The short answer: By working from the evidence inward: the marks inside first, then the roof itself, then water tests on one section at a time to reproduce the leak, moisture readings to map the wet area, and where needed a small test cut to see the layers. The mark inside seldom sits under the fault, because water travels.

Why is the stain often not under the leak?

Because water that gets through the covering rarely drops straight down. It runs along the boarding under the covering, the deck, follows the joists or soaks sideways through the insulation, and only appears inside where it finds a joint, a light fitting or the lowest point of a ceiling. On a block, that can mean the stain is in one flat and the fault sits over the next.

That is why patching the roof directly above the mark so often fails. Finding the leak means tracing the path back from where the water shows to where it gets in, and treating the stain as the end of that path rather than the start.

Where do flat roofs usually let water in?

At the joints and edges far more often than in the open field of the covering. The places an investigation looks at first are these:

  • Upstands and abutments: where the covering rises against a wall, parapet or neighbouring building, and the flashing or trim that closes its top edge.
  • Outlets: the roof's rainwater drains, where a blocked or badly joined outlet lets water back up and in.
  • Rooflights: the kerb each one sits on and the seal between the covering and the frame.
  • Pipes, vents and fixings: anything that passes through the roof, each one a seal that has been weathering since it went in.
  • Laps and seams: the joints between sheets, which can open as the material ages or moves.
  • Parapet copings: the cap along the top of a parapet wall, where water can get into the wall and run down behind the upstand.

Some leaks are not the roof at all. Water can come through a wall or a window, from plumbing in the flat above, or from moisture inside the building condensing on a cold ceiling, and each of those needs a different fix. If the marks come in cold, dry spells rather than with rain, read whether it is condensation rather than a leak first.

How is the source traced, step by step?

In a fixed order, from what is already visible to what has to be uncovered, so that each step narrows the search for the next.

  1. The evidence inside. The marks are looked at and photographed: where they are, how far they spread, whether they are fresh or old, and what sits above them.
  2. The roof itself. A visual inspection of the covering and every detail above and around the marks, again photographed, looking for the places listed above.
  3. A water test. One section of the roof at a time is wetted or flooded while someone watches inside, so that the leak is reproduced and tied to a particular area rather than guessed at.
  4. Moisture readings. A moisture meter maps where the build-up under the covering is wet, which shows how far the water has spread as well as where it is concentrated.
  5. A test cut. Where the readings leave a question, a small cut is opened to see the layers and what they hold, then made good afterwards.

Not every leak needs every step. A failed seal round a pipe can be plain to see from the roof, and the later steps are there for the leaks that hide.

What do you get at the end?

The cause and the repair: the point where water entered, why, and what putting it right involves, with the photographs that show it. For most leaks that is a repair to one detail or one area, priced in writing.

Sometimes the readings tell a bigger story. Where wet insulation turns out to be widespread, mending the entry point no longer finishes the job, because the build-up underneath has failed, and the decision moves to the refurbishment options for the whole roof.

What helps the investigation along?

Information gathered before the visit, most of which only the people in the building have. It is worth collecting:

  • When it shows: in heavy rain, in wind-driven rain from one direction, or only after days of rain.
  • Where it shows inside: which flat or unit, which room and which part of the ceiling or wall.
  • Photographs: the marks, ideally with dates, and the roof if it can be seen safely from a window.
  • Access: the way onto the roof, and whether the flat or unit below can be opened on the day.
  • The history: earlier repairs, who carried them out, and any guarantee papers for the covering.

For a managing agent, access to the space below is often the hardest part to arrange, so raise it early. How that fits with the rest of looking after a block's roofs is set out for property and block managers.

What does it cost?

If the cause is plain to see, an inspection of the roof and a written figure for putting it right are free. Where it has to be traced, the water test, the readings and any test cut make a chargeable visit. Not every leak needs one, and we decide from the first look or the photographs; when one does, we say so first, agree its fee before the visit is booked, and it counts towards the repair if that goes ahead. If you also need the findings written up as a report, for a freeholder, a board of directors or an insurer, that report is a survey, with a fee of its own agreed the same way. Once the cause is known, the repair is priced in writing before anything starts.