Your dormer was designed for the weather of its day

A dormer's little flat roof catches a lot of water for its size, and it has to leave through one or two small outlets. That works as long as the inflow matches what it was once calculated for.

That is precisely where something shifted. Dutch consumer authority Milieu Centraal writes that KNMI has calculated that the amount of precipitation has risen by 20 percent over the past 60 years. So a dormer from the 1980s has drainage sized for a different climate than today's — and it is not only about the total, but about how it falls.

What you notice is not dramatic but creeping: water still standing on the roof too long after a heavy shower, an overflow that occasionally runs down the cheek, green growth in one corner. Each is a sign that drainage is only just keeping up — and standing water is exactly what ages the covering and edge finishing prematurely.

In practice: look an hour after a heavy shower, not during it. If water is still there, something is off with the fall or the outlet. Keep the outlet free of leaves and moss — the cheapest maintenance there is — and during a dormer renovation (€ 1,500-6,000) have the fall and drainage capacity explicitly assessed, not just the covering. Every guide rate is in the price index.