Where could this damp spot be coming from?

[jamiegw]

In the top corner of a currently unoccupied cold room, there is always a damp area in the top corner of the wall. I have run a dehumidifier in the room to try and reduce the damp spot, but it gets damper again. There are more spots underneath that are also damp. The bottom of the wall isn't damp.

The outside wall brickwork needs repair, however the damp area starts a bit higher than the poor brickwork, closer to where the tiles are and the white skirting.

I don't see any cracks in the tiles so I don't know if the water is somehow getting in under the tiles.

 

[DAZB]

You may find the bottom few course and the verges have been nailed fully as per some specs for wind uplift etc but other tiles are possibly just laid on with every 3rd course nailed so just push one up until you work out which ones move.
Just a thought but the right hand side of the roof is quite a large area and unless you have another outlet to the right it is all discharging via the small spout right onto where you are having an issue. It could be that the volume of water during a downpour is travelling under the side of the tiles in that area and if there are any breaches in the felt it will enter below.
You could extend the spout into a downpipe on the roof and turn it into the gutter right over the lower outlet as it will sit in the same channel.

 

[jamiegw]

You could extend the spout into a downpipe on the roof and turn it into the gutter right over the lower outlet as it will sit in the same channel.

I've tried to have a look at that area from inside the barn and I can see the felt isn't quite going all the way to the verge tiles. So perhaps when it rains there is some water getting between gaps in the tiles and the felt is not doing it's job.

Something like this you mean?

 

[Wayners]

No cavity in that wall and you need to use lime mortar by looks of it

 

[jamiegw]

It's going to rain the next few days so I won't be taking up any tiles, but I could put a pipe there to carry the water straight down to the gutter instead of running over the tiles. Then if the damp starts reducing, I'll know it's an issue with the water from the top gutter.

If the damp doesn't clear, perhaps it's rain getting in through gaps in the tiles.

 

[jamiegw]

No cavity in that wall and you need to use lime mortar by looks of it

How do you know there isn't a cavity? The room is converted from the barn area so you are surely right about there being no cavity

 

[DAZB]

Yes, the pipe will run as you show and also add a bend turned into the gutter to direct it down the lower pipe like this...

68mm Offset Bend

EN 12200.

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