Load system
Load system mounting is the ballasted, non-penetrating way to hold a photovoltaic array on a flat roof: dead weight resists wind uplift, so nothing is drilled through the waterproofing. Ballast mass follows from the wind uplift calculation and is capped by the permitted roof load, with protection mats under every contact point. Built up from rail profiles, ballast blocks or trays and roof accessories. Guide
Related flat roof categories
Load systems: ballast, wind uplift and the permitted roof load
A ballasted structure holds the array down with mass instead of anchors, so two numbers decide the design: the wind uplift on the tilted modules and the permitted additional roof load the building can carry. Uplift follows from the wind zone, building height, terrain category and the position of each module in the roof plane, and is answered with concrete ballast blocks or gravel trays placed where the calculation asks. The permitted load, in kg/m² and confirmed by a structural engineer, is the ceiling that plan must stay under, and a protection mat between every foot and the PVC-P, EPDM or bitumen membrane is part of the specification, not an accessory.
FROM THE FIELD
"The mistake I see most often is ballast spread evenly across the roof. Uplift is not uniform: the edge and corner zones carry several times the suction of the field area, so those rows need extra blocks while the middle often needs fewer. Follow the zone plan, not the eye."
Ladislav Proc · Mounting systems specialist, ONSA Plus
How to choose a ballasted load system
Work in this order: confirm the permitted roof load and the membrane type, run the wind uplift calculation for the actual site, then pick the layout and tilt that keep the resulting ballast inside that limit. Choosing hardware first and calculating afterwards is how projects end up redesigned on site.
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Ballast, zones and the load limit
Ballast is sized per position, not per project. The calculation splits the roof into a field zone, an edge zone and corner zones, and gives each position the weight it needs under the design wind. Concrete blocks give a compact, predictable mass, while gravel trays spread the load and suit a tighter kg/m² allowance.
- Permitted roof load: the hard limit. Get it in writing from the structural assessment before selecting anything, and remember snow load shares the same reserve.
- Edge and corner zones: set the array back from the parapet where possible. Setback reduces the suction the outer rows see and the ballast they need.
- Aerodynamic profiles: wind deflectors and closed rear panels turn the module row into a shape that presses down rather than lifts, cutting ballast on exposed roofs.
- Protection mats: a mat under every foot and ballast block prevents point loading, abrasion and, on PVC-P, incompatible contact.
Tip from practice
Check the membrane before the ballast. A PVC-P sheet and untreated polystyrene or bare bitumen do not tolerate direct contact, and the roofing warranty usually names the separation layer required. Ask the roofer which mat is approved, then order it with the roof accessories.
East-west or south facing
Layout decides how much roof each kilowatt occupies and how much ballast the array needs. A south layout sets every row at one tilt and needs spacing to avoid mutual shading. An east-west layout pairs modules back to back in a shallow zigzag, so rows shelter each other aerodynamically and the roof is used densely.
| Property | East-west layout | South layout |
|---|---|---|
| Typical tilt | Shallow, roughly 10 to 15° | Steeper, roughly 15 to 30° |
| Roof area used | Dense, little spacing between pairs | Wider spacing to avoid shading |
| Ballast needed | Lower, rows shelter each other | Higher, each row meets the wind |
| Output profile | Morning and evening peaks, flatter curve | Higher midday peak, more per module |
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Frequently asked questions about load systems
Does a ballasted load system penetrate the roof?
No. A load system is held down by weight alone, so the waterproofing is never drilled and the roofing warranty stays intact. The only contact with the membrane is through protection mats under the feet and ballast.
How much ballast does a flat roof array need?
It comes from the wind uplift calculation for that specific site, not from a rule of thumb. Wind zone, building height, terrain, parapet setback and tilt all change the figure, and edge and corner positions need considerably more than the field area.
What if the roof cannot carry the ballast?
Reduce the load rather than ignore the limit. A shallower tilt, an east-west layout, deflectors and a setback from the edges all cut the weight needed. If it still exceeds the limit, a penetrating or bonded fixing on flat roofs is the alternative.
Why do edges and corners need more ballast?
Wind separating over the parapet creates strong local suction along the perimeter and especially at the corners. Those zones can see several times the uplift of the field area, so the calculation assigns them extra ballast or the array is set back.
Do I need protection mats under the system?
Yes, in practice always. Mats spread point loads, stop abrasion as the structure moves with temperature, and separate the membrane from incompatible materials. Match the mat to the membrane, since PVC-P, EPDM and bitumen differ.
Do aerodynamic profiles really reduce the ballast?
Yes. A closed rear panel or wind deflector reshapes the airflow so the row is pressed down instead of lifted, lowering the uplift the ballast must counter. The saving is largest on tall, exposed buildings and is quantified in the wind calculation.
Ballast quantity and placement follow a wind load calculation and the permitted additional load of the specific roof.
About ONSA Plus
Why installers across Europe order ballasted systems from us
ONSA Plus is a European B2B distributor of photovoltaics, battery storage, heat pumps and EV charging stations. In our own EU warehouses we keep over 1,500 products from 25+ brands, including Huawei, Solis, SolaX and Dyness.
We are not just a trading company. We design and service energy systems ourselves, so we answer questions on ballast calculation and membrane protection from engineering practice, not from a catalogue. You handle a warranty claim directly with us and you buy per job.
