This is normal. There is usually 100mm (4 inches) or 150mm (6 inches) of insulation in place. If the existing insulation between the timbers is good enough, it is normally left in place. If not, it is removed and replaced. Additional is rolled on top between the timbers and then across the timbers as required, at 90 degrees to the layers below to provide better coverage. We normally increase the depth of attic insulation to 400mm (15¾ inches). If a floored area for storage is required in the attic we counter batten to raise the new floor area above the new insulation level.