Structural expert opinion

What it answers

• How much reserve capacity does the existing structure have?
• Can it be built on, and if so by how much?
• Can it be altered: can a wall come down, can an opening be cut?
• What causes an existing defect — a crack, movement, damp?

Why it is the first step

Because it says what is achievable at all. Without it the design rests on assumption, and the assumption collapses at the strip-out.

For a rooftop addition especially: the reserve capacity of an existing building is tight, and it is often exactly what decides that light gauge steel is an option. Rooftop additions in light gauge steel.

What has to be surveyed

The old drawings are not enough — it may not have been built that way, there may have been undocumented alterations, and the condition is not revealed by a drawing.

• geometry: exact dimensions, floor levels, grid lines,
• structural system: what carries load, over what span,
• materials and condition,
• foundations: this often requires trial pits,
• existing services, if the extension connects to them.

Surveying an existing building.

What the opinion does not solve

It does not remove the uncertainty, only reduces it. However thorough the survey, risk always remains with an existing building — so we record in the contract what happens if something unexpected emerges during the strip-out.

What follows

If the opinion says there is reserve, design can start. If not, two routes remain: strengthening, or a lighter structure.

This is the point where self-weight decides — and where light gauge steel is strongest. Self-weight and imposed load.

The point

For an existing building the survey is the most important input to the design. What is not surveyed is an assumption — and you should not build on one.