Signs of a leaking flat roof

1Stains, damp and smells indoors

Most owners first notice a ceiling stain. It is often a brownish ring that grows after rain and fades a little as the plaster dries, leaving a tide mark at its edge. Fresh drips are the last stage, not the first. Before them you may see paint that bubbles or flakes, a patch of ceiling that looks slightly darker in damp weather, or wallpaper lifting along the top of an outside wall.

A musty smell in a room below a flat roof, or black mould spotting in a corner, points the same way. Light fittings are worth a look too. Water that reaches the ceiling void can follow cables and appear around a downlight, and in that case switch the circuit off and have it checked before using it again.

A stain that stays the same size through a dry spell and grows in a wet one is behaving like a leak. A mark that spreads during a cold, dry week may be condensation instead, and that needs a different cure.

2What the roof surface shows

Up on the roof, look for a covering that has lifted or split. On felt, watch for blisters, which are bubbles where moisture or air is trapped, and for cracks across the surface or along the laps where sheets overlap. Granules washing off the top layer expose the bitumen to sunlight. On an EPDM roof, lifted edges at the upstands, the short vertical faces where the roof meets a wall, are a common starting point. On GRP, hairline cracks at the joints or around the edge trim let water in.

Check the perimeter and every penetration: flashings against walls, rooflight kerbs, vent pipes and the outlets that carry water away. Moss and silt in an outlet tell their own story, because a blocked outlet leaves water standing against details that were never meant to sit wet. Only look from a safe position, from a window or a ladder at the eaves, and never walk a roof you have not tested.

3Standing water and soft spots

Ponding, where shallow water stays on the roof after the rain has stopped and the rest of the surface has dried, is both a sign and a cause. It tells you the water has nowhere to go, and it keeps every seam and joint wet for longer. We cover the reasons in our page on water pooling on a flat roof, which also looks at the fall a finished roof should have.

Soft spots are more serious. If the covering gives or feels spongy under a foot, the boards beneath have probably absorbed water for some time. Stop walking on it at once. Sagging in the ceiling below, or a visible dip, can point the same way.

Not every dip is a fault, though. Some roofs carry small puddles that clear within a day or so and sit away from any joint. What deserves attention is water in the same place after each rain, or a spot that is spreading.

4How the entry point is traced

Water on a flat roof moves along the line of least resistance. It can run across the deck, along an insulation layer or down a joist before it finds a gap in the ceiling and drips, so the stain may be a metre or more from the hole. That is the reason a patch laid over the stain so often fails.

We start indoors, noting the pattern of the stain and any tide marks, and opening a hatch or the void above where we can safely do so to see whether the boards are dry or dark. We then work outside, comparing what we saw inside with the details on the roof: the upstands and abutments, the outlets, the rooflights, the pipe penetrations and the edges. We go through the details one at a time rather than guessing.

The survey photographs show you the fault in your own roof, and the quote is written to match what they reveal.

5What to do while you wait

Move furniture and belongings away from the damp area and put a container under any drip. Take dated photographs of the stain, then more each time it changes. Clear leaves or debris from an outlet only where you can reach it safely from the ground or a window.

Do not climb onto a wet roof or push a sealant tube at the nearest crack. A surface repair can trap water in the boards and hide the real leak. If the ceiling is bulging, keep clear of that area and ask someone to make the room safe.

Tell your insurer early if you plan to claim, and speak to it before any permanent work. Our page on why insurers ask about flat roofs explains what records help. Ordinary upkeep, set out under flat roof maintenance, catches many of these signs while they are small. For the range of work on offer, see our flat roofing page, where repair and replacement are both covered. If you are unsure which applies, a survey settles it.