Northern Light
Superstar
And they reclad the roof above the Great Hall during the last round of renovation (though if I recall correctly, all they did was laying blueskin over the existing copper roof and then putting new copper on top). Like how?
AoD
Let me preface the following by saying I did not see the last roof re-do in person, up close.. so I'm merely speculating.
Blueskin (or any other vapour-lock seal) can lead to condensation build-up due to improper ventilation.
That's the most common reason I've heard for the product's water-tightness failing. But I have no idea if that is the case here.