There are many misconceptions about flat roof replacement that are common and should be addressed; one of them is that they are actually flat. While there is not a pronounced angle in the design of flat roofs they are definitely not flat. If the roof was actually even all the way across then it would be virtually impossible for all of the water to be drained from the roof, which would do nothing but create problems from the beginning. When you are having a flat roof replacement done on your home then you should inquire as to what angle of attack the roofing company has planned to address the drainage of water. Plano Flat Roof has some nice tips on this.
Flat roof replacement has to be done in a manner that is going to resolve stagnant water issues; otherwise you will be in the exact same predicament within a short period of time. The last thing you want to do is pay for the same job twice. Where your flat roof is located and how it is built is going to dictate which tactic is going to be best for you.
If you have a flat roof that has at least one open side, you can have your roof built up to include a natural slope that will be installed in a fashion to propel the water towards the open edge. 1/8 of an inch slope per foot is what has been common in the past and worked fine to this point, but to insure that the water takes the hint and flows in the right direction it is suggested that you have a flat roof replacement that includes at least a ¼ inch slope per foot of roof. Having that extra slope will not create any visual difference but it will help minimize the ability of water to pond.
Your other option is to have a drain system on the roof that promotes the clearing of water off of the roof. A drain system is easy to install when you are getting a flat roof replacement done and should be discussed before the work begins. If your roof has a retaining wall around all four sides or otherwise blocks water from having access to the edges of the roof then you are going to have to go with this option. The two ways of accomplishing a proper drain system are through the use of scuppers and adding drain pipes with inlets. Scuppers are metal flashing that are inserted into an opening that has been cut into the retaining wall.
The scuppers will be sealed around all sides to prevent leaking and crickets will be installed to usher water to the scuppers for draining. Installing drain pipes is something that should be done before the actual roofing material gets applied, all that will be visible are the pipe inlets that are covered with a metal screen of some kind to prevent clogging issues. Crickets and strategically placed slopes will help the water go to where it needs to be. Drains will often work with the current gutter system already in place. Whichever way you decide to go, you should know that the slope of your flat roof replacement is crucially important to its success.