What's caused this wet patch?

[Radian]

Up in the loft I pulled back some pink insulation around this sewer stack and found a wet patch:

 

 

The card layer of the plasterboard was disintegrated and mouldy. The stain doesn't go much further than you can see in the photo so is not any kind of leak coming in from the edges of the region shown. It can only have come from above or possibly down the pipe. Neither of those options make sense to me as there are no signs of a leak anywhere above and the nearest plumbing is a meter away. The pipe is a little dirty and if the seal through the roof were to blame I'm sure it would show. But the stain does go right up to the pipe in one place.

I'm baffled.

[JohnMo]

Best guess would be the point where the vent stack goes through the roof.  If it is damp now it is a current issue or dry could be old or just not leaking now.

[TonyT]

Condensation from the stack?

[markc]

I would go with condensation forming on the stack, then running down

[Radian]

7 hours ago, JohnMo said:

Best guess would be the point where the vent stack goes through the roof.  If it is damp now it is a current issue or dry could be old or just not leaking now.

 

That's what I looked at first but the exit point is well sealed and shows absolutely no sign of water  ingress.

 

6 hours ago, TonyT said:

Condensation from the stack?

 

6 hours ago, markc said:

I would go with condensation forming on the stack, then running down

 

I think this is it... nice one!

 

I kind of ruled it at at first because there is an identical stack on the opposite side of the loft and that was perfectly dry around the plasterboard.

 

But the clue is in the direction the pool goes off in - notice the wet patch was on one side only, and there's an open vented CH header tank at the end of an imaginary line drawn between the stack, pool and nearby tank. And I was having a problem with warm system water circulating up the feed pipe that caused lots of condensation inside the tank. I'd forgotten all about this.

 

I think I fixed that issue when I recently rebuilt most of the CH system but this has made me look more closely at how the stack comes up through the plasterboard ceiling. Below is a service box in the corner of a room that goes down two floors so plenty of warm air can come up. I will gun some foam around the gap for starters. But would it be necessary to lag the pipe where its exposed in the loft? The loft is ventilated but always that little bit warmer than outside so the pipe can reach the dew point first.