1) Standing Water. For a flat roof it will certainly help to clear any significant standing water in the areas that the roof seems to be leaking. Often it is the weight of the pooling water that is a contributing factor for the leak. It is often a simple process of clearing water a flat roof, as you just need to sweep it away. However, this should only be done if you are able to gain safe access to the area. If water seems to be pooling on a slanted roof, then this often means a blockage of some sort is in play, which might relate to the guttering system and something is stopping the rainwater from flowing down the downspout. Clearing the obstructions should soon help to end a problem of this nature.
2) Roof Patching. In some cases it might be practical to use a specific wet patch material on the leaking area of a roof. Plastic-based roof cement is highly effective at sealing the areas of a roof where the leaks seem to be most persistent. It can however be difficult to identify the exact location of the leak. It is often necessary to inspect the rooftop for signs of damage or penetration around the tiles and flashing, as well as any rooftop equipment. It helps to start the search as close to the original leak as possible, this should hopefully steer you in the right direction to locating the roof damage
3) Using Tarp. If rainwater coming into a property seems to be a significant problem, then this is often rectified by stretching a sheet of plastic tarp across the area of the roof which is leaking. Wood strips are used to help with keeping the tarp in place. Tarp should be seen as a short-term repair until you are able to contact a reliable roofing contractor to repair the roof in a correct and proper manner.