Maybe they are smarter than we think . Build the nest on a ladder and then get it up to the roof using the ladder !Those can't be the nests, they are not organised and anyway aren't swallows' nests usually up under eaves or roofs? I bet that's just a convenient place they're using for sorting and storing what they've brought back. Look around high up for the nest.
Ladders out of action means more time for cycling. Baby swallows to come home to. Sounds like a win-win situation to me!I'd disturb them now before they mess up your whole summer....