What Is Positive Flat Roof Drainage and Why Is It Important?

Achieving positive “flat roof drainage” is VITAL and is the total objective on any quality flat roof drainage plan. 

At first glance you may say this is a “given”. Oddly enough, it doesn’t explain why major roof drainage manufacturers do not have a roof drain sump pan (not a drain pan) available to the roof drain buying public. The task of “sumping” (lowering) a roof drain fell to the architect, engineer and contractor. The contractor in turn assigned to his field personnel to handle.  

The only way to get a “positive” flow is by making sure that the roof drains are the lowest point on the flat roof. Being lower than the flat roof surface, gravity will then push the rainwater into the roof drain. It is conveyed to a disposal point (underground storm drain system, a planter box or to the street) at ground level via the roof drain piping system. That’s how it should work but is not necessarily the case. Let’s take a look at the extant technologies in place…

What technologies are used to achieve positive drainage?


Roof drain manufacturers provide a flat plate sump receiver with their roof drains. This puts the roof drain flat on the roof. Further, when the roofer adds the roof covering to the drain, it becomes higher than the roof surface. This means rainwater must travel uphill to enter the roof drain. This is “negative” drainage and can have disastrous results.   

image of a roof drain sump receiver
sump receiver


This has been the main method used by the construction industry for decades. The installer used wood products to build a sump box. This is a very laborious, time consuming and ultimately expensive process. Further, the installer had to try and make the sump receiver and the roof drain fit between the roof joists. This couldn’t be done without cutting huge pieces from the sump pan down to make it work. Cutting the sump pan down reduces the beneficial effect of it. 

wood roof drain sump box
wood sump box and chopped roof drain pans


This method includes laying insulation on the flat roof. It is generally 2″ thick but can be thicker. When the insulation approaches the roof drains, it is now 2″ above the roof. It is then tapered to the roof drain. This is an extremely effective roof drainage system and results in positive drainage. This is used for both new construction and for recover projects.

But, what if you are changing out or replacing just a few roof drains? Do you want to spend the money to completely reslope your flat roof? No, as it would be very expensive as there are related tasks such as raising plumbing vents and AC curbs.  The below photo shows an example of a sloped roofing system. Any plumbing vents, AC curbs, heating vents etc. in the tapered area will have to be raised. 

Positive flat roof drainage using the tapered insulation roofing system.

The Old Saying Holds True - Necessity Is The Mother Of Invention

The roof drain sump pans on this website are designed and engineered to achieve positive flat roof drainage. NO EXCEPTIONS

How could such vital technology be missing from the industry for so long? It’s hard to say why exactly. But the good news is that a plumbing contractor developed these products to work on nearly any roof. Who better to solve the problem than someone that is confronted with the problem on a regular basis? This is probably the case with the development of many new ideas and inventions. 

These products have passed rigorous field testing over a number of years and they work exactly as stated and will achieve positive drainage on every job, every time, no exceptions when installed correctly. 

In addition to being a major industry advancement, they are engineered to be EASY to install by the novice or pro in minutes, not hours.  

While used on new construction projects, they particularly shine on retrofit applications. After all that’s what they were originally designed for.  

Try them out for yourself. 

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