It is not a given that your garage will be watertight. Sometimes garages aren’t built to the same standard of the rest of the house or they may have been built alongside the house but the roof is in desperate need of repair. This can lead to your garage letting in water which can be detrimental if you are not aware. If you have electrics in your garage such as lighting and sockets, then a water leak can be dangerous. If you use your garage for storage then items can quickly get damaged if they get wet. It can be a breading ground for mould and bacteria.
IF you notice a wet patch in your garage or can see water coming in then you need to try and identify where the water is coming from. If it is the roof then you will need to call out a roofing company who will come and investigate and fix the problem. If the roof is tiled then it may be that a few of the tiles need to be replaced.
Flat roofs are renowned for leaking over time, so if this has happened a number of times before you may decide to have it removed and have a pitched roof installed instead. This is obviously more costly but it may be that you are spending a lot on repairs anyway.