Removing moisture from a roof is the single most long term factor for keeping the interior dry. Ponding water, water not removed by the slope of a roof - after the rain has stopped - has increased the chances for a leak by 100%. Ponding water has time to find and work on any defiencies in the roofing system. A poor roofing system on a well sloped structure will leak less than a poor roofing system on a flat or dead level structure.
Steep slope structures shed water by overlapping individual units (shingles, tile, slate, etc.) passing it down from one unit onto the other until the moisture reaches the drainage system, this is known as shingling. Shingling is not waterproofing.
Dead-level, flat or low-slope roofs require waterproofing. Flat roofs do not have the luxury of quickly removing moisture like steep slope roofs. Because of the slow rate of drainage on a flat roof, moisture is constantly testing the integrity of the roofing system. For flat roofs, roofs with no slope built into the building's structure, tapered board insulation can be installed to create slope to move the moisture to draining devices.
The vast majority of flat roofing systems are single ply membranes or built-up-roofs (BUR); there are pros and cons to each system. Flat roofs can have no air spaces, they must be completely waterproof/watertight. When creating slope on a flat structure is not possible there are "Built-Up Roofs" designed to combat ponding water by remaining fluid throughout their lifetime. The tar used between the plies of felt melt during warm weather creating a flow of tar which levels out within the plies filling any voids. These types of roofs are generally known as 'Coal Tar Roofs'.
Other common concerns for choosing a roof system are the amount of pressure the roofing system will have, for example, the effects of heavy rainfall, snowfall, or roof traffic. A quality underlayment is required if a steep sloped structure is located in an area where ice and snow are a constant threat. A firm cover-board, walkway system, heavy single-ply sheet, or a quality BUR roofing system (or a combination of them) will help protect your flat roof if there is light to heavy roof top traffic. While roofing is fairly simple in design, your design and the components used in your roofing system need to pair up with what the roofing system is protecting you against.