Never let them rest on a roof, no matter how far from your house they land get after them and throw something suitable at them until they fly off. If they go to another do the same, the only place they should get peace to sit is on the loft. If you consistently chase them from the roofs they soon learn it is not a place to land. Give them 2 beans short rather than 2 extra, keep them keen to be around the loft and your trapping problems will be sorted. My late father used to keep them to this motto, Young birds can only be in 3 places, the Loft the Basket or the Sky if you achieve this you will beat all before you.