By Jennifer Kirstens · 2026-08-31 · 18 min read

Ballasted racking is the default choice for commercial flat roofs with TPO, PVC, or EPDM membrane because the array is held down entirely by weight — no membrane penetrations, no roof-warranty conflicts. PES stocks the three ballasted platforms that own the commercial market:

Ballasted arrays resist wind uplift purely by dead weight and by friction against the membrane. ASCE 7-22 § 29 governs the pressure calculation, and every OEM's design tool (BX Design Assistant, U-Builder, K2 Base) implements it. The four inputs are:

Consider a 100 kW commercial rooftop in Cincinnati, OH — 110 mph V_ult, Exp C, 30 ft mean roof height, 20 psf ground snow, TPO membrane on 4" ISO board.

Not every ballasted system is compatible with every membrane. Older EPDM (rubber) roofs are prone to abrasion; some TPOs plasticize under long-term contact with steel. PES uses this matrix for compatibility check:

Understanding the roof assembly is prerequisite to ballasted solar design. A typical commercial low-slope membrane roof has 5–7 layers, and the racking system contacts only the top membrane, but the load path travels through every layer down to the steel deck.

Structural deck — steel Type B deck (most common), concrete deck, or wood deck. Steel deck typically supports 20–40 psf live load; concrete supports more. This is where the array + ballast + snow + live load has to end up. The structural PE calculates the reserve capacity per IBC-2021 § 1607.14.

Insulation — ISO board (polyisocyanurate) or EPS foam, typically 3–6" thick, 20 psi compressive strength for standard commercial. The ballast trays sit indirectly on this via the membrane and slip pads. Older ISO can compress permanently under sustained load — factor of concern for the structural review.

Cover board — 1/4" gypsum-fiber or 1/2" HD ISO on top of the insulation. Provides puncture resistance and a firmer surface for the membrane.

Membrane — TPO, PVC, or EPDM. This is the water barrier and the surface the racking touches. TPO and PVC are thermoplastic; EPDM is rubber. All three require slip pads under the ballast tubs to prevent abrasion.

Slip pad / walkway pad — an OEM- or membrane-specified pad that sits between the tub and the membrane. This is the single most important component of the ballasted-roof interface. Skip it, and you void the roof warranty.

Cable / conduit routing layer — many commercial designs route DC and AC cable in surface-mounted trays or in tub perimeters, on top of the membrane. Wire management protects the membrane from UV degradation of exposed cable and prevents cable-abrasion damage.

Ballast weight can be delivered in four common forms. Each has its own logistics and cost profile:

Solid CMU block (8x8x16, 40 lb). The default — cheap, dense, available locally at every masonry supplier. Downside: hand-carry weight limits and rooftop delivery via crane or ballast-lift. Load 1 block per tub for interior zones, 2–3 for edge/corner.

Concrete paver (2", 30 lb, or 4", 60 lb). Lower weight per unit, easier to carry, sometimes dimensionally optimized for OEM tub trays. Slightly higher cost per pound. Popular where crane access is limited.

Cast ballast tray inserts. Some OEMs (IronRidge, Unirac) offer purpose-cast concrete tray inserts sized exactly for the tub geometry. Removes assembly-time weight-count errors. Higher cost per pound but faster install.

Steel plate / custom ballast. Rare — only when structural PSF budget is tight and you need to concentrate weight in specific zones. Custom steel is expensive and heavy per volume; usually a last resort.

PES freight optimization: for jobs east of the Mississippi, we usually recommend you order tubs from PES and source CMU or paver ballast from a local yard. Concrete is cheap; concrete freight is expensive. A single 100 kW BX system with solid CMU ballast can weigh 17,000 lb — shipping that on freight from Louisville to Denver adds $2,000+ to the job.

The wind-uplift math for ballasted arrays is the single most important safety calculation in commercial rooftop solar. A conservatively ballasted array will sit fine for 30 years; an under-ballasted array in a strong wind event can lift, shift, or fully depart the roof, taking the membrane with it.

ASCE 7-22 § 30.7 governs the roof-zone calculation for wind uplift on components and cladding. The three zones are:

• Corner zone (Zone 3) — the four rectangles at each corner of the roof, dimensions a = min(0.1 × roof width, 0.4 × building height, 3 ft). Uplift coefficient is typically 2.0–2.5× the interior value.
• Edge zone (Zone 2) — strips along each roof edge from corner to corner, of dimension 'a'. Uplift coefficient is typically 1.5–1.8× the interior value.
• Interior zone (Zone 1) — everything not corner or edge. This is where the majority of the array sits and where the base ballast per module is specified.

Every OEM design tool implements the zone calculation and outputs ballast per module for each zone. Reviewing the output: expect corner-zone ballast to be roughly 2× interior, edge-zone ballast roughly 1.5× interior. If those ratios aren't showing up, the exposure or building height inputs may be wrong.

Perimeter wind deflectors (also called wind skirts or wind fences) are optional aluminum or steel skirts installed at the north-facing edge of a ballasted array. Their purpose is aerodynamic: they redirect the wind flow up and over the array instead of underneath, cutting the uplift pressure on the front row of modules by 15–30%.

Cost-benefit: a wind deflector kit for a 100 kW array runs $600–$1,200. The ballast weight savings from adding a deflector is typically 10–25% of the front-row and corner-row ballast — for a 100 kW array in Exp C, that might mean 1,500–3,000 lb less concrete on the roof. If the roof structure has tight reserve capacity, that ballast reduction can be the difference between 'design passes' and 'need structural retrofit'.

Which sites should absolutely spec a wind deflector:

• Any site with V_ult ≥ 130 mph
• Any Exp D (coastal or open-water) site regardless of speed
• Any building with mean roof height ≥ 40 ft
• Any site where the roof structural PE has flagged a tight PSF budget

Ballasted commercial arrays fall into two dominant layout patterns: south-facing tilted rows (most IronRidge BX and Unirac RM DT deployments) and east-west dual-slope rows (K2 D-Dome, and some IronRidge East-West configurations).

South-facing. Modules tilted 5–10° to the south. Individual module yields maximum energy at midday. Row-to-row spacing must account for shading — typically 2–4 ft of gap per row. Result: array density around 11 W per square foot of roof. Higher single-module yield, lower overall roof utilization.

East-West. Modules alternate between east-tilted and west-tilted at 10° each. Individual module yield is 5–10% lower than a south-facing module, but there's no row-to-row shading (adjacent rows have opposing tilts). Result: array density around 15 W per square foot — a 30–40% increase in installed kW on the same roof. Peak generation shifts from midday to a broader morning + afternoon curve, which better matches most commercial load profiles.

The K2 D-Dome system is the most refined East-West product on the market. PES stocks the D-Dome East-West standard configuration and has quoted many hundreds of kW of it on commercial retail and industrial roofs where roof area is at a premium and demand-charge reduction is the primary economics.

After ballast is loaded and modules are mounted, run this pre-commissioning checklist. It catches the 80% of AHJ inspection reject reasons on commercial flat-roof arrays:

1. Ballast count per tub matches the design ballast plan. Photograph every corner and edge tub with the ballast visible; keep in the project file.
2. Slip pad or walkway pad is present under every tub. This is the single most common miss and the fastest way to void the roof warranty.
3. Setback dimensions match IFC-2021 § 1204. 4-ft perimeter pathways, 4-ft smoke ventilation cutouts, and access lanes for firefighting equipment.
4. Grounding path validated. Continuity check from a random module frame to the array's ground rod or main service ground. Should read < 1 ohm.
5. Cable management is complete. All DC and AC cable is in tray, clip, or conduit — no cable resting on the membrane or draped over module frames.
6. Perimeter wind deflectors installed as designed. If the design called for deflectors and they're missing, the ballast plan is invalid.
7. Roof penetrations (if any) are properly flashed and sealed. Even 'penetration-free' ballasted arrays often have 1–2 penetrations for AC feeder and grounding — those need to be flashed by the roofing contractor before energization.

The slip pad between the ballast tub and the roof membrane is the single most important interface in a ballasted install. The wrong pad wears the membrane, and once the membrane wears through, the roof leaks — usually years after the array is installed and the leak source is hard to identify.

Slip pads come in three families:

Roofing-manufacturer-approved slip pads. The membrane manufacturer (Carlisle SynTec, GAF, Firestone, Johns Manville, Sika) publishes the specific pad it approves for each membrane product. TPO manufacturers typically approve a 40 mil TPO cover strip; PVC manufacturers approve a similar PVC cover; EPDM manufacturers approve a mm-thickness rubber sheet or a reinforced-fabric mat. Using the manufacturer's approved pad preserves the roof warranty.

Racking-OEM slip pads. IronRidge, Unirac, and K2 all ship optional slip pads with their ballasted racking kits. These pads are engineered for the racking-specific footprint but are NOT the roof manufacturer's approved pad. Best used as a supplementary pad in combination with the manufacturer-approved pad — belt-and-suspenders.

Generic walk pads / traffic pads. Available at any roofing supplier for around $10–$20 per pad. Fine for foot-traffic protection; usually NOT approved for permanent solar-mount contact by any membrane manufacturer. Do not substitute for a specified pad on a warranted install.

The decision matrix: on a fully-warranted membrane roof (< 20 years old, active manufacturer warranty), always use the membrane manufacturer's approved pad. On an older or already-worn roof where warranty is moot, the OEM-supplied pad is typically sufficient — but confirm with the roof owner/PM before install.

Ballasted arrays touch the roofing contractor's warranty scope. The install sequence, warranty coverage, and post-install responsibility all need explicit coordination with:

• Roof owner / property manager. Signs off on the roof-load increase, receives the structural PE letter, gets copies of all warranty documents.
• Original roofing contractor. Preserves warranty by verifying the slip pad and tub layout comply with manufacturer specs. Sometimes performs the perimeter flashing work if any penetrations are required for grounding or conduit exits.
• Membrane manufacturer. Issues a warranty-preservation letter confirming the slip pad and racking system are compatible with the specific membrane product and installation age.
• Roof structural engineer. Independent PE (often the building's original SE of record, if available) confirms the roof can carry the array weight per IBC-2021 § 1607.14.
• Solar installer / EPC. Owns the racking install, ballast loading, module mounting, and array energization.

Two-page 'roof-solar coordination memo' should be signed by all five parties before install starts. PES contractors have this template available through the Axis portal — it's saved many warranty disputes.

Commercial membrane roofs have a design life of 20–30 years; solar arrays have a 25–30 year design life. There is a high probability that either the roof or the array will need service, patching, or replacement during the array's operational period.

Best practices for maintenance planning:

• Annual inspection. Walk the array once a year: verify ballast tubs are undisturbed, slip pads are still under every tub, no cable damage, no membrane wear visible around any tub. Include this in the O&M contract.
• 5-year membrane inspection. Have the original roof manufacturer's certified inspector review the array footprint at year 5. Preserves warranty on most modern membrane products.
• 10–15 year mid-life reroof planning. When the membrane reaches mid-life, plan the reroof strategy: some arrays can be temporarily lifted (module by module, tub by tub) to allow reroofing underneath, then re-set. This is expensive but usually cheaper than deconstructing and rebuilding the array.
• Repowering at year 15–20. As modules age past 15 years, efficiency drops and warranty claims become harder. Repowering with modern modules on the existing ballasted racking is often the highest-return capital project.
• End-of-life recycling. Racking is 100% aluminum/steel and recyclable through any scrap metal buyer. Modules go to a certified PV recycler (SEIA maintains the current list).

Two specialized commercial roof types require modified ballasted approaches:

Green roofs (living vegetation on a growing medium over the membrane) can host solar arrays via elevated frame systems. The plants provide cooling and biodiversity; the array provides energy and afternoon shade. IronRidge BX and K2 D-Dome both offer green-roof-compatible tub variants that sit above the growing medium. PES quotes these as project-specific — send site details through the Axis portal for a quoted BOM.

Blue roofs (roofs designed for controlled stormwater retention) have similar tub-elevation considerations. The tub must clear the maximum retention depth (typically 4–6 inches) and the drainage plane must not be obstructed. Confirm with the stormwater engineer before finalizing the array layout.

Both cases benefit from a taller-profile ballast tub or a fixed-frame elevated rack — neither is standard catalog stock but both are available through manufacturer special-order in 4–6 week lead time.

Larger commercial ballasted arrays (typically 250 kW+) increasingly include roof-load monitoring — strain gauges or wireless load sensors that detect unexpected loading events. Snow-drift accumulation, ice buildup, and worst-case wind events can add substantial temporary load to a roof structure, and monitoring provides an early-warning signal to trigger snow-removal or wind-uplift response.

Typical DAS (data acquisition system) additions to a ballasted array:

• Strain gauges at key structural nodes (roof column tops, primary beam junctions) — usually installed by the structural engineer during the design phase.
• Wireless load cells under a representative sample of ballast tubs — detect changes in downward load caused by snow drift or ballast displacement.
• Weather station (wind speed, wind direction, temperature, snow depth) mounted on the roof or adjacent — provides real-time environmental context.
• Snow-depth sensor at the roof surface — critical for heavy-snow-region installs.
• Alarm and notification integrated with the site EMS or building automation — triggers on load thresholds or wind speeds.

This is not standard for residential or small-commercial ballasted work, but is increasingly common on 500 kW+ commercial rooftop portfolios and on any array over 100 kW installed on a building where the structural reserve capacity was tight at design time.

Coastal ballasted arrays face two compounded loading extremes: high V_ult wind speeds (140–180 mph in some Gulf and Atlantic zones) combined with Exp D roughness (1.6× the pressure of Exp C). The base ballast requirement can climb past 200 lb per module in corner zones — enough to make a purely-ballasted solution structurally impractical on most existing commercial roofs.

Practical strategies for coastal ballasted installs:

• Hybrid attachment. Combine ballast with mechanical anchoring in corner and edge zones. IronRidge and Unirac both offer edge-strap or penetrating-anchor accessories for this purpose. Interior zones stay purely ballasted.
• Perimeter wind deflectors. Non-negotiable in Exp D or high-V_ult. Cut the front-row uplift by 15–25%.
• Tightened array setback. Move the array further from the roof edge — 8–10 ft rather than the minimum 4 ft — to reduce corner-zone exposure.
• Reduced tilt. Consider 5° instead of 10° — lower sail area, significantly reduced uplift.
• Full ASCE 7-22 wind tunnel or CFD study. For extreme sites, commission a project-specific study to validate the design rather than relying on the OEM design tool's generic assumptions.

In HVHZ (High-Velocity Hurricane Zones — Miami-Dade and Broward Counties in Florida), local code adds additional requirements beyond ASCE 7-22. The Florida Building Code 2020 HVHZ chapters govern; a Florida-licensed structural engineer must review the design, and NOA (Notice of Acceptance) documentation is required for the specific racking system. IronRidge BX, Unirac RM10, and K2 D-Dome all have current NOAs; PES ships with the current NOA package on request for Florida installs.