Is it a rain leak or a plumbing leak?
A roofer sent to a burst pipe finds nothing, and a plumber sent to a slipped tile finds nothing either. Ten minutes of watching sorts it out.
Rain leaks track the weather and plumbing leaks do not. Keep a dated note for a week of when it rained and when the patch grew. If it only grows after rain, ring a roofer. If it grows through a dry week, ring a plumber. The water meter settles most of what is left.
The short version
- A rain leak follows the rain, often a few hours behind it, and stops once the weather does. A plumbing leak carries on whatever the sky is doing.
- Rain through a roof picks up dust and tannin from the timber above, so it stains brown or yellow. A clean water supply leak often runs clear and marks far less.
- Turn off the stop tap, then read the water meter with everything off and again half an hour later. Neither check needs anybody on the roof.
- Water that turns up after a shower, a bath, a wash cycle or a flush is plumbing or waste, not weather.
- The mark is rarely under the source. Rain water and pipe water both run along timber before they drop.
Start here
Calling the wrong trade costs you a visit and tells you nothing
Calling the wrong trade to a leak costs you a visit and tells you nothing, which is the whole reason for this page. A roofer called out to a plumbing leak walks the roof, finds a dry covering and charges for the visit.
A plumber called out to a rain leak pressure-tests sound pipework and does the same in reverse. Neither has done anything wrong and you are no further forward. Ten minutes of watching first usually settles which one you need, and every test below is one you can do indoors with a pencil and a torch.
Water and electricity first. If water is anywhere near a ceiling rose, a downlighter or the consumer unit, switch that circuit off at the consumer unit, not the wall switch, and get an electrician before it goes back on. Water tracks along cables and comes out somewhere other than where it went in. Do not put a hand into a wet ceiling void. In the loft, stay on the joists with a torch: the plasterboard between them will not hold you, and work at height is not a thing to improvise. Nobody needs to go on the roof to answer this question.
The strongest test
Does it track the weather?
Whether a leak tracks the weather is the single test that decides most cases, and it needs a week and a biro. Rain that gets past a roof has to soak through, run along something and reach the ceiling, so the mark often appears hours or a day after the downpour, not during it. Judge the pattern, not one drip. Rain leaks go quiet in settled weather. Plumbing leaks do not.
Keep a dated log for a week
Three columns on the back of an envelope: the date, whether it rained and how hard, and whether the patch grew. Draw round the mark in pencil first, date it and photograph it. Reading the stain itself has its own page. All you need here is the log.
- It grows in the day or two after heavy rain. Water is arriving from outside: a roof, a flashing, a gutter or a wall.
- It grows steadily through a dry week. The water is already in the house and nothing on the roof will fix it.
- It grows on some rainy days and not others. Note the wind as well as the rain. Water blown hard at one elevation gets past details that shed vertical rain perfectly well, which is water coming in only during driving rain, and it is still weather.
One caveat: a dry fortnight proves nothing if there was no real downpour in it.
The free checks
Turn off the stop tap, then watch the meter
Turning off the stop tap and reading the water meter take half an hour between them and settle a live supply leak outright.
The stop tap
The internal stop tap is usually under the kitchen sink or wherever the main enters the house. Turn it off and watch the drip for an hour or two. If it slows, the water is coming from the pressurised side of your plumbing. Rain does not stop because you turned a tap.
The meter test
Turn off every tap and appliance, the washing machine, the dishwasher and the outside tap included. Read the water meter and write the number down. Use no water for half an hour, then read it again. If it has moved you have a live supply leak and no roof work will touch it. If it has not, a whole category is ruled out.
The heating gauge
A sealed heating system whose pressure gauge keeps dropping and needs topping up is losing water somewhere, often a pipe under a floor or in a ceiling void. That is a heating engineer, not a roofer.
The sequential use test
Neither check catches a waste leak, because waste pipes carry no pressure and turn no meter. So on a dry day run one thing at a time with a gap between each and watch the patch. Fill the bath and empty it. Run the shower for ten minutes with the screen closed. Flush the WC several times. A shower tray or a failed seal shows on the shower and not the bath fill, and a waste leak shows on the empty rather than the fill. Government guidance names these faults: traumatic damp, caused by leaking water from waste and heating pipes, overflowing baths or sinks, burst pipes or defective water storage vessels.
Reading the water
Clean or dirty, warm or cold, and what it smells like
What the water looks like narrows it down, though it never settles it alone. Rain that has come through a roof crosses dust, old soot and bare timber and dissolves tannins out of the wood, which is why a rain leak so often leaves a brown or yellow mark with a hard tide line that bleeds back through fresh emulsion.
Mains water from a burst or a weeping joint starts clean and often leaves little more than a grey damp shadow, so a ceiling visibly wet without being visibly stained is worth taking seriously as a plumbing leak. Clean water crossing a filthy void picks up colour too, so brown does not prove rain. Clear is the more useful of the two.
Two more readings. Touch the wet area with the back of your hand, then clean ceiling a metre away: warmer points at a hot water or heating pipe, and nothing outside the house is warm. Then smell it. Rain and mains water have no smell, and anything sour or drainy is a waste pipe and a plumber's job.
Position
Where it is, in three dimensions
Position is evidence, as long as you remember that water moves sideways before it drops. Under a bathroom, a kitchen, an airing cupboard, a tank or a known pipe run points one way.
Under a valley, a chimney breast, an abutment, a rooflight or a soil vent pipe points the other, and so does a mark hugging the edge of the ceiling against an outside wall, which is often a gutter overflowing onto the wall head and not the roof at all.
The trap is assuming the source is directly above the mark. Water landing on a rafter, an underlay or a joist runs downhill until something stops it, so the entry point sits uphill and often several feet sideways. That is as true of a weeping compression joint as of a slipped tile. A damp patch on a chimney breast has its own set of causes, and half of them are neither.
| What you check | Points at rain | Points at plumbing |
|---|---|---|
| Weather over a fortnight | Grows within a day of heavy rain, quiet in settled weather | Grows through a dry week |
| Colour of the water | Brown or yellow, hard tide line, bleeds back through emulsion | Often clear or grey, wet without much staining |
| Timing against use | No relation to baths, showers or the washing machine | Follows a shower, a bath emptying, a wash cycle or a flush |
| Meter with everything off | Does not move | Moves, which proves a live supply leak |
| Position in the house | Valley, chimney, abutment, rooflight, vent pipe, ceiling edge | Under a bathroom, kitchen, tank or pipe run |
| Temperature and smell | Cold, no smell beyond musty | Warm means hot water. Sour or drainy means waste |
The awkward ones
Three cases that break the rules
Condensation on cold pipework in a loft
A cold mains pipe in a humid loft sweats, drips onto whatever is below and looks exactly like a pinhole leak. It is neither rain nor plumbing. Wipe the pipe dry, wrap kitchen roll round it and come back in an hour: wet evenly all round is condensation, wet at one point with a run below is a leak.
The same reasoning covers the whole roof void, and BRE puts it plainly, that dripping from roofs into the rooms below is often caused by a build-up of condensation in the cold roof voids, rather than by leaks which may initially be suspected. If the loft is wet all over rather than in one track, read condensation or a roof leak before you price any roofing.
An overflow discharging into the loft
This is plumbing that behaves like weather. A storage tank or a cistern with a sticking ball valve runs at unpredictable intervals with no relation to anything you do, and it can put a lot of water into a ceiling quickly. Look at the tank and at the overflow pipe outside: a stained wall under an overflow, or a tank whose level sits at the top of the pipe, is your answer.
Rain that only shows in driving rain
A leak that needs wind behind the rain can go a month without showing, then fill a ceiling in one squall. Because it skips ordinary wet days people read it as intermittent and hunt for something they did indoors. Record wind direction alongside rainfall and the pattern appears. BRE's warning holds throughout: dampness is very often misdiagnosed, and a remedy bought on a wrong diagnosis leaves the real defect running.
Next
What to do next, in order
- Isolate the circuit if the water is near a light fitting, and keep out from under a bulging or spongy ceiling.
- Pencil round the mark, date it and photograph it.
- Read the water meter with everything off, wait half an hour and read it again.
- Keep the log for a week: date, rain, wind direction, whether it grew.
- On a dry day, run the use test one fixture at a time.
If it points at plumbing, this is not a roofing job and we will tell you so. You want a plumber. Anything involving a gas appliance needs the trade registered for it, because nobody may work on a gas fitting unless they are competent and a member of a class of persons approved by the Health and Safety Executive.
Wet electrics are for an electrician, and fixed electrical work in a home sits under Approved Document P. None of those is us, and ringing them first will be quicker.
If the log points at the weather, that is roofing work, and it is far more often one junction, one lap or one cracked tile than a covering at the end of its life. Thomas comes out and looks himself, goes into the loft, follows the water uphill to where it is getting in and puts in writing what he found. The site visit and the written itemised quote are both free. You can ask for a quote or send a photograph first.
Whichever trade it is, leave the redecorating until the ceiling has dried out. A ceiling skimmed while still holding water cracks and stains through, and soft plasterboard is a plastering repair that follows the fix.
Questions
Common questions
How do I tell a roof leak from a plumbing leak?
By whether it tracks the weather. Draw round the mark in pencil, date it, and keep a week of notes on rain and growth. A rain leak grows within a day of heavy rain and goes quiet in settled weather. A plumbing leak grows through a dry week and takes no notice of the forecast.
What is the water meter test for a leak?
Turn off every tap and appliance, including the washing machine, the dishwasher and the outside tap. Read the meter and write the number down. Use no water for half an hour, then read it again. If the reading has moved you have a live supply leak, and no roof work will fix it.
Can a roof leak appear a day after it has rained?
Yes. Water that gets past a covering has to soak through, run along a rafter or the underlay and reach the ceiling, so the mark often shows hours or a day after the rain, not during it. Judge the pattern over a fortnight instead of timing any single drip.
Why is the water clear rather than brown?
Rain coming through a roof crosses dust, soot and bare timber and dissolves tannins on the way, which is what makes the classic brown mark. Mains water starts clean, so a supply leak often leaves a faint grey shadow with little colour. Clear water is a reason to look at pipework first.
Do I need a plumber or a roofer for a leak after a shower?
A plumber: water that appears after a shower, a bath emptying, a wash cycle or a flush is coming from the plumbing or the waste, and it will do the same on a dry day. Run one fixture at a time with a gap between each and watch the patch to work out which one it is.
Read next
Related advice
The guides below pick up where this one leaves off, and the roofing page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Ruled out the plumbing and want the roof looked at?
Thomas comes out himself, goes into the loft, follows the water back to where it is getting in and puts in writing what he found. If it turns out to be a pipe, he will say so and you can ring a plumber.