In the world of damp, there are two completely different enemies that often look the same to the naked eye, but require entirely different treatments. Enemy A is an Active Leak: water coming from outside (e.g. rain through the roof) or a burst pipe. Enemy B is Condensation (Mould): water created from inside - household vapour (breathing, cooking, showering) that liquefies on a cold wall.
If you confuse the two, you'll spend thousands on the wrong contractor. Here's how to tell them apart.
Your eyes are the first diagnostic tool. Water leaves a different "fingerprint" depending on how it got there:
An active leak (e.g. from the roof) typically creates a stain that looks like a "map". It has a clear, dark-coloured (often brown or yellowish) ring on its perimeter. This happens because water carries salts and dirt from the cement and deposits them at the edges as it dries. The centre may be swollen or dripping.
Condensation rarely leaves brown rings. It usually appears as a "cloud" of tiny black or green specks (mould). There's no dripping water. Paint may peel lightly, but the plaster underneath is usually sound, unlike a leak which "rots" the wall deep down.
Where and when does the problem appear?
Can appear in the middle of the ceiling or at random spots. If it's from the roof, it appears or worsens during or right after rain. If it's from a supply pipe, the stain grows continuously - even after a month of sunshine!
Appears exclusively in winter (cold outside, heating inside). It loves the coldest spots - thermal bridges: upper room corners (where columns meet beams), around window frames, and (most classically) behind wardrobes or furniture against north-facing, uninsulated walls.
Still not sure? Try the oldest and most reliable engineering trick:
Cut a square piece (approximately 20×20 cm) of household aluminium foil or thick transparent nylon sheeting. Tape it tightly over the problematic, damp wall with insulating tape, sealing all four edges completely so no air can get through. Leave it undisturbed for 24 to 48 hours.
If the wall-side is wet → Leak (moisture is being pushed through from a pipe or roof). Call a contractor. If the room-side is covered in droplets → Condensation (room humidity is liquefying on the cold surface). Open your windows!
It's January. We spot black blotches high in the corner of the living room.
We assume the roof membrane tore. We call a contractor who charges €800 for applying brand-new polyurethane membranes across the entire roof surface. Two weeks later, the black blotches return in exactly the same corner! What happened? The family had been cooking every single day without switching on the kitchen extractor, and the poorly insulated house filled with steam and moisture. The vapour hit the freezing concrete corner, condensed into liquid water and grew mould. €800 wasted on a completely unnecessary roof repair.
We run the tin-foil test. Droplets are on the room side. We stop drying clothes on radiators, ventilate for 10 minutes every morning, and wipe the corner with diluted bleach. Problem solved for €0.
Bottom Line: Don't shoot the messenger (the wall). Mould is usually a ventilation and insulation problem, while a yellow stain is a structural waterproofing or plumbing issue. A correct diagnosis is half the (and cheaper) cure!
Return to category.
Go to categoryReturn to the central guide.
Go to guide
