Alectrona
Commercial mountingOn a flat commercial roof the default method is a non-penetrative ballasted frame, held down by weight rather than fixings, so the array goes on without drilling through the waterproof layer.
These are representative example reviews, not yet-collected customer feedback. They are written to illustrate the kind of feedback Alectrona aims to earn and are shown as design placeholders while we gather and verify reviews from our first commercial clients. Alectrona is the commercial solar trading brand of RVTC LTD.
What set Alectrona apart was the documented design pack. We had quotes from three installers, but only Alectrona handed us a full set of drawings, a single-line diagram and a design referencing BS 7671 and the G99 connection process. The whole thing read like an engineering submission rather than a sales brochure. Our M&E consultant reviewed it and signed it off without a single query. That gave the board the confidence to release the capital.
Other firms priced our roof off a satellite image and a desktop guess. Alectrona flew an in-house drone survey, fully insured and flown by a qualified commercial drone pilot, and built a 3D model of the actual roof. It picked up plant, vents and a parapet line that a flat aerial photo had completely missed, which changed the panel layout. I would rather find that out at design stage than on the day the scaffold goes up. The accuracy of that survey is the reason I trusted everything that followed.
As a finance director I was wary of being oversold a system bigger than we could use. Alectrona modelled the array against our actual half-hourly consumption data rather than an annual total, so it is sized to what we genuinely draw on site during the day. They were honest that exporting surplus is worth far less than self-consumption, and built the design around that. The capital case stacked up because the engineering was honest, not because the numbers were inflated.
We were undecided between buying outright, leasing and a PPA. Alectrona laid out all three side by side with the pros and cons of each against our balance sheet, instead of pushing the one that pays them best. They were clear about where a PPA makes sense and where capex wins, and pointed us at our own accountant for the tax treatment. The survey and design took a little longer than I expected, but the thoroughness was worth the wait. Genuinely consultative.
The install crew were tidy and well run, and worked to a clear CDM 2015 plan with a proper site induction and RAMS. What impressed me most was the handover. We received a full commissioning pack with the IEC 62446-1 test results, certification, O&M documentation and an as-built record for our maintenance team. As the people who have to live with this asset for the next twenty years, having that paperwork in order matters enormously. Nothing was left loose.
I expected the usual hard sell and got the opposite. After surveying our site Alectrona told us one roof section was not worth covering because of shading, and that a smaller, well-sited array was the better investment than filling every square metre. There was no commission-driven upselling and no pressure. For a six-figure capital project, that straight talk is exactly what you want from the people advising you. We will be using them again on our second site.
Most large industrial and commercial buildings have a flat or near-flat roof finished with a single-ply membrane, felt or asphalt. The method that suits this roof in most cases is ballasted mounting. The frames sit on the roof and are held in place by weight, usually concrete blocks or pavers, rather than by screws or bolts through the covering. Nothing penetrates the waterproof layer, which is the line that keeps the roof and its warranty intact.
This page explains the engineering behind that choice for a building over 50 kWp, which sits outside the MCS scheme and is assured instead through component standards, a structural assessment, the G99 grid connection and CDM 2015 site management. The amount of ballast, the layout and whether the roof can carry the array are not figures we can give from a desktop. They come from the structural survey and the wind-load calculation for your building, and the yield comes from a PV*SOL model of your roof.
Non-penetrative mounting where the roof allows.
Ballasted mounting is a non-penetrative method. The frame rests on the roof surface, often on protective mats that spread the load and guard the membrane, and it is held against wind and movement by added weight. Because nothing is fixed through the covering, there are no new holes to seal and no breaches in the waterproofing for the building owner or a landlord to worry about.
That is the reason it is the usual choice on a single-ply membrane or felt roof. The alternative on a flat roof is a penetrative fixing through the deck, which can be the right answer where ballast weight is a problem, but it puts holes through the waterproof layer and brings the membrane warranty into question. On a metal roof the choice is different again: a standing-seam roof takes a clamp that grips the raised seam with no penetration at all, and a trapezoidal profile takes a sealed mechanical fixing. The right method follows the roof, and we confirm which applies after the survey.
The weight on a ballasted system is not a fixed number and it is not chosen for comfort. Wind is the governing force. A low-profile array on an open roof still acts as a surface the wind pushes and lifts, and the uplift is strongest at the roof edges and corners. The ballast has to hold the frame down against that uplift across the whole roof, which is why a reputable design follows a wind-load calculation to BS EN 1991-1-4, the Eurocode wind standard, with the figures set by your building height, exposure, parapet and roof zone. Many manufacturers back this with wind-tunnel testing of the specific frame.
The structure has to carry the result. A flat roof was designed for its own loads plus snow and access, and the array plus its ballast is an added distributed and point load on top. A structural engineer checks whether the existing deck and frame can take it, where the load can sit, and whether any strengthening is needed. We do not quote a load per square metre as a universal fact, because the safe figure is specific to your roof. It is confirmed by the structural survey and the wind-load calculation for the building, not assumed.
A flat roof gives a choice of array geometry that a pitched roof does not. South-facing rows tilt every module towards the sun for the highest output per panel, but the rows have to be spaced apart so they do not shade each other, which limits how many modules the roof holds. An east-west layout sets the modules back to back in low-tilt pairs, one side facing each way. It fits more capacity on the same area and flattens the generation across the day, with a morning and an afternoon shoulder rather than a single midday peak, which often suits a building that uses power across working hours.
East-west also tends to need less ballast per module, because the lower tilt presents less area to the wind. Neither layout is the right answer everywhere. The trade-off between peak yield, total capacity, ballast weight and how the generation matches your demand is modelled for your roof in PV*SOL, and we design to what the building actually needs.
The module technologies in the commercial market each have a place. PERC is the older p-type cell and is now largely a legacy choice. TOPCon is the n-type cell that most current large-format commercial modules use, with good efficiency and temperature behaviour. HJT is another n-type design that can give strong yield and low-temperature losses. None of these is the best in every case. The right cell depends on the roof area, the budget and the performance the model shows for your site, and we select on bankability and fit rather than a single headline number.
Bifacial modules can add output on a flat roof because they collect light reflected off the roof surface behind the array. The size of that gain depends on the reflectance, or albedo, of the surface and on the row spacing and tilt. A light, reflective membrane returns more light to the rear of the module than a dark one, so the uplift is real on the right roof and small on the wrong one. We model the bifacial gain for your specific roof and surface in PV*SOL rather than apply a marketing percentage, and we specify bifacial only where the model shows it pays.
A ballasted design depends on the roof under it, so the survey reads the roof itself before any layout is drawn. The membrane is checked for type, age and condition, because a covering near the end of its service life is the wrong base for a twenty-five year array that would have to come off to replace it. The falls and drainage are mapped, since a flat roof is never truly level and the array must not sit where water collects. The position of outlets, rooflights, plant, AC units and access hatches is recorded, because the frame has to leave them clear and reachable. The parapet height and roof zones are measured for the wind calculation, and the deck build-up is established for the structural assessment.
This is also where access is worked out. A roof reached only by an internal hatch sets a hard limit on how concrete ballast and frame components arrive, and a crane or hoist lift has its own siting and road-closure questions. None of this is desktop work, and it is why we do not price a ballasted array from a postcode and a roof area. The survey and the structural assessment turn the roof into a real design, and the yield comes from the roof-space and PV*SOL model for your building.
A non-penetrative array still has to connect to the building, and the DC cabling, the inverter feeds and any monitoring run have to leave the roof. The discipline that keeps a ballasted job non-penetrative is to use the openings the roof already has, routing cable through an existing service penetration or a purpose-made weathered curb rather than drilling fresh holes through the membrane. Where a new penetration is genuinely unavoidable, it is detailed and sealed by a roofing trade to the membrane maker's specification so the waterproofing and its warranty are preserved.
On the roof surface the cable is carried in trays or trunking set on the same protective bearers as the frame, kept off the membrane so it cannot abrade the covering or trap water. The layout also leaves maintenance walkways between rows and clear routes to every roof outlet, rooflight and plant item, so future access for the building does not mean lifting panels. These details feed the grid connection too, since the cable route ends at the inverter and the metering that the G99 connection to Northern Powergrid governs.
A ballasted system is held by weight and friction, so its failure modes are different from a fixed array, and a sound design is one that answers each of them. The first is uplift: if the ballast is under-specified for the wind zone, the frame lifts at the edges and corners where suction is strongest, which is the case the BS EN 1991-1-4 calculation exists to prevent. The second is sliding or creep, where thermal movement and repeated wind loading walk the array across the roof over time unless the friction and any mechanical interconnection between rows are designed for it. The third is concentrated point loading, where ballast blocks bear too hard on a single spot and indent or stress the membrane, which the protective mats and load-spreading bearers guard against.
Two more are about water. Ponding happens where the frame or its feet obstruct the roof's falls and water backs up, so the layout has to respect the drainage the survey mapped. Scour and debris build-up under a low array can block outlets, which is why maintenance access and a clear path to every drain are part of the design rather than an afterthought. A planned operations and maintenance regime keeps these in check across the array's life.
Ballast is the usual choice on a flat roof, but it is not the right one on every flat roof, and an honest design says so. The clearest case is a roof that cannot carry the added weight: where the structural assessment shows the deck has little residual capacity, the ballast a wind-safe design would need may exceed what the building can take, and a lightweight penetrative fixing into the structure becomes the better engineering even though it puts sealed holes through the covering. A roof with a worn or short-life membrane is another, because committing a long-life array to a covering due for replacement stores up a costly future removal.
Steep falls, an obstructed or heavily plant-laden roof, or a very exposed high-rise position where the required ballast becomes impractical can all push the answer away from a simple ballasted system. This is the same logic that sends a pitched trapezoidal metal roof down a fixed route entirely, since weight alone holds nothing on a slope. We compare the methods against your survey rather than fit one by default, and the wider trade-offs sit on the commercial mounting overview.
No. A ballasted mount is non-penetrative. The frame sits on the roof surface and is held down by weight, usually concrete blocks, so nothing is fixed through the waterproof layer. That is the main reason it suits a single-ply membrane or felt roof and keeps the membrane warranty intact.
There is no single answer, because wind is the governing force and it varies with the building. The weight is set by a wind-load calculation to BS EN 1991-1-4 using your roof height, exposure, parapet and the edge and corner zones where uplift is strongest. We give the figure for your roof from that calculation, not as a universal number.
That is what the structural survey establishes. A structural engineer checks whether the existing deck and frame can take the array plus its ballast as an added load, where that load can sit, and whether any strengthening is needed. We confirm it for your building before designing the system, rather than assume a load per square metre.
It depends on what the building needs. South-facing rows give the highest output per panel but have to be spaced to avoid shading, so they fit less capacity. East-west low-tilt pairs fit more capacity, spread the generation across the day and often need less ballast. We model both for your roof in PV*SOL and design to your demand pattern.
Sometimes. Bifacial modules collect light reflected off the roof behind the array, so the gain depends on the surface reflectance and the row spacing. A light, reflective membrane returns more light than a dark one. We model the gain for your specific roof in PV*SOL and specify bifacial only where the figures show it pays, rather than apply a marketing percentage.
It depends on the size of the array and the roof, so we give a programme after the survey rather than a fixed figure. The survey and structural assessment come first, then design, the G99 connection application to Northern Powergrid and procurement, and only then the roof works. The installation itself is often quicker than a penetrative job because there is no drilling and sealing of the deck, but craning concrete ballast and frame to roof level, and any restricted internal access, can be the longer part. We set out the realistic stages and lead time for your building once the survey has read the roof.
There is no single figure, because a ballasted design is survey-led. The cost turns on how much ballast the wind-load calculation to BS EN 1991-1-4 calls for, the array size, the layout chosen, and whether the structural survey finds the roof needs strengthening or the membrane needs replacing first. Craning concrete ballast and frame to roof level and any restricted access shape the labour too. We price it against your structural survey and PV*SOL model rather than a per-kWp rate, and the way commercial pricing is built up is explained on our commercial solar cost guide and finance pages. As a system over 50 kWp this sits outside the domestic schemes, so the figure is engineered for your specific building rather than taken from a fixed price list.
Tell us about the roof or the site. We survey it, confirm the structure, then specify the mounting system that fits, with no penetrations where the roof allows.