Roof Question tiny Drips

Make sure the attic door is well sealed.

A run of masking tape it quickest

At the end of the day there shouldn't be much if any rain hitting the felt anyway, it's there as a secondary barrier and the tiles should be the first protection.

Is there a velux window on the same roof face? What cardinal direction is the roof facing?

I was initially thinking that it's almost too well defined for that. Looking again now I think it's the felt overlap which is creating the line where the DP differs. I'm thinking now that condensation is happening where there is only one layer of felt and that the line is where the felt overlap occurs. So the attic itself was warm & humid and the heatloss was coming in from outside (as it does), and the DP was reached on the single layer of felt before the double-layer creating the defined line.

OP is there is slight ridge where you can detect that the felt overlap starts at or around the moisture-mark? Example here where the overlap ends are shown as 4" where yours appear to be around 10" to 12":

Anyway OP warm moist air is getting into the attic slightly too quickly, so as above increase the insulation and seal any obvious leakage areas.

You have a point. Back to my initial thoughts then.

I dont follow you.

We are looking at the felt from the inside, so the top is over lapping the bottom.

The other side of the overlap will be even lower down.

There will an overlap outside that we cant see but it will be higher again, unless they are overlapping by 75% which I doubt!

Might be one of those half cross your eyes 3d photos, but to me at least, the bottom of the photo corresponds to the bottom of the roof. The lower layer is sagging at the top, but this is pretty typical and this section will be covered by overlap on the outside.

i.e. the darker layer is above and overlapping the lighter layer.

Actually OP, those layers of felt look completely different to me, was there some roof alteration/repair in the past?

If it was due (dew har har) to the overlap, wouldn't it be equally along the single layer portion?

A simple thing to do is just make sure that any attic insulation is back from the edges of the roof. You want your attic to be cold and windy, this removes any condensation from the leaky warm house below. As such, insulation needs to be kept back from the edges to allow air/ventilation to come in under the eaves/soffit/fascia and blow around within the attic space. This is how cold attics (what we have 99% of the time in Ireland) are designed to work.