Finding a leak in your roof can be a seriously frustrating, expensive and time consuming business. The rule of thumb is that if your leak is say in a corner of your sitting room, do not expect the leak to be directly above it. The reason for this is that your house like all houses if it's say over ten years old is more than likely to be not quite level any longer, due to earth move so there your leak could be starting anywhere up to twenty feet or so from where it's actually dripping.
If your roof is reliably easy to get up onto and walk around, then either you should have a look yourself before calling in the experts. The very first thing you'd want to look for is your guttering. Make sure that's absolutely clean of all leaves and rubbish and there is flowing like it should be. It's very common for blocked guttering to cause a build up of water and there before an overflow and the result will be your leak.
Secondly have a good look out for old rusted nails if you have an iron roof and replace these if they are. Remember that water will find its way through a pinhole, then pool and build up your your ceiling that will result in quite a flow an hour or so later inside your home.
If you have a tiled roof, then look out for cracks and seal them with clear silicone available from your hardware store.
The last place to look out for can be the worst of all and that is the older flat roof with Butanol as its lining. This is a black tar like sheet. When this would have been laid, it was laid in long strips and one strip would have been overlaid over the other. Make sure all those overlaid strips are still glued down. once again your hardware store has a repair kit for Butanol.
If you can not see any problems with your roof, then head for an inspection of the barge boards.
Barge boards are located just below your roof line and if you live in an older home, they should be checked periodically for damage, wear and rot.
Because they are so high up they tend to not be as obvious as say your weatherboats or whatever you have but they are important. So, if you have a single story home then get up your ladder and first check the conditions of the corners. Usually any sign of rot can be obvious close up as you will be able to see 'soft spots' – These should be attended to immediately as rot in the wood will then more then likely lead to leaks down internal walls or in your ceiling that can cause all sorts of problems.
There are two ways of tackling this job, depending on your skills and how bad the rot is. The easiest and fastest way is to use 'builders bog' this is a compound that is cheap to buy from your hardware store and is rather like fiber glass. This is only suitable if the rot is only small – say three or four inches diameter at most.
Clean all the dead wood out of the hole. Do this absolutely thoroughly. Mix up your bog and put deep into the hole. You will only have about a 5 minute working time before the bog starts to harden. For a clean looking job, apply a spatchula to get a clean finish. Once it's hard it's quite difficult to sand.
If the rot is larger than say 3 -4 inches then you will have no option but to replace the entire barge board.
After this if you still have a leak then best to consult a qualified builder or plumber for their services.