As well is two words!
How does a sage know everything about everything? or does he? or does he just think he does?
Remember, if you buy something you bought it, not brought it.
You don't actually say what make of rooflight or what seal, however, assuming it is normally the largest rooflight then it's like;y to be a Heki. As far as I'm aware they don't use a dedicated 'seal' as such, Like mast items mounted through a hole in the roof they use either a squirty tube type sealant, usually non-hardening like say Soudal RV61 or sinilar, or a Roll type mastic which is also a good choice as it's easy to work with or even a comination of the two using a final bead around the edges.
Dave.
------------- Give a man a fish and he will eat for a day,
Teach a man to fish and you can get rid of him for a whole weekend.