Mould on the ceiling under a flat roof: the roof is not necessarily leaking

The symptom: dark or fuzzy patches on the ceiling under a flat extension or roof structure, often in the corners or along the junction with the wall. Worse in the cold months, and no visible drip comes down. That last point is the clue that this may be something other than a leak.

The first possible cause is the insulation choice itself. Dutch consumer authority Milieu Centraal calls insulating a flat roof from the inside a bad idea because of the risk of moisture and wood rot, stating that the risk of moisture in the roof construction or mould on the ceiling is high. If insulation was ever fitted between or under the joists, that is the first suspect — not the covering.

The second lies in the build-up. Milieu Centraal advises ensuring a vapour-retarding layer on the warm side, that is, beneath the insulation; if it is missing or sits on the wrong side, warm indoor air is drawn into the package and condenses there. Worse is the combination: wood trapped between two vapour-retarding layers can, according to Milieu Centraal, become too damp and start to rot — at which point the problem has become structural. And the frequently heard remedy backfires: Milieu Centraal specifically advises against a ventilation space between the insulation layers, because it removes too little moisture and can even introduce extra moisture.

What you can do yourself is limited but useful: note when the patches worsen (cold weather versus after rain), ventilate the room better, and check whether the patch sits above a cold bridge. What you should not do is have the roof construction opened on suspicion, or have the covering replaced first — with a condensation problem that solves nothing. Have the build-up assessed before anything is ordered. If it follows that the package must be redone, that happens from the outside: roof covering (€ 1,500-7,500 per roof surface) and external insulation (€ 6,000-16,000 per home) in one go. Every guide rate is in the price index.