This modern plastic coupling was allowed to leak for a few years.

 
The constant trickle of water from the gutters in the same spot eventually eroded the paintwork, and then the mortar-pointing.

 
Over time the heat inside the building will suck the moisture inwards due to evaporation. The wall is solid brick (230mm) without a cavity so it would not take long for a ‘damp’ patch to appear internally; first, the wallpaper peels away, then a dark stain on the plaster.

The matter is further complicated with a 3mm gypsum plaster skim applied over the original lime plaster. Lime plaster is able to absorb and release moisture as vapour but it becomes trapped beneath the gypsum skim which does not ‘breath’ as well. So the wall becomes cold, wet and sweating. In time this will cause condensation to occur which in time will allow black mould to grow.

 
Another issue is that as the moisture migrates through the bricks it absorbs all the contaminants from the clay (which of course is dug out of the ground) i.e. magnesium, nitrates, sulphates, potassium, chlorides etc. These are collectively known as hygroscopic salts, which means they absorb moisture from the internal atmosphere.

“What moisture?” I hear you say. Well, when asleep at night two adults will emit a few litres of moisture just from breathing. Other sources include drying wet clothes on a radiator and taking a shower with no outward ventilation. Hygroscopic salts are also acidic on the pH scale so usually damage paintwork. You cannot treat salt-contaminated plaster; you have to hack it off, usually back to the brickwork…but not always.

In this case, fortunately, this is not a north-facing wall that would be permanently cold with no direct sunlight. Otherwise, the problem would have been a lot worse.

The moral of this story is to keep an eye on your gutter couplings, the joint between two sections of gutter.

A £5 tube of gutter sealant would have nipped this in the bud a long time ago!