Damp patch at the dormer: leak or condensation?

The symptom: discolouration on the ceiling or the inside of a cheek, often in the corner where the dormer meets the roof slope. Sometimes with a clear edge, sometimes as a vague haze that worsens in winter.

Four possible origins, from outside in. One: the junction with the roof surface — the lead or zinc work where the cheeks meet the roof. Two: the sealant joints around the frame, which harden and crack over the years. Three: the little flat roof itself, with open seams or blisters in the covering. Four — the confusing one — condensation from inside. Dutch consumer authority Milieu Centraal warns that insulating a roof incorrectly carries a high risk of moisture in the roof and rotting wood or mould; a dormer packs a lot of insulation into little space, so that risk is real.

Two questions make the distinction. Does the patch grow during and just after rain, and especially with wind from one particular direction? Then it is outside, at a junction. Is it there in the cold months instead, across a larger area, and does it dry out when you ventilate more? Then it points to condensation — and the vapour-retarding layer is the suspect, because Milieu Centraal advises placing it on the warm side, beneath the insulation. If it is missing or on the wrong side, indoor air condenses inside the package.

What you can do yourself: note for a week when the patch worsens and in what weather, ventilate the room better, and photograph the junction from the ground. What you do not do is have the construction opened on suspicion, or have sealant joints replaced first when the problem is condensation — that solves nothing. Note one frequently heard piece of advice that backfires: Milieu Centraal specifically advises against a ventilation space between insulation layers, because it removes too little moisture and can even introduce extra moisture. Have the cause established before anything is ordered; repairs fall under dormer renovation (€ 1,500-6,000) and every guide rate is in the price index.