Line of yellow discoloured paint appearing on original wall/new loft wall boundary.

stuart45

It's probably been dot and dabbled.with 12.5 mm plasterboard and no insulation to bring it to the same thickness as the solid wall below.

Doozergirl

If it isn't damp then I don't think it's an ongoing issue. 

I'm wondering if it's some expanding foam on the other side that's been used to 'seal' the join and has leached through.  

I's really just be incline to put stain blocker and paint over it at this point and see how you get on. 

And I'd like to see the render sorted out. I think that is a matter of when, not if that join becomes a moisture trap.  

stuart45

I think it would have been better to finish the gable end with the tyrolean gun, and gone over the joint with a finer finish to blend it all in.
They might have used solid dabs at the bottom.