Keeping birds off the house roof and only trap on the loft is not easy and I think a lot has to do with your loft location and position in connection with house roofs.
if your garden is small and the loft is close to the roof its hard to keep the birds from using it but if your loft is far enough away from the house roofs then you have a chance. in our old house all our birds landed on the roof before trapping but at our current house they always land on the loft 1st time.
Back to the young birds flying,the longer they stay hanging around the loft roof the more comfortable and safe they will feel about sitting there,but when they are at a point were they need to be taking off and flying a bit we let them out and scare them ONCE only, use a ball or a flag whatever get them in the air and let them land when ready don't chase them again after that, do this every time you let them out chase them once and they will start to build up the time in the air and eventually kit together.