Surveying an existing building

Why the old drawings are not enough

For three reasons:

1. It may not have been built the way the drawing shows.
2. Later alterations may have been made, undocumented.
3. The condition — cracks, damp, corrosion — is not revealed by a drawing.

So the old drawing is a starting point, not evidence.

What has to be surveyed

Geometry

Exact dimensions, floor levels, openings, grid lines. The basis of all further design.

Structural system

What carries load: walls, columns, type of floors, spans, directions. It determines where load can be applied.

Materials and condition

Type of masonry, floor construction, condition of existing steel or timber members.

Foundations

Critical for an extension or a rooftop addition: what foundation there is, how deep, with how much reserve capacity. This often requires trial pits.

Existing services and electrical installation

If the extension connects to them.

The special case of a rooftop addition

Here the reserve capacity of the existing structure decides: how much additional load the foundations and the vertical structure can take.

This is the situation where light gauge steel is at its strongest: the low self-weight is often precisely what the existing building can still take. Rooftop additions.

But even for that you have to know what the reserve is — you cannot build on an assumption.

How to survey

• topographic and building survey for the geometry,
• non-destructive testing where possible,
• targeted opening up where it is unavoidable — foundations, floor construction,
• material testing where the capacity is in question,
• and recording the survey, because it becomes the basis of the design.

Managing the risk

However thorough the survey, uncertainty always remains with an existing building. So the contract has to state what happens if something unexpected emerges during the strip-out.

The point

For an existing building the survey is the most important input to the design. What is not surveyed is an assumption — and a design built on assumptions collapses at the strip-out.