Advice re dampness on wooden ceiling?

[Windinherhair]

We have a small wooden den/ summer house in the back garden.
There is dampness on the ceiling, which is turning black. We're pretty sure it's not coming from the outside because we painted the shingles with a thick, gloopy tar stuff.
The wood is pretty soft where the dampness is.
There is no wall/ ceiling cavity, just the one layer of wooden planks which I think are pine.
The den window is always ajar, even throughout winter, and in the summer the door is left open most of the time. There are 3 condensation crystal units inside too, and they do take a lot of water out of the atmosphere.
Any ideas on how I can treat the ceiling?
Thanks

 

[Burnerman]

The den is sure to be damp at most times of the year, and this mould is due to a high moisture content of the air within, possibly aided and abetted with the type of floor that you have.
The roofing planks will often be cold, and this is where warmer moisture condenses.
So, when the weather warms up, brush and wipe the mould away and treat the timber with a preservative - it will help a reoccurrence to some degree.
The main issue and solution is with ventilation though - which is quite a problem if the outside air is particularly moist, which it is at the moment.
John