Roof leaks are tough to diagnose and they can cost thousands to repair. If you have a roof leak, it may not be as bad as you think though. Most roofs that leak do so because they have worn out or have been damaged over time.
If you are re-roofing, you may have to apply for building notice. The rule is that when more than 25% of a roof is to be removed and replaced, you must upgrade your insulation to the current Government regulations.