Research · Paper 09 · August 2026
When load is added at roof level, attention goes to the roof. But the load does not stop there. It travels down through the frame to the ground, and on multi-storey and altered stock the governing element is frequently somewhere along that path rather than at the top of it.
Load added at roof level is carried by every element beneath it, down to the foundations.
On single-storey shedding the path is short. On multi-storey stock it is not.
A roof assessed in isolation answers a question that stops one storey too early.
01 · The principle
Anything placed on a roof imposes a reaction at its supports, which imposes a reaction at theirs, and so on to the foundations. Each element in that chain has its own capacity and its own remaining margin.
Covering, purlins, rafters or portal frame, columns, bases. The whole path can normally be recorded in one visit and the governing element is usually near the top, which is why the shorthand of assessing the roof works reasonably well on this stock.
A converted mill or a multi-storey former industrial building carries roof load down through floors, walls and columns that have themselves been altered, penetrated and loaded over a long life. The roof may have ample capacity while an element three storeys below does not, and nothing at roof level reveals it.
By the time an array is proposed, the foundations are carrying everything added since construction. That total is rarely recorded anywhere, and it is the one element that cannot be inspected without excavation.
02 · Where it goes wrong
Altering a floor to form a stair, a lift shaft or a services riser changes how load distributes to the elements around it. Where several such alterations have been made at different dates, the distribution is not what any original drawing shows.
Holes cut through beams and walls to route services are common on converted stock and are frequently undocumented. Their effect depends entirely on position, which means they cannot be assessed as a category.
Plant or an array concentrated over one bay imposes its reaction on a specific set of members and, below them, on a specific line down the building. A load that is trivial when averaged across a roof plane may not be trivial where it actually lands.
03 · What to require
On single-storey shedding this may add little. On multi-storey stock it is the difference between an answer and half an answer, and it is much cheaper to include at instruction than to revisit.
A report should identify the governing element and its margin. If the answer is a purlin, that is useful. If the answer is a column at second floor, that is considerably more useful, and it will not have been found by looking at the roof.
Foundations are almost never inspected. Concealed elements frequently are not. Both may be reasonable to accept, but they should be recorded as accepted rather than passed over in silence.
04 · Basis and limitations
05 · Citation
Suggested citation
Raihan, S. (2026) The Roof Is Not Usually the Constraint: Load Paths in Existing Buildings. Solar Surveys Research, Paper 05. Solar Surveys Ltd. Available at: https://solarsurveys.co.uk/research/the-roof-is-not-usually-the-constraint
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