How Can You Tell Where a Roof Leak Is Coming From?

The Stain Is the Exit, Not the Entrance

Water that gets past a covering lands on the underlay and runs down the slope, then onto timber, which carries it sideways. By the time it reaches a ceiling it can be metres from the defect.

So the stain never tells you where the water got in. What it does tell you is which category of fault you are looking for, and that narrows things a great deal.

One local qualification: on the older, unlined slate roofs common around Neston and Parkgate, there is often no underlay at all. Water has less far to travel, so the stain is a better guide than it is on a modern roof - though still not a precise one.

What the Position Tells You

Ceiling, in the middle of a room, upstairs. The covering or a penetration through it. This is the classic and it is the least informative, because the water has had the furthest to travel.

Ceiling, at the junction with an external wall. Usually the eaves - water going behind a gutter, a blocked gutter overflowing, or the underlay not carrying into the gutter properly. Often not the covering at all.

On a chimney breast. The stack: its flaunching, pointing, brickwork, lead or flue. And worth ruling out the two causes that are not leaks - soot salts and condensation in a sealed flue.

Around a roof window. Almost always the flashing kit rather than the window - though on a complex roof it can be water from higher up finding the first hole in the ceiling.

On a wall rather than a ceiling, as a vertical strip. Rainwater goods. A gutter joint, a downpipe, or water going behind the gutter.

Below a flat roof. The perimeter, an upstand or an outlet, in that order.

What the Shape and Behaviour Tells You

A round or oval stain with a defined edge is a drip landing in one place. Something above is releasing water at a point - a nail, a lap, a specific defect.

A spreading, irregular stain is water running along something and soaking in over a length. Usually a rafter or a joist carrying it.

A stain that has a dark, soft edge and feels damp is live.

A dry stain with a sharp brown tide line is historic. It may be a leak that was fixed, and it may be a leak that only happens in weather you have not had recently.

A stain that keeps growing outward with each rainfall in the same place is a persistent single source.

Several separate stains in different places is either a covering at the end of its life, or two independent faults, and those need distinguishing.

The Timing Question

Alongside position and shape, when it appears is the third piece of evidence and arguably the most useful.

In wind-driven rain from one direction: a detail on that face.

In prolonged heavy rain regardless of wind: drainage - a gutter, a valley, a back gutter, an outlet.

Immediately as it starts raining: a direct route with nothing slowing it.

Hours after the rain has stopped: water that has been absorbed and is working its way through - usually masonry, often a chimney.

In cold dry weather with no rain at all: not a leak. That is condensation, and the answer is ventilation rather than roofing.

What This Gets You

Not a diagnosis - the roof still has to be looked at, and the loft is where the real answer usually is.

What it gets you is a much better conversation. "Damp on the ceiling by the external wall in the back bedroom, only after long spells of rain, been there three winters" points somewhere specific before anybody arrives.

It also protects you. If somebody quotes for the tiles directly above a stain without asking any of these questions, they have priced a guess.