Two ways to keep an array on a flat roof: weigh it down, or bolt it down. The choice is usually made on structural grounds, but it has consequences for warranty, maintenance and risk that outlast the calculation.
Concrete blocks or trays hold the array in place through mass and friction. No holes in the membrane.
The appeal: the roof stays intact, roofing warranties usually stay valid, installation is fast, and the array can be relocated or removed with little trace. On leased buildings this often decides it.
The constraint: weight. Ballasted arrays typically add 15-25 kg/m² distributed, with higher point loads at perimeters and corners where wind uplift is worst. Older industrial roofs frequently do not have that spare capacity, and finding out requires a structural engineer rather than an assumption.
Wind is the design driver. Perimeter and corner zones experience substantially higher uplift than the field of the roof, so ballast is not uniform – edges get significantly more. Any layout showing evenly distributed ballast across the whole roof has skipped the zone analysis.
Mechanical fixings through the membrane into the structural deck or purlins, with flashed and sealed penetrations.
The appeal: far lower added weight, typically 8-12 kg/m². Higher wind resistance. Often the only viable option where structural capacity is tight, and generally preferred in high-wind or hurricane-exposed regions.
The constraint: every penetration is a potential leak, and there may be several hundred of them. Done properly – correct flashing, compatible materials, competent workmanship – they last as long as the roof. Done carelessly, you will find out during the first serious storm, and by then the array sits on top of the problem.
Have it before you choose, not after. Most commercial membrane warranties contain provisions about penetrations and about attaching anything to the roof surface. Some require the original roofing contractor to perform or supervise any penetration work. Some are voided outright by third-party attachment.
Practical sequence: get the warranty document, read the relevant clause, contact the manufacturer or roofer in writing, and get their requirements before the mounting system is specified. This routinely adds cost and a few weeks. Discovering it after installation adds considerably more.
A solar array has a twenty-five year design life. A single-ply membrane typically runs twenty to thirty years. If the roof is already fifteen years old, you are installing a long-lived asset on top of a covering that will need replacing well inside the array's life.
Removing and reinstalling an array to replace a roof is expensive – frequently a substantial fraction of the original installation cost. If the roof has under ten years left, the honest options are to replace it first or to accept that cost in your model. Ballasted systems are cheaper to lift and refit, which is a genuine point in their favour on older buildings.
Many commercial installations end up mixed: ballast across the field of the roof, mechanical fixings at perimeter and corner zones where uplift demands it. This keeps penetration count low while satisfying wind requirements, and it is often the pragmatic answer.
If a bidder treats these as formalities to be produced after contract award, that is informative. The structural answer can change the design fundamentally, which means it should come before the price, not after.