Existing building drainage surveys: what a good investigation should establish

In most existing buildings the drainage drawings, where they exist at all, record an intention rather than the installation. Routes are altered during construction, later fit-outs add connections, and repairs replace sections without any record being updated.

Designing a refurbishment on the assumption that the drawings are correct is one of the most common routes to late redesign. Before an existing system is reused, extended or relied on for a new use, its route, levels and condition need to be established on site.

A survey specified as 'CCTV all drainage' produces a lot of footage and not necessarily the answers. The better starting point is a short list of questions the design cannot proceed without.

CCTV is very good at showing the internal condition of a pipe where the camera can reach: cracks, displaced joints, root ingress, deposits, deformation and connections entering the run. Condition grading gives a consistent way to record what was seen.

It is not a complete picture. A camera cannot always pass tight bends, traps, blockages, small-bore pipes or changes in material. Standard CCTV does not in itself establish the capacity of a system, the precise depth of a pipe without separate level measurement, or the condition of the pipe exterior and surrounding ground. Where the survey could not reach, the report should say so, and the design should treat those sections as unconfirmed rather than assumed good.

Sonde tracing, level surveys at chambers, dye testing and targeted intrusive opening-up each fill specific gaps. A good investigation combines these deliberately rather than relying on a single technique.

In occupied, operational and heritage buildings the limit on a survey is rarely the equipment. It is access: chambers under fixed finishes, stacks hidden in boxing, runs beneath slabs, and short windows outside trading or performance hours.

Planning access in advance, agreeing what can be lifted or opened, and accepting where a phased investigation is needed will usually produce better information than a single rushed visit.

The output should be a record drawing that the design team can use: routes plotted against the building, invert and cover levels at chambers, pipe sizes and materials, and a clear distinction between what was confirmed on site and what is inferred.

A condition summary should be written for decision-making, not only as a list of defect codes. It should say which sections can be reused, which need repair or replacement, and which remain uncertain.

Drainage investigations affect the architect's layouts, the structural engineer's slab and foundation work, and the contractor's programme. Sharing findings early, particularly where routes clash with proposed structure or levels rule out a gravity connection, avoids those issues surfacing at construction stage.

The purpose of the survey is to make design decisions with evidence. Confirmed levels determine whether new connections can fall by gravity or need pumping. Condition findings decide reuse against replacement. Unresolved sections become explicit risks with a proposed route to closing them out, such as further investigation during enabling works.

When the investigation is scoped by the engineer who will use the results, the survey becomes part of the design rather than a separate report on the shelf.