Research · Paper 11 · August 2026
On a ballasted flat roof the array is held down by weight rather than by penetrating fixings. That weight is not a product specification to be looked up. It is an output of a calculation about the specific roof, and where it lands on the structure matters as much as how much of it there is.
Ballast resists uplift. Its quantity follows from the wind demand at that site, not from a catalogue.
It is permanent load the deck must carry, in the position it sits.
Two identical arrays on two identical roofs in different places require different ballast.
01 · The two questions
A ballasted system creates a requirement that pulls in opposite directions, and both directions have to be satisfied simultaneously on the same roof.
How much restraint does the array need? This follows from peak velocity pressure at the site, derived under BS EN 1991-1-4 and its UK National Annex with modification for terrain, altitude and distance to shoreline, and from the array-specific coefficients in BRE Digest 489. A more exposed site needs more ballast for the same array.
Can the deck carry it? Ballast is permanent dead load, and it is concentrated where the ballast trays sit rather than spread across the roof plane. The check is not whether the roof can carry the average, but whether it can carry it where it actually is.
An exposed site needs more ballast; a weak deck can carry less. Where those two collide the answer is not more ballast, and it is not less. It is usually a different layout, a different mounting approach, or a mechanically fixed system.
Figure 1
Ballast is not a uniform layer
02 · What goes wrong
A quantity that worked on a previous roof is applied to this one. It has no validity here, because the wind demand and the deck are both different. This is the most common error and the least visible in the paperwork.
Uplift is highest at edges and corners and lower in the field of the roof. A uniform arrangement is simultaneously too light where it matters and unnecessarily heavy where it does not, which loads the deck without buying restraint.
An assessment cannot be completed without knowing the ballast arrangement, because that arrangement is a substantial part of the load being assessed. Where a proposal arrives without one, the appraisal stalls after the site visit rather than before it.
03 · What to require
Position and mass, tray by tray or zone by zone. The product datasheet does not contain this, because it is a property of the design for this roof rather than of the product.
Stated, so the basis of the ballast quantity is on the record and can be checked. A ballast figure without the derivation behind it cannot be verified by anyone.
Where the load lands, assessed against the structure beneath rather than averaged over the roof area. Averaging is what makes a ballasted array look lighter than it is.
How Solar Surveys approaches this
Ballast is the one load in a rooftop array that is entirely within the designer control and most often reaches the structural engineer as a single averaged figure. Solar Surveys asks for the real distribution, because the average is not what the deck carries.
Wind suction is highest at corners and edges, so that is where ballast concentrates. A single kilogram-per-square-metre figure describes a roof nobody is proposing to build and hides the peaks that matter.
Corner, edge and interior zones are assessed against the ballast intended for each, so the check reflects the loading pattern the array will actually impose on the deck and the members beneath it.
Where the distribution does not work, saying so while the layout can still change is far cheaper than discovering it once the ballast is on the roof. We would rather have that conversation early.
An averaged ballast figure describes a roof nobody is building. The deck carries the peaks, not the mean.
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04 · Basis and limitations
05 · Citation
Suggested citation
Raihan, S. (2026) Ballast Is a Designed Load, Not a Delivered Quantity. Solar Surveys Research, Paper 05. Solar Surveys Ltd. Available at: https://solarsurveys.co.uk/research/ballast-is-a-designed-load
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