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Plan & Takeoffs

What to check, and in what order

There is a water stain on my ceiling

Water runs along things before it drips, so the leak is often not above the stain. The shape and the timing narrow it fast: a ring that grows after rain points outside, a stain that ignores the weather points at plumbing, and a stain that appears in cold weather only may be condensation rather than a leak at all.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

A ceiling stain marks where water LEFT the structure, not where it entered. Water arriving at a rafter, a pipe or a duct runs along it — sometimes for metres — before finding a low point to drip from, so the first instinct, to look directly above the stain, is often looking in the wrong place.

Two observations narrow it more than anything else. The TIMING: does it follow rain, follow use of a bathroom or appliance above, or ignore both? And the SHAPE: a defined ring with a darker edge is a repeated wetting and drying cycle, a diffuse patch with no edge is usually slow or condensation-related, and a stain that runs in a line follows something the water was travelling along.

The third question is whether it is still active. A dried historic stain from a leak fixed years ago looks much like a current one, and the distinction decides whether anything needs doing. Marking the edge in pencil with the date is the cheapest test available and it settles the question within a few weeks.

The one urgent case is a ceiling that is bulging, sagging or heavy — that is water held above a saturated ceiling, and it is a falling hazard with real weight behind it.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. A roof covering or flashing defect

    Most common

    Appears or grows after rain, especially wind-driven rain from one direction, and is often near a chimney, a valley, a roof junction or a penetration rather than in open roof.

    How to check: Note whether it tracks the weather, then look in the loft with a torch during or just after rain to find where the water is actually arriving.

  2. A plumbing leak above

    Most common

    Independent of the weather and often tied to use — worse after a bath or shower, or continuous and slowly growing. Sometimes the water is warm.

    How to check: Whether it changes with rain or with use. A stain that appears on a dry week after somebody showers identifies itself.

  3. Condensation in the roof space

    Common

    Worst in cold weather, diffuse rather than a defined ring, often widespread or in several places, with damp or mouldy timber and cold-side surfaces in the loft.

    How to check: Look in the loft on a cold morning. Droplets on the underside of the roof covering, rather than a single wet path, is condensation.

  4. A blocked or overflowing gutter or valley

    Common

    Near an external wall or an eaves line, strongly tied to heavy rain, often with staining visible outside on the wall below.

    How to check: Watch the gutters during heavy rain. Water spilling behind a gutter rather than over its front goes straight into the eaves.

  5. A leak somewhere else, travelling

    Common

    Nothing above the stain accounts for it, and the loft is dry directly above but wet somewhere uphill along a rafter, a pipe or a duct run.

    How to check: In the loft, trace UPHILL from the wet point rather than looking above the stain. Water runs along timber and pipework before it drips.

  6. An overflow or a failed seal on a tank or appliance

    Less common

    Intermittent and unrelated to weather, sometimes with a large volume at once rather than a slow growth.

    How to check: Whether there is a tank, a cylinder or an appliance above. A one-off large stain that then stops points at an overflow event.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It comes first because it is the only answer that changes what you do in the next hour rather than the next week. Water held above a saturated ceiling is a falling hazard.

  2. 2

    It is the only reliable way to tell an active leak from a historic one, and the two look identical on the day. Everything after this is cheaper if you know which you have.

  3. 3

    This single observation splits the causes into three groups — outside, plumbing, condensation — and eliminates two of them.

  4. 4

    It shows where water is arriving rather than where it is leaving, which are usually different places. Looking directly above the stain is the commonest wasted search.

  5. 5

    Water runs along rafters, pipes and ducts before dripping, so the entry point is up-slope of the wet timber, not at it.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Attic and Loft Ventilation Calculator (Intake, Exhaust and Vent Count) →Gutter Calculator →Vapor Barrier Dew-Point Condensation Risk Calculator →

When to stop and get somebody in

  • Immediately, if the ceiling is bulging, sagging or feels heavy — that is standing water above a weakened ceiling, and it can come down as a single mass.
  • Where the stain is anywhere near a light fitting, a downlight or a ceiling rose, since water and electrical accessories in the same place is a hazard regardless of the leak's source.
  • Where the leak is on a roof that cannot be inspected safely from inside the loft, or where the covering itself has failed rather than a flashing.
  • Where the roof timbers in the affected area are soft to a probe, discoloured or carry fungal growth, which turns a leak into a structural question.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.