Ballasted vs mechanically-fixed solar mounting on a flat roof

The single biggest decision on a flat-roof solar project is not the panel brand — it is how the array is held down. There are two established methods, and the choice between them determines whether your roof warranty survives, whether the deck is overloaded, and how the array behaves in a storm. Here is how ballasted and mechanically-fixed mounting compare.

Ballasted, penetration-free mounting

A ballasted system holds the array in place with weight. An aluminium frame carries the panels at a shallow tilt and sits on the membrane on protective slip-sheets, held down by concrete ballast blocks placed on the frame. Nothing is drilled or bolted through the roof.

The advantages are decisive on most commercial roofs. Because there are no penetrations, there is nothing to leak and nothing to void the membrane manufacturer’s waterproofing guarantee — the roof stays exactly as watertight as it was. Installation is faster because there is no drilling and sealing of hundreds of fixings, and the whole thing can, in principle, be removed without trace.

The limitation is weight. Ballast is load, and a ballasted array typically adds 15 to 25 kilograms per square metre — more on an exposed, high-wind roof that needs heavier ballast at the perimeter and corners. The deck has to be able to carry it.

Mechanically-fixed mounting

A mechanically-fixed system bolts the frame through the membrane and into the structure — the purlins of a metal deck, or the concrete. The ballast weight is largely removed, so the dead load on the deck is much lower.

The advantage is that it suits decks that cannot carry ballast, and it resists wind uplift through the fixing rather than through weight, which can matter in very high wind zones.

The limitation is the penetrations. Every fixing is a hole through the waterproofing, so each one has to be detailed and sealed to the membrane manufacturer’s own specification by an approved method — and doing that correctly is what keeps the waterproofing guarantee valid. Done badly, a mechanically-fixed array is a future leak. This is why it should never be treated as the cheap default; it is a considered choice for a specific roof.

How to choose

Factor	Points to ballasted	Points to mechanically-fixed
Deck load capacity	Ample residual capacity	Marginal or weak deck
Membrane	Live single-ply/felt guarantee to protect	Manufacturer approves sealed fixings
Wind zone	Standard exposure	Very high / exposed site
Priority	No penetrations, fastest install	Lowest dead load on the structure

In practice, the large majority of commercial flat roofs — warehouses, industrial units, offices on modern decks — take a ballasted array, because the deck can carry the weight and keeping the membrane intact is worth a great deal. Mechanically-fixed and low-ballast hybrid systems come into their own on weaker decks and in extreme wind zones.

The honest answer on any specific roof comes from two checks that happen before any design: a wind-uplift calculation to BS EN 1991-1-4, which sets the ballast weight and distribution, and a structural residual-capacity assessment, which confirms what the deck can actually carry. A specialist runs both and lets them decide the method. A generalist quotes the same racking for every roof — which is exactly how roof warranties get voided and decks get overloaded.

If you want a straight answer for your building, send us the roof details and we will tell you which method it needs, and why.