It is something of a myth to say that you have a flat roof. The simple fact is that no roof is flat. There is always a slight incline to either one side, or even two sides for the runoff from rain water. This is because the last thing that you want on a roof is standing water. Standing water can become brackish and seep into the roof and the ceiling below causing discoloration and staining. Worse, it can create mould and mildew. This can set into wooden beams and result in a costly replacement and repair bill.
The best approach is to apply a Roof sealant to the flat roof first and foremost. One of the best ones on the market is www.ct1.com/product-applications/ct1-the-ultimate-roof-sealant/. When this is applied to any cracks in the roof it is sealed for good. It also works on the edging of the roofs where the tiles meet the eaves.
Flat roofs are still a fine alternative to have on a house or an extension. It also follows that a drain and guttering is another important addition to the flat roof. One of the best advantages is that if you have adequate support underneath you could take a telescope up and do some star gazing. You might even be able to convert it into a balcony or outdoor garden area and may make a whole new space for you.