Where is the water coming from?

Patterns

What each pattern often points to

Where the water shows and the weather it arrives in are the two clues worth having before anyone looks at the roof. Each pattern below links the guide that explains it.

• Damp on cold, still mornings that stays away through a mild wet week. This often points to condensation rather than a leak: moisture from the rooms below settling on the coldest layer of the roof. It tends to get worse once the heating comes on, or after showers and cooking, and it feeds mould along cold edges and corners. How to tell condensation from a leak.

  What gets looked at

  • Outside: the covering, its upstands and outlets, and anything that passes through it.
  • Inside: whether the marks match a path water could actually follow.
  • Moisture readings inside the roof, and its ventilation where it has some.

• A stain on a ceiling under the flat roof that grows while it rains. This often points to the covering itself: a split, a lap that has opened, or a blister that has burst. The mark is rarely right under the fault, because water runs along the deck before it drops. The signs of a failing flat roof.

  What gets looked at

  • The whole covering, not only the area above the stain.
  • Splits, open laps and burst blisters, and old patches that have parted.
  • Moisture readings, to tell whether the layers under the covering are wet.

• Water that keeps coming after the rain has stopped. This often points to one of two things. Water held on the roof behind an outlet (the roof's rainwater drain) that is slow or blocked rises until it gets in at a detail. Or water that got in during the rain has travelled along the deck and is still finding its way down, a fair way from where it got in. Why water stays on a flat roof and why the stain is rarely under the fault.

  What gets looked at

  • Each outlet, and whether it drains freely.
  • Where water stands on the roof, if it does, and why.
  • The path from the stain back to the detail above it.

• Water that comes in only when the rain is heavy or blown by the wind. This often points to the details and edges rather than the open roof: rain driven by the wind gets under an edge or trim that has lifted, past an upstand or flashing that has cracked, or over a kerb built too low. Why the weather it leaks in matters and what failed upstands and flashings look like.

  What gets looked at

  • Upstands and flashings along walls and parapets.
  • The edge trims and the drip beneath them, and whether any edge has lifted.
  • The kerbs round rooflights and anything else raised above the roof.

• Damp beside a wall, parapet or chimney that rises above the roof. This often points to the upstand, where the waterproofing turns up the wall, or the flashing over its top edge. Movement between the materials cracks this detailing before anything else, and a good many leaks blamed on the roof as a whole begin here. What cracked upstands and flashings look like.

  What gets looked at

  • The top edge of each flashing, for hairline gaps.
  • The mortar joint the flashing is tucked into.
  • Whether the waterproofing has come away from the wall.

• Drips round a rooflight. In rain, this often points to the kerb the rooflight sits on: a kerb built too low, or waterproofing that stops short of the frame. In still, cold weather, a cold frame collects condensation that marks the ceiling in much the same way. Why rooflights are the awkward case.

  What gets looked at

  • The height of the kerb the rooflight sits on.
  • Whether the waterproofing climbs the kerb and closes under the frame.
  • The corners, where three planes meet.

• Water by a door that opens onto a terrace or balcony. This often points to the threshold, the junction of the door sill and the surface outside: too little fall away from the door, or waterproofing that was never carried up behind the frame. A blocked outlet outside makes it worse. Where a balcony lets water in.

  What gets looked at

  • The drop from the door sill down to the surface outside.
  • The outlets, and whether water actually leaves through them.
  • Any fixing through the deck, and where the surface meets the walls.

• Water round a pipe, a vent or anything else through the roof. This often points to the detail round it. Each one was sealed when it went in and that seal has been weathering since; a stain also often leads back towards things of this kind. Why every fixing through the waterproofing matters and what points to a leak.

  What gets looked at

  • The seal round each pipe, vent or fixing through the covering.
  • The covering immediately round it, for splits or lifting.
  • The marks inside, to see whether they line up with it.

• Drips inside a garage. A garage seldom has a ceiling or insulation for water to run along, so the drip usually sits near its cause. This often points to an edge that has lifted, a gutter spilling back, the junction with a wall, or splits in old felt. Where a garage lets the rain in.

  What gets looked at

  • The edges, the gutter and the junction with the wall.
  • The boards from inside the garage as well as the covering from above.
  • Which covering the roof has, before any fix is discussed.

• Water you cannot place. That is normal, and it often points to an entry some way from the mark: water travels along the deck or the joists before it drops, so the stain marks the lowest point it reached rather than where it got in. Patching the stain is how people end up paying twice. Why chasing the stain fails.

  What gets looked at

  • The whole roof: covering, upstands, outlets and every penetration.
  • The marks inside, to see which path water could have followed to reach them.
  • Moisture readings where nothing shows on the surface.

• Damp that is there whatever the weather. If the water takes no notice of rain at all, the roof may not be the source. Where it is the roof, this often points to water held in the insulation from an earlier leak and released slowly in dry spells, well after the rain itself. When a roof has both problems.

  What gets looked at

  • Moisture readings, to tell whether the insulation is wet.
  • The covering and its details, for the point where water first got in.
  • Your record of the weather against the marks, if you have kept one.

None of these is a diagnosis. Water travels before it shows, so only the roof itself confirms where it gets in, sometimes from a look and sometimes only once the water has been traced.

Coverings

What each covering tends to do

The same leak looks different on different roofs. If you know what yours is covered with, this is what that covering usually does as it ages, each from the guide linked beside it.

• Felt. As it ages it stiffens and cracks, blisters form where the layers have come unstuck, and the laps between sheets can lift and open. The signs on a felt roof.
• Liquid-applied. Its thickness is set on the roof rather than in a factory, so thin or damaged areas matter, and so do the edges and laps where it meets another material. How liquid and felt differ.
• Rubber or single ply. The sheet usually outlasts the joints between sheets, so seams are where it tends to fail; a thin membrane punctures easily, and at upstands and edges it can shrink and pull away. Single ply roof repair.
• Asphalt. Fine crazing stays in the surface and shows that the finish has weathered; a crack right through, often along a junction or a line where the deck moves, is what lets water in, and blisters are the other common sight. What mastic asphalt is.
• Not sure what it is. That is fine: the inspection establishes what the roof is covered with, and that comes before any talk of repairs. Telling coverings apart.

First steps

What to do while you wait

• Contain it. Put a bucket under the drip, move or cover whatever the water could spoil, and turn the power off in that room if a light or a socket is anywhere near the wet.
• Photograph it with the date. The ceiling, the wall beneath it and the room around it, then the same shot again each time the mark changes.
• Stay off a wet roof. Wet coverings are slippery and domestic roof edges are seldom protected, so no look at the roof is worth the risk. Leave any sealing until the entry point is known.

What helps us

• Photographs of the ceiling and, from a safe window, of the roof.
• Roughly how old the covering is, if you know.
• When it started, and whether it is getting worse.

More on each, including what to note down before you get in touch, is in our guide to a leak's first hour.