Rooftop aerials are always best, so give the rooftop one a try.
Make a note of where the aerial is pointing and then look at Wolfbane's
Coverage Predictor
Put in your postcode and from this you will be able to work out where you are receiving from and the size and group (Gp) aerial required. Then follow the link to the analogue coverage predictor. If the aerial group shows the same letter as the digital predictor then you should receive all the freeview channels available if the aerial is big enough. If it is different you will be in my situation of having a partial service.
The situation may change after switchover, so have a look at
mb21's table to find whether the aerial will need replacing post DSO as regards the (PSB) public service and commercial (COM) mux.