How to find a roof leak, and why it is never where the drip is

The single most useful thing to understand about a roof leak is this: the wet patch on your ceiling is almost never underneath the hole.

Water comes in through the roof covering, lands on the felt or on a rafter, and then runs. It runs along the underside of the felt, down a rafter, across a ceiling joist, and it drips off at the first place gravity gives it a reason to. That can easily be three or four metres from where it got in, and on a house with a valley or a purlin it can be further.

Which is why “the leak is above the landing” is not information a roofer can act on, and why somebody who goes straight to the point above your damp patch and starts replacing tiles is guessing.

Where leaks actually start

In rough order of how often I find them:

1. Flashings. The lead where the roof meets something else: a chimney, a wall, an extension roof running into the house. This is the biggest single category by a distance. Lead lifts out of the mortar joint it was tucked into, or splits along a fold where it has fatigued, and water gets straight in behind it.

2. Valleys. Where two roof slopes meet. Valleys carry a lot of water, they collect debris, and on the Victorian housing round here they are frequently original lead that has done well over a century.

3. Around chimneys generally. Not just the flashing but the flaunching on top and the pointing in the stack itself. Water can enter at the top of a stack and come out well below it.

4. Slipped or cracked slates and tiles. The obvious one, and less common than people expect. One slipped slate does cause leaks, but it is not the usual suspect.

5. Roof windows. Almost always the flashing kit rather than the window. Fitted with mastic instead of the correct kit for the tile type, which works for two years.

6. Where a lower roof meets a wall. The back addition on a terrace, or an extension. This is the abutment, and it is the same flashing failure as number one in a different location.

7. Ridge and hip tiles. Bedding mortar cracks and falls out, and water gets under the ridge.

Notice how many of those are junctions rather than the roof surface itself. Roofs mostly do not leak in the middle. They leak where they meet something.

How to trace it, from inside

You will learn more from twenty minutes in the loft than from an hour looking at the roof from the pavement.

Go up during or just after heavy rain, in daylight if you can, and take a torch anyway.

Then work backwards from the wet:

1. Find where it is dripping into the ceiling. That is your starting point, not your answer.
2. Look up and follow the water back. Look for a wet track on the felt, on a rafter, on a purlin. Water leaves a visible route.
3. Follow it uphill. Water only goes down. Whatever is wet, the source is higher up the slope than that.
4. Keep going until the wet stops. The highest point of wetness is where it is getting in, or very close to it.
5. Look for daylight. Torch off, eyes adjusted, look for pinpricks of light. Any daylight is a hole.
6. Check the junctions specifically. Where the roof meets the chimney, where a valley runs, where the extension roof meets the wall. Look at those even if the wet track does not obviously lead there.

If the felt is intact you may not see the entry point at all, because the water is running on top of the felt and only coming through where the felt is torn or lapped. In that case the highest wet point on the felt is still the best clue you will get.

Take photographs. Of everything. It saves a lot of time on the phone and it means whoever comes out is looking for a specific thing.

The clues from the pattern

When it leaks tells you a lot.

• Only in driving rain from one direction points to a flashing, an abutment, or a wall detail rather than the roof surface. Wind is pushing water somewhere gravity would not take it.
• Every time it rains, proportional to how hard is usually a genuine hole in the covering.
• Only after prolonged rain suggests something that has to saturate first, often a chimney stack or a solid wall.
• In winter and not in summer, regardless of rain, is condensation, not a leak. That is a different problem with a different fix.
• Just after a storm, suddenly, having been fine is nearly always a slipped or lifted slate.

Where it appears tells you the rest.

• Damp on a chimney breast, upstairs or in the loft: flashing or the stack, almost always
• Damp on the inside of an outside wall: could be the roofline, the gutter, or the wall itself rather than the roof
• A mark in the middle of a ceiling well away from any wall: covering or felt
• Water near the top of a wall on an upper floor: often the gutter overflowing rather than the roof at all

The things people mistake for roof leaks

Worth ruling these out before anybody goes up a ladder:

A blocked or overflowing gutter. Water running over the back of a gutter goes straight down the wall and can appear inside. Look at your gutters during heavy rain. If water is coming over the edge, that is your problem and it costs almost nothing to fix.

Condensation. Winter damp in a loft that is dry in summer.

A plumbing leak. A cold water tank in the loft, or a pipe in a ceiling void. It does not care whether it is raining, which is the giveaway.

Penetrating damp through a solid wall. Very common on the older Leeds housing, which is solid stone or brick with no cavity. A saturated wall in a wet spell can show internally in a way that looks like a roof leak.

A failed pointing joint on a parapet or a coping stone. Water enters the top of a wall and travels down inside it.

What makes it urgent, and what does not

Ring somebody today if: water is near a light fitting, a socket or the consumer unit; a ceiling is bulging or sagging with held water; there is a visible hole in the roof and more rain coming; or slates are on the ground and more are loose above a path or a car.

It can wait for a dry day if: it is a stain that appears slowly during rain and dries out afterwards, with no active dripping, and nothing electrical near it.

Most leaks are the second kind, and there is no reason to pay an emergency callout premium for a two hour job on a dry morning. Anyone who tells you a slow stain is an emergency is selling urgency.

What a roofer should do

When somebody comes out for a leak, they should go into the loft as well as onto the roof. A roofer who only looks from outside is working with half the evidence, and the half that matters most is the water track on the inside.

They should also tell you what they found, ideally with photographs. You cannot see your own roof, and taking somebody’s word for what is up there is how people end up paying for work nobody did.

The one thing worth doing before winter

Clear your gutters. Every autumn, after the leaves have come down.

More of what looks like roof leaks in this city is actually gutter overflow than any other single cause, and clearing them is the cheapest maintenance there is. It costs a fraction of a leak investigation and it prevents a category of problem entirely.