A quarter of a century changes the question

Most aluminium structures are designed for a machine that will be modified, moved or replaced within a decade, and maintained in the meantime. A solar mounting structure has none of those escape routes. It is expected to stay outside, unattended, for a design life commonly taken as 25 years.

That reframes the specification. Decisions that would be marginal in a machine frame become consequential here, because there is no scheduled re-tightening, no repainting and no convenient replacement. A coating choice, a fastener material or a joint detail that would survive ten years of industrial service can fail well short of 25 years of weather.

It also means the specification has to be about degradation mechanisms rather than about initial strength. The mounting structure is rarely close to its load capacity. It is far more likely to be limited by corrosion, by loosening, or by the slow loss of a protective layer.

Wind: the governing case pulls up

The intuitive picture of wind loading is pressure pushing down on a panel. The governing case is usually the opposite.

A tilted panel behaves like an aerofoil in a flowing airstream: flow accelerates over the upper surface, pressure there drops, and the result is a net force that lifts. Uplift is frequently larger in magnitude than the downward pressure case, and it acts on a structure whose resistance comes from its own weight and its anchorage rather than from the ground beneath it.

That has two consequences for an aluminium structure. The first is that the anchorage governs: the fixing into the roof or the ground has to resist a sustained, reversing, cyclic pull-out load for 25 years. The second is that fatigue enters, because the load cycles with every gust. A structure sized only for the static downward case will be under-designed for the case that actually occurs.

The earth path is not automatic

This is the detail that catches aluminium mounting structures out, and it is a direct consequence of the finish that makes aluminium suitable for outdoor use.

Panel frames and the mounting structure are required to be at the same electrical potential, and the array has to be bonded to earth. With steel, that happens largely by itself: bolted steel-to-steel contact conducts. With anodised aluminium, it does not. An anodic oxide layer is an electrical insulator, and two anodised parts bolted together are separated by a non-conducting film on both sides.

The result is a structure that looks continuous and is not. Bonding has to be deliberate: a toothed or serrated washer that cuts through the layer at the joint, a designated bonding point with the coating removed, or a separate bonding conductor. Which method is used has to be stated, and it has to be one that still works after 25 years of weathering.

Galvanic corrosion: the couple you create by choosing stainless

The natural instinct for a 25-year outdoor structure is to specify stainless fasteners. That is usually right, and it creates a problem that has to be managed.

Aluminium and stainless steel are far apart in the galvanic series. Join them electrically, add rainwater — which is a perfectly good electrolyte — and the aluminium corrodes to protect the stainless. The bolt stays bright and the hole around it disappears, which is the worst kind of failure because it is invisible until something moves.

The controls are standard and they have to be specified rather than assumed: isolate the two metals where practical, keep the cathodic area small relative to the anodic one, exclude water from the joint, and specify a coating on the aluminium. Where isolation is used, remember the conflict with the previous section: an isolation washer helps corrosion and breaks the earth path, so the bonding has to be provided separately.

Thermal movement along a long row

Aluminium expands measurably with temperature, and a solar array is a structure that sees the full range: cold nights, hot days, and direct sun on dark surfaces that raises the metal well above air temperature.

A short array can absorb this. A long row cannot. Over tens of metres the total movement is substantial, and if the structure is restrained at both ends it will either bow or transfer the load into the anchorage. Long rows need provision for the movement, either through expansion joints in the rail run or through fixings that permit sliding.

The same reasoning applies to the anchorage spacing. Fixing points that are too close together over-constrain the run; too far apart and the rail lacks support. The arrangement follows from the movement range as well as from the load, and it is worth stating on the drawing so it is not decided on the roof.

What to send with a solar mounting enquiry

Solar enquiries are unusually specific, because the load depends entirely on the site. Six items, and the ones most often missing are the site data and the bonding method.