Screw piles vs concrete footings: solar carport foundations

The foundation is the least glamorous part of a solar carport and, more often than not, the part that decides your programme. The steel, the panels and the inverters are broadly the same job on any site. What varies, sometimes dramatically, is what sits under the tarmac and how you get load into it without shutting the whole car park for a fortnight. Get the foundation choice right and the works barely disrupt trading. Get it wrong, or let an installer guess before anyone has looked underground, and you inherit delays, spoil heaps and change-order costs that were entirely avoidable.

There are three practical options for a solar carport installation: helical screw piles, concrete footings, and ballasted (weighted) foundations. This guide explains where each belongs, and how a proper ground survey decides between them rather than a salesperson’s habit.

Why the foundation decides your programme

A solar carport is a canopy. Unlike a roof array, it has to carry its own wind, snow and imposed loads down through columns into the ground, and the governing case is usually uplift rather than downward weight. A large canopy behaves like a wing in a gale, so the foundation has to resist the structure trying to lift and rotate, not just push down.

That structural demand meets a commercial constraint: the ground it goes into is a live, revenue-generating car park. Every option below is really a trade between how well it handles your specific ground and how much of the car park it takes out of use, for how long.

Helical screw piles: the default for roughly 90% of sites

Helical screw piles are steel shafts with one or more helical plates near the tip. A hydraulic torque head on a compact rig winds them into the ground like an oversized screw. The installed torque is monitored in real time, which gives a direct, verifiable measure of load capacity as each pile goes in. No trench, no pour, no cure.

For the majority of open, level car parks on reasonable ground, this is the sensible default, and it is why a large share of solar carport installations end up pile-founded.

Why installers reach for them first

• No concrete, no cure time. You are not waiting days for footings to gain strength before steel can go up. The pile is load-bearing the moment it hits its torque figure.
• All-weather. There is no wet pour to protect from frost or rain, so the programme is far less exposed to a bad-weather week.
• Minimal spoil. A pile displaces ground rather than excavating it, so there is very little muck to cart away and dispose of. That alone can remove a line of lorry movements from a busy site.
• Fast. On clean ground a rig can install a run of piles quickly, keeping the whole build tight.
• Reversible. At end of life they can be unwound and the ground reinstated, which matters on leased sites and helps the sustainability case.

What it means for a live car park

Because there is no excavation or curing, screw-pile works can usually proceed bay by bay. You cordon a section, install, move on, and hand bays back progressively instead of losing the whole car park for the duration. Kerbs, drainage and surfacing stay largely intact. For a retailer, hospital or office estate that cannot simply close its parking, that phasing is often the deciding advantage.

Concrete footings: when the ground forces your hand

Concrete footings, whether mass/pad bases or piled-and-capped foundations, are the right answer when the ground rules screw piles out. The common triggers are:

• Buried services. You cannot drive a pile through a gas main, an HV cable, a foul sewer or a shared drainage run. Where columns have to land near known services, an engineered concrete base spreads load without penetrating deep.
• Membranes and structural decks. Over a podium deck, a basement, a car park built above another use, or any waterproof tanking, driving a pile would breach the membrane. A footing (or ballast) that sits on the slab protects the waterproofing.
• Contaminated or heavily made ground. On former industrial land, capped fill or ground full of obstructions, pile driving becomes unpredictable, refusals are common, and disturbing contamination carries its own liabilities. A designed footing is more controllable.
• Obstructions and shallow rock. Where buried slabs, old foundations or rock sit just below the surface, screwing to depth may not be viable.

The trade-off is real. Concrete means excavation, spoil to remove, formwork, a pour and a cure period before steel can follow. That occupies the affected bays for longer and adds lorry movements. It is the correct tool for the situations above, but it is a slower, more disruptive one, which is exactly why it should be reserved for ground that genuinely needs it rather than used as a default.

Ballasted (weighted) foundations: the niche third option

Ballasted foundations use precast concrete blocks or kentledge to hold the structure down with mass alone, without penetrating the ground at all. They suit podium decks, capped landfill, or membranes that must not be breached under any circumstances.

They are the exception, not the rule. Ballast is bulky, heavy and eats into the parking footprint, and the deck or slab below has to be verified to carry the concentrated point loads. On a normal open car park a ballasted solution is usually the wrong economics. It earns its place only where nothing can be driven or dug.

How a ground survey and trial holes decide

None of the above should be chosen from a photograph. The foundation schedule comes out of a proper investigation:

1. Desk study and utility survey. Records and a GPR/CAT scan locate buried services and known obstructions before anything goes near them.
2. Trial holes and trial pits. Physically opening the ground in a few positions shows the real make-up: surfacing depth, sub-base, fill, water table.
3. Geotechnical testing. Probe tests, standard penetration tests or a test pile establish how the ground behaves and, for piles, the torque-to-capacity relationship.

The output is a foundation design, and on a single car park it is often mixed: screw piles across the clean majority, concrete footings over the run of services along one edge. This survey step is the one cowboys skip. If a quote names a single foundation type before anyone has looked under the tarmac, treat that number as a guess, and expect it to move once reality intervenes. Different property types bring different ground risks too, which is part of why we set out the picture by sector on our verticals pages.

Engineered to the loads, not to a rule of thumb

Whichever foundation is used, it is designed to the actual loads under BS EN 1991 (Eurocode 1), which sets the wind, snow and imposed actions the canopy must resist. For a carport, wind uplift typically governs, and the pile embedment or footing size follows from that calculation, verified by a structural engineer, not from “we always use this size”. A credible solar carport installer will show you a stamped foundation schedule tied to the survey data. If the design cannot be traced back to loads and ground tests, it has not really been engineered.

Cost and timeline trade-offs

Foundations and steel dominate the capital cost of a carport; the mounting steel alone is commonly around 40 to 50 per cent of the build. Screw piles are usually the cheaper and faster route once installed, because you avoid excavation, muck-away and cure time. Concrete footings add all three, which is part of why they are reserved for ground that requires them.

For context on the whole system, a solar carport at scale tends to land around £1,100 to £1,600 per kWp, roughly £6,000 to £12,000 per space, and that is before VAT; commercial buyers should always work from ex-VAT figures because the 20 per cent VAT is generally recoverable. The foundation choice moves the groundworks line within that, not the panel or inverter cost. Capital allowances (solar is special-rate plant, so the annual investment allowance plus a 50 per cent first-year allowance for companies may apply) can change the net picture, but take your own tax advice on your position. Our cost breakdown sets the numbers out in full.

Where foundations sit in the wider programme

Foundations are rarely the long pole in the tent. The Distribution Network Operator usually is. Anything above 3.68kW per phase, which covers essentially all commercial carports, needs G99 pre-approval before energisation, and that process often runs 8 to 16 weeks. Start it early and the groundworks choice becomes a scheduling detail rather than a bottleneck.

Planning is generally lighter than people expect. In England, since 21 December 2023, most non-domestic solar canopies fall under Class OA prior approval rather than full planning, with the council able to consider siting, design and glare, determined within 56 days. Notably, sustainable drainage (SuDS) for run-off is a mandatory condition, so how water leaves the site is part of the job regardless, which ties back to the ground survey. This is a standing permitted-development right with a three-year window to complete, not a limited offer, so there is no reason to rush the groundworks investigation to beat a deadline. Wales and Northern Ireland still generally require full planning case by case. If you are adding EV charging under the canopy, the Workplace Charging Scheme contributes £500 per socket toward the infrastructure; our grants and funding guide covers the current rules and limits.

The short version: let the survey pick the foundation, insist on a load-based design, and phase the works so the car park keeps earning while it happens.

If you want an honest read on which foundation your site actually needs, tell us about your car park and we will start with the ground, not the sales pitch.