Why Flat Roofs on New Extensions Leak Early

The short answer: an early leak on a new extension usually comes from how the roof was built, not from the material: falls so shallow that water sits, upstands too low where the roof meets the house, rushed outlets and edges, a sagging deck, or condensation from a build-up that traps moisture. Find which before anyone covers it over.

Why does a leak in the first years point at the build?

Because the materials on a new roof have barely started to age. A covering laid recently has had no time to wear through, so when water gets in this early, the cause is almost always in the design or the installation rather than in the sheet or the membrane itself.

That changes the question worth asking. On an old roof the sensible first thought is what has worn out. On a young one it is what was missed: a slope that was never there, a junction finished in a hurry, a detail the drawings left vague and the builder filled in on the day. Every section below is one of those places.

Do the falls actually take the water away?

Often they do not, and standing water is the clue. A flat roof needs a gentle but real slope towards its outlets, and a deck laid level, or one that bends between the joists once it carries weight, leaves water lying in the same hollows after every shower.

The standard the new-build industry works to asks for a structure designed so the finished roof drains, with no back falls and no ponding, and that is a fair test to hold any extension against even where it does not formally apply. Water that never leaves keeps the covering wet, works at every lap and seam it sits over, and finds the weakest one first. The guide to ponding on flat roofs covers what lasting pools mean and what can be done about them.

Is the water coming through the roof at all?

Not always, and at the wall it frequently is not. Where the extension roof meets the house, the waterproofing must rise up the wall high enough to stay above splashing water, and a flashing has to be let into the brickwork above it so rain running down the wall is sent out over the upstand rather than behind it.

When either is skimped, water appears along that junction inside. But rain can also enter the cavity of the house wall higher up, through open joints, a missing cavity tray or a cracked sill, and run down inside the wall until it reaches the roof line. It then shows on the extension ceiling and gets called a roof leak, when the roof is the one thing letting no water in. A leak at the junction may not be coming through the roof at all, and anyone who patches the covering without checking the wall above it may be fixing nothing.

Can the water get off the roof, and stay out at the edges?

Only if the drainage was sized and finished with care. Too few outlets, outlets that are too small, or no overflow for the moment an outlet blocks, and a heavy shower has nowhere to go but back up the roof towards the details that are least able to cope.

The edges matter as much as the outlets. A drip that does not project far enough, or a trim fixed so the covering finishes short of it, lets wind-driven rain creep back underneath, and the damp shows along the outside wall of the room below. Where gutters take the water, they have to sit low enough and fall the right way; the roofline page deals with gutters and the edge boards behind them.

What about rooflights and anything else through the roof?

Every opening is a detail, and details are where hurried roofs fail. A rooflight needs a kerb tall enough for the waterproofing to be dressed up its sides properly, and a seal at the top that the rooflight itself completes.

Pipes, extract ducts and cable entries need a proper collar or sleeve bonded into the covering, not a ring of mastic around a hole. These are small areas on a small roof, and they are exactly where a builder under time pressure is tempted to improvise. The rooflights page covers the kerb and the upstand around a unit.

Could it be condensation rather than a leak?

Yes, and on a new extension it is easy to mistake one for the other. A build-up that puts its insulation in the wrong place, leaves out a vapour control layer or seals in a damp deck can produce water inside the roof with no rain involved at all.

Telling the two apart, and why it matters which you have, is set out in condensation or leak, and the build-ups themselves in warm roof or cold roof. The short version for here: a new layer on top cures a leak through the covering and does nothing for water forming underneath it.

Who built the roof, and what does a completion certificate prove?

On many extensions the roof is laid by the general builder who put up the walls, not by a roofing contractor, and that alone can explain a lot. Flat roofing is a trade of its own, with details that look simple and are not.

A completion certificate from building control shows that the work met the regulations when it was inspected. It does not show that the roof cannot leak, and an inspector looking at a finished roof sees its surface rather than every junction beneath it. What the regulations ask of a flat roof, and who signs the work off, is the subject of the guide to building regulations for flat roofs. If you are choosing who looks at it now, the questions worth asking a flat roofer apply just as much to a diagnosis as to a new roof.

What should you do now?

Record it, tell the builder if the work is recent, and then have the roof properly inspected before anything is covered over. In that order, because each step makes the next one easier.

• Photograph it and keep notes. Where the water appears, how much, and after which rain: a steady downpour, a storm from one direction, or a cold dry morning with no rain at all. That record does more to locate the fault than any amount of description afterwards.
• Speak to the builder first if the work is recent. Put it in writing with the photographs attached, so the conversation starts from evidence rather than memory.
• Then have it inspected properly. Someone should look at the falls, the junction with the house and the wall above it, the outlets, the edges and every opening, and say where it is entering before talking about a fix. Signs worth watching for in the meantime are listed in signs a flat roof is failing.

Do not let anyone brush or pour a new layer over the roof before the cause is known. Whatever goes on top can only perform if what lies beneath it is dry and sound, and laid over a fault in the falls, a low upstand or a wet deck it hides the problem while keeping it. If water is coming in right now rather than in theory, what to do when a flat roof is leaking covers the first hours, and our repairs page explains how leaks are tracked down and repaired.