says required minimum weight comes from the project design plan. It also discusses nominal block dimensions and inspection for degradation in the context of those products. This is a strong procurement lesson: block capacity and stacking are product-specific, and block identity belongs in the BOM. Unirac ballast-block technical bulletin.
Use this ballast schedule format:
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| Grid/bay ID | Roof/array zone | Chassis or tray part | Block specification | Quantity and verified minimum mass | Placement detail | Local roof reaction | Status |
|---|---|---|---|---|---|---|---|
| B-14 | Returned by design report | Exact part number | Controlled size/material | ___ | Drawing detail ___ | ___ | Open/accepted |
Never “borrow” a block from an interior bay to solve a shortage at the perimeter. Never consolidate several blocks at a convenient location unless the revised design explicitly permits it. Weight in the wrong place may fail both wind resistance and roof load distribution.
Wind resistance is a system result, not ballast mass alone
Ballast resists sliding and overturning as part of an interconnected aerodynamic and structural system. Module geometry, tilt, gaps, deflectors, rack flexibility, friction, array size, roof height, parapets and openings can affect behavior. More total weight does not prove that the required load path exists at every location.
The April 2026 FM Property Loss Prevention Data Sheet 1-15, checked on 2026-09-21, includes current guidance for roof-mounted PV panels and a section on boundary-layer wind-tunnel testing and ballasted systems. Its supporting discussion explains that load sharing is important to determining array loads and required ballast, and that a test array should represent the actual structure and ballast configuration. It also distinguishes corners, edges and interiors. FM guidance applies within its stated loss-prevention and project context; it does not replace the governing code or a manufacturer’s project design. FM Data Sheet 1-15, April 2026.
Ask the designer to identify the wind basis for the exact system: code analytical method, approved wind-tunnel coefficients, evaluation report, product certification or another accepted route. Retain the model geometry limits and any required array size, gap, deflector or connectivity conditions. A wind-tunnel report for one tilt and module aspect ratio does not automatically cover another.
If the system changes from fully ballasted to hybrid anchored, update the analysis and roof interface. IronRidge’s current BX manual shows an optional attachment arrangement and directs users to the flat-roof attachment instructions. It also states that the attachment kit is not part of the grounding path and calls for sealing or flashing according to roofing-manufacturer guidance. This is model-specific evidence that an anchor is more than a ballast-reduction line item.
Ask for separate checks for:
- uplift and overturning at each critical bay or group;
- sliding and the friction assumptions, including membrane interface;
- load sharing and connectivity required by the calculation;
- component forces, chassis or tray limits and module clamp loads;
- anchor tension and shear where hybrid attachments are used;
- seismic displacement and interaction where applicable;
- snow, rain and other controlling load combinations; and
- roof-deck, support, insulation and membrane reactions.
If the report produces warnings, preserve them. A clean BOM exported after someone manually deletes a warning page is not acceptable evidence.
Check average load, local reactions and the roof load path
An array-wide average dead load can hide concentrated ballast. The structural review should map each block or tray reaction through the membrane, cover board and insulation into the deck and supporting structure. Check local bearing and insulation compression as well as deck, joist, beam, column and foundation effects under the governing load combinations.
The total roof load includes modules, mounting components, ballast, cable supports, equipment and sometimes temporary construction loads. Existing roofing aggregate, retained equipment and ponded water also matter. Ask the engineer to state whether reported capacities are additional allowable loads or total loads, and whether they apply uniformly across the roof.
Roof pads or slip sheets can protect a membrane or spread contact, but only within their approved system and roof-manufacturer conditions. Record pad material, dimensions, thickness, placement and compatibility. Do not assume a larger pad cures an overloaded deck or compressible insulation. Obtain written roof-manufacturer or warranty-provider acceptance where required.
Construction sequencing deserves a separate plan. Staging many ballast blocks in one roof area before distributing them can impose a temporary concentration absent from the final calculation. State delivery route, pallet limits, lifting locations, temporary storage zones and the sequence for placing blocks. The responsible structural party should approve those loads.
For mounting hardware interfaces and environmental evidence, use the solar mounting fastener guide. For module attachment boundaries, use the solar module clamp-zone guide. The module and rack documents both need to accept the final clamp position and loading.
Protect primary and overflow drainage
A ballasted array must not obstruct primary drains, scuppers, overflow openings, crickets, valleys or designed flow paths. Record these elements on both the roof plan and installation layout. Define project-specific no-place zones around drains and access clearances for cleaning; do not invent one universal distance.
Survey actual roof slopes and existing ponding before final design. “Flat” roofs still use slope to drainage, and field conditions can differ from drawings. A rack foot, pad, cable tray or ballast unit placed across a shallow flow path can trap debris and water. Added dead load can also affect deflection and ponding behavior, which the structural and roof professionals must review.
The drainage review should answer:
- Are all primary and overflow outlets shown, dimensioned and kept accessible?
- Does any tray, pad, conductor or ballast cross the route by which water reaches an outlet?
- Can maintenance staff remove strainers and clear debris without moving modules or ballast?
- Are roof crickets, sumps, expansion joints and low points represented in the array model?
- Has the engineer considered rain and ponding loads under the governing rules?
- Will any new penetration or attachment preserve the roofing system and warranty requirements?
ASCE’s current ASCE 7-22 overview explicitly includes rain among the covered hazards. That high-level fact does not supply the project calculation. Require the responsible engineer to document the adopted rain/ponding basis and the roof professional to approve physical clearances.
Cable routing belongs in the same review. Keep conductors supported above expected water paths using approved components, without creating debris dams or resting connectors on the membrane. If glands or enclosures are exposed, the IP67 versus IP68 guide helps distinguish component ratings from installed protection.
Use a bounded hypothetical comparison
Hypothetical screening example — not a real project design. Assume a licensed engineer has approved a roof capacity schedule and a mounting-system design report for a 120-module array. The report divides the layout into referenced bays and returns 220 concrete units of at least 14 kg each, with higher counts in specified perimeter bays. It also identifies four optional anchors, shows two drains with project-defined clear zones and limits temporary pallet storage to marked structural lines.
Bid A returns the exact report revision, a grid-matched map, 220 blocks meeting the stated minimum verified mass, all pads and four anchor assemblies. Bid B prices “two blocks per module,” yielding 240 blocks, but provides no location map or local-reaction table. Bid C prices 210 blocks using a heavier local unit and moves them toward the roof center for easier installation.
Bid A is comparable to the stated design package, subject to full technical review. Bid B is not automatically safer because its total mass is higher: the distribution and system calculation are missing. Bid C cannot be accepted merely because its total weight appears similar; changing unit mass and location changes reactions and wind resistance. Neither extra nor redistributed weight should be approved without a revised design.
Now assume an obstruction forces the installer to remove two modules and open a new aisle. The original map should be treated as superseded until the mounting-system design is rerun and the roof/structural review is updated. Array edges and load sharing have changed. The example does not prescribe 14 kg blocks, a block count, anchor count or drain clearance for any real roof.
Hypothetical local-load screen. Suppose the returned map shows four 14 kg blocks in one tray. Their block mass alone is 56 kg before the tray, module and other components are added. Dividing 56 kg by the area of the whole array would hide that local concentration. The responsible engineer should use the actual contact geometry and load combinations, not this simple mass arithmetic, to check the roof assembly.
Build a comparable RFQ package
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| RFQ deliverable | Supplier or designer return | Acceptance question | Hold point |
|---|---|---|---|
| Input register | Site, code, loads, geometry, roof and module data with revisions | Are all project facts traceable? | Default assumptions replace missing facts |
| System identity | Manufacturer, family, exact component parts and current manuals | Is the calculation for the quoted system? | Mixed-family parts without approval |
| Roof plan | Dimensioned array, obstacles, zones, drains, access and references | Does it match the surveyed roof? | Aerial image only |
| Engineering report | Method, inputs, outputs, reactions, warnings and professional review required by project | Are limits and assumptions intact? | Summary page only |
| Ballast map | Unique bay IDs, unit type, count/mass and exact positions | Can an inspector verify every location? | Average blocks per module |
| Anchor map | Exact anchors, reactions, substrate and waterproofing detail | Is the roof/load path approved? | Anchor used as unexplained ballast substitute |
| Roof interface | Pads/slip sheets, compatibility and warranty acceptance | Is membrane/insulation protected? | Generic rubber pad |
| Drainage plan | Primary/overflow paths, clear zones and maintenance access | Can water and service personnel move as designed? | Drains hidden under modules |
| Commercial return | Itemized BOM, spares, price, MOQ and lead time | Does price cover the approved package? | Technical gaps treated as cost savings |
Require a controlled BOM tied to drawing callouts. Include chassis, trays, deflectors, clamps, connectors, bonding hardware, cable supports, pads, blocks and optional attachments. State who supplies each item and whether the ballast unit must be procured locally. Ask bidders for actual commercial terms rather than using estimates from this article.
Inspect installation against the map
Before work starts, confirm that the roof survey, calculation report, plan, ballast map and BOM share one revision. Mark a physical grid or another durable location reference. Train installers to stop when a drain, curb, roof step or dimension differs from the drawing; field improvisation can invalidate the design.
Receiving inspection should verify component identity, block dimensions and mass under the approved sampling plan, pad material, module compatibility and document revision. Quarantine damaged or degraded blocks, cracked polymer parts, distorted metal, missing labels or substituted hardware. Protect roofing materials during delivery and staging.
During installation, record representative photographs and required inspection sign-offs for chassis orientation, array gaps, deflectors, interconnections, clamp positions, ballast location, anchors, pads, grounding and cable routing. A photo of a completed row does not prove the concealed ballast in every bay, so use the map identifiers and staged inspections.
At completion, walk every drain and overflow route. Confirm service access, remove debris and temporary materials, and reconcile the installed block count by location rather than only by total. Record approved field changes on an as-built plan. The owner’s maintenance file should define periodic inspections after severe weather and roof service, subject to the system and roof-manufacturer instructions.
The current Unirac RM5 installation guide available through Unirac’s document page and checked on 2026-09-21 states that project construction drawings, when provided, take precedence over its general installation guide. That model-specific statement supports a wider purchasing discipline: preserve project drawings and do not let a generic manual erase project details. Unirac RM5 Installation Guide document page.
Source boundaries checked on 2026-09-21
The ASCE, FM, Unirac, IronRidge and K2 pages linked above were checked on 2026-09-21. Product instructions and calculation tools change. Preserve the exact approved report and document revision in the project file. Manufacturer examples are used only to show evidence structure; this guide does not combine their parts or apply one family’s ballast rule to another.
If you are preparing a flat-roof mounting enquiry, assemble the surveyed roof plan, module schedule, design inputs, roof assembly, structural limits and required calculation deliverables. SINAWATTS can use that package to identify the requested hardware and documentation scope; project structural, roofing and code approval remains with the responsible parties.
Send your ballasted flat-roof mounting evidence package for an RFQ
Buyer FAQ
Can I compare ballasted systems by kilograms per module?
Not reliably. An average can help an early feasibility screen, but purchasing and installation require a project-specific map. Wind demand, array edge conditions, load sharing, roof geometry and local structural capacity cause ballast to vary by location. Compare complete approved systems and local reactions.
Does adding more ballast always make the system safer?
No. Extra weight can overload the roof or insulation and still be ineffective if placed in the wrong bay. It may also change tested or calculated behavior. Any added or moved ballast should be evaluated through the approved system design and structural review.
Can ballast blocks be sourced locally?
Only if the mounting-system and project documents permit it and the blocks meet the controlled size, material, mass, durability and placement requirements. State the verification method and reject degraded units. A nominal masonry-unit description does not prove actual mass.
When should a hybrid roof anchor be used?
Use anchors when the approved design calls for them—for example, to address a system limit, roof capacity or project requirement. The exact reason and reactions must be documented. Substrate attachment, flashing, membrane compatibility and grounding treatment require their own approved details.
How close can the array or ballast be to a roof drain?
There is no universal distance in this guide. The roof professional and project documents should define clear zones and maintenance access using actual drainage geometry, local rules and manufacturer requirements. Show primary and overflow paths on the issued roof plan.
What should trigger a new ballast calculation?
Changes to module model or orientation, tilt, row spacing, array boundary, roof dimensions, parapet, obstruction, aisle, design loads, mounting system, ballast unit, anchor layout or roof interface can be material. Route them through the defined change-control process before installation continues.
What evidence should remain in the closeout file?
Keep the input register, calculation report, wind/evaluation evidence used by the design, roof plan, ballast and anchor maps, structural and roof approvals, controlled BOM, installation records, as-built deviations, block verification, drainage inspection and maintenance instructions. Those records allow later roof work or module replacement to be reviewed against the real design.