Flat roofs

Step 1: the structural check comes first

A ballasted array adds permanent dead load across the whole roof area, and it adds it on top of the snow load the roof was already designed for. The question for the structural engineer is not "can the roof take solar panels" but "what additional uniformly distributed and point load is available, in kg/m², after the existing design loads".

• Distributed load: a ballasted east-west system typically lands somewhere in the region of 10 to 25 kg/m² over the array footprint, depending on wind zone, building height and tilt. High tilt and exposed sites push it up quickly.
• Point loads: ballast is not spread evenly. Corner and edge rows carry the most, and lightweight decks such as trapezoidal steel are sensitive to concentrated loads over a single span.
• Snow acts at the same time as the array's dead load and drifts against rows and parapets. It does not simply replace the panel weight.
• If the reserve is too small, the options are a penetrating or bonded system, a lighter aerodynamic layout with deflectors, a lower tilt, or a smaller array. Adding slabs is not one of them.

Step 2: ballasted or penetrating

Both are legitimate. The roof, not preference, decides.

• Ballasted: no penetrations, so the membrane warranty is usually preserved and installation is fast and reversible. Needs load reserve and needs a friction check, because the system relies on weight and on the friction coefficient between mat and membrane to resist sliding.
• Penetrating: anchors into the deck or slab, so ballast drops to a fraction. Every anchor needs a sealed and flashed detail executed by the roofing contractor, and the pull-out capacity of the deck has to be tested or calculated.
• Bonded: feet adhered to the membrane with a compatible adhesive or welded patch. No holes and low weight, but it depends on membrane age, cleanliness and the adhesive being approved by the membrane manufacturer.
• Hybrid: a mostly ballasted field with mechanical anchors only in the corner and edge zones, where uplift peaks. Often the cheapest way to satisfy a marginal load reserve.

Step 3: wind uplift, edge zones and the ballast calculation

Wind actions are derived from EN 1991-1-4 with the national annex, using the site's basic wind velocity, terrain category, building height and roof geometry. The result is not one pressure across the roof. The code divides the roof into a middle field and much more heavily loaded strips along the perimeter, with the reference length e taken as the smaller of the building width and twice its height.

• Corner zones see the highest suction of all. Arrays are often simply kept out of them, which is cheaper than ballasting them.
• Edge zones along the parapets need extra ballast or anchors. A layout that ignores zoning and applies one average ballast figure everywhere is under-ballasted at the edges and over-loaded in the middle.
• Parapet setback: most system suppliers require a clear strip inside the parapet. It keeps the array out of the worst turbulence, gives maintenance access and keeps the drainage route open.
• Aerodynamic deflectors: a rear wind deflector closes the gap behind the module so airflow passes over the array instead of under it, converting uplift into downforce. It cuts required ballast substantially, at the cost of a larger surface catching snow and a larger effective area in the shading model.
• Sliding and friction: uplift is only half the check. Horizontal wind force must be resisted by friction between the protection mat and the membrane, which is why the mat material is part of the calculation and cannot be substituted freely.

Step 4: membrane compatibility and protection mats

The mounting system touches the waterproofing at every foot, permanently, under load, at temperatures that can swing from below freezing to well over 60 °C on a dark membrane in summer.

• PVC-P single ply: plasticisers migrate. Direct contact with bitumen, EPS insulation or some rubbers causes embrittlement or softening. Use a separation fleece or a mat the membrane manufacturer approves, and never let a bitumen-based pad touch it.
• EPDM: chemically stable but soft and easily indented by point loads. Wide-footprint mats spread the pressure. Avoid oil-based and bitumen-based products in contact with it.
• Bitumen: softens in heat, so a narrow foot can slowly imprint and eventually wear the cap sheet. Slip sheets plus wide bearing plates are the standard answer, and the roof surface temperature under a dark array runs higher than the open roof.
• Gravel and green roofs: ballast bears on the structural deck through the build-up, so the mat and base must sit on a proper bearing layer and not float on loose gravel or growing medium.
• Existing membrane condition: an old, shrunken or brittle sheet may need local reinforcement patches before anything is placed on it. This is the roofer's call, in writing.

Step 5: east-west or south, tilt and row spacing

The layout decision is a trade between peak yield and how much of the roof you can actually use.

• East-west: modules mounted back to back in a shallow A shape, typically at 10 to 15 degrees. Rows nest with little or no gap, so the roof carries far more kWp per square metre. Ballast is lower because the closed geometry reduces uplift, and the production curve is broader, with morning and afternoon peaks that suit self-consumption.
• South facing: higher yield per installed kWp, typically at 15 to 20 degrees on a flat roof rather than the optimum for latitude, because tilt drives both wind load and row spacing. Rows need a clear gap behind each one.
• Row spacing and self-shading: for a south layout, spacing is set so the rear row is not shaded at the design sun elevation, commonly taken around midwinter midday. Too tight and the bottom cell rows shade in winter, dragging down the whole string through its bypass diodes.
• Tilt is not free: every extra degree raises the projected area to the wind, so it raises ballast, and it widens the shadow, so it widens row spacing. On a load-limited roof the optimum tilt is usually lower than the yield model alone would suggest.
• Bifacial gain: a bifacial module over a light-coloured membrane collects reflected light on its rear side. Flat roofs are the best case for it, given adequate clearance under the module, a high-albedo surface and a mounting system whose rails and ballast do not shade the back. On a bright white membrane the extra yield is meaningful, on dark bitumen it is close to nothing.

Step 6: drainage, access and maintenance

A flat roof is a drainage system with a building underneath it. Ballast trays, base plates and deflectors sit directly in the water path, and anything that dams water creates ponding, which adds load exactly where the array already sits.

• Keep outlets and emergency overflows clear. No component, no ballast slab and no cable route may sit over or beside a drain in a way that restricts flow.
• Do not block the falls. Rows run so that water passes between and under the bases. Where a base crosses a fall, the system must leave a flow gap at deck level.
• Leaves and debris collect against deflectors and under the lowest module edge. Plan an access route for cleaning and for gutter and outlet inspection.
• Cable management: DC cabling is routed in trays or clipped to rails, never lying in the water path and never resting on the membrane where it can abrade it.
• Fall protection and access to the array for inverter and string checks has to be designed in, not improvised after handover.

East-west against south on a flat roof

Swipe the table to the left